Eval injection in the JScript event-script dispatcher in Progressive Robot Ltd's hMailServer, versions 6.0.0 through 6.3.3 on Windows, allows a remote, unauthenticated attacker to run arbitrary JScript inside the hMailServer service process, with the privileges of the service account, via a password containing a backslash followed by an apostrophe, sent in any logon (SMTP AUTH, POP3, IMAP) that names an existing, active account. Exploitation requires a non-default configuration: event scripting enabled (off by default), the script language set to JScript (the default is VBScript), and an OnClientValidatePassword handler defined in the event script. The server wrote event values into the handler call as JScript string literals, escaping the apostrophe but not the backslash, so such a value closes the literal and the rest of it is parsed as script. The same flaw is reachable by a remote POP3 server through the message UID it returns, where an OnExternalAccountDownload handler is defined, and by a remote SMTP server through the error reply it rejects a delivery with, where an OnDeliveryFailed handler is defined. Before 6.2.25 the injected script can create any COM object, and from 6.2.25 it can with the default ScriptAllowedObjects value of ''; WScript.Shell among them gives command execution as the service account. VBScript event scripts and the Linux builds of Progressive Robot Ltd's hMailServer are not affected.

SQL injection in the database layer of Progressive Robot hMailServer 6.0.0-B1 through 6.2.18, when the server uses MySQL/MariaDB or PostgreSQL, allows a remote attacker to alter SQL statements the server runs. On those two engines parameters are not bound: SQLStatement::GenerateFromCommand wrote each value into the statement's text as a quoted literal with a separate string replacement over the whole text per parameter, so a value holding the name of a parameter substituted after it had that name replaced, inside the value, by the later value with its own quotes, which ended the literal early and placed the later value in the statement as SQL. The row the server writes for each recipient of a queued message carries the recipient's address twice, so in the default configuration an authenticated user who may send mail reaches it by sending to an address of their choosing. A remote unauthenticated sender reaches the same statement through a message the server sends to the envelope sender, such as a delivery failure notice or an automatic reply, and, when greylisting or message indexing is enabled (both are off by default), through the greylisting record's envelope addresses or through the message metadata the server stores for IMAP SORT, which carries four header fields of a received message. An altered statement runs with the privileges of the server's database account; on PostgreSQL, whose client call runs every statement in the text it is given, further statements can follow it, while on MySQL/MariaDB, where one statement is run per call, the change is confined to the statement it is in. SQL Server and SQL Server Compact, where parameters are bound, are not affected.

Missing authorization in the REST and COM administration APIs of Progressive Robot hMailServer 6.0.0 through 6.3.6 allows a remote authenticated user whose credential is confined to named domains to change mail routing and domain configuration beyond those domains on a multi-tenant server. A domain administrator, an account or API key given another role that manages accounts in named domains, or a full-scope API key issued for named domains could create a domain alias for any name (so that other tenants' mail for that name is diverted to an address the attacker controls, or denied), rename a domain to any name the server does not host, set a rule's bind-to-address, send-using-route or script-function action to reach one of the server's own addresses, a configured route's stored credentials or a function of the server's event script, and set the domain's relay host - which delivery then connects to with no address rule - to an address on the server's own network; and over the COM API any account could set the three rule actions on its own rules, where the script-function action runs at delivery. The authorization gate did not refuse these routes to a confined credential and the handlers were not given the caller; over COM the domain-alias save performed no authorization check at all, the domain rename required only a domain administrator, and a rule save required at most that the caller could change the rule's owner.

Cross-site scripting in the webmail of Progressive Robot hMailServer 6.3.2 through 6.3.5 allows a remote attacker who can send a user an encrypted message to run script in the webmail's origin with that user's session. When the webmail decrypted an S/MIME message (from 6.3.2) or an OpenPGP message (from 6.3.4) in the browser, it offered each decrypted attachment as a blob URL of the media type the message declared for it. A click saved the file, but if the user opened the attachment in a new tab, a part declared as text/html was rendered as a document of the webmail's origin and its script could read the mailbox, send mail and change the account through the REST API. The webmail is served only when the REST API is enabled, which it is not by default.

Heap-based buffer overflow in the legacy Blowfish decryption routine (BlowFishEncryptor::DecryptFromString) in Progressive Robot hMailServer 6.0.0 through 6.3.3 on Windows allows a local interactive user with no hMailServer credentials to write bytes of their choosing past the end of a 255-byte heap buffer in the hMailServer service process, which runs as LocalSystem by default. The user does this by passing a long hexadecimal string to the COM method Utilities.BlowfishDecrypt, which checked no authentication. The routine converted hexadecimal input of any length into a fixed 255-byte buffer before decrypting it in place. The result is a denial of service (service crash), and possibly code execution with the privileges of the service account.

The Linux live-update apply helper (hmailserver-update) of Progressive Robot hMailServer 6.3.4 and 6.3.5 runs as root on a request file written by the unprivileged hmailserver service account, and took from that request the program used to verify an AppImage update's signature and the systemd unit to stop before reading the service account's files. An attacker who already runs code as the hmailserver service account, for example through another flaw in the mail server, can therefore have arbitrary code executed as root, on any Linux installation where the live update's path unit is active - the default for the project's .deb and .rpm packages - and on AppImage installations run under that unit.

Missing authorization on COM objects in Progressive Robot hMailServer 6.0.0 through 6.3.5 (Windows only) lets a local interactive user with no hMailServer credential read and write arbitrary files as the service account and queue mail as any sender. The service registers its COM classes with no DCOM access or launch permission and calls CoInitializeSecurity with no security descriptor, so any user logged on at the console or over Remote Desktop can activate the classes in the running service; a hMailServer.Message, its Attachments and Attachment, and a hMailServer.FetchAccount created this way carry a credential that never authenticated. Attachments.Add(path) and Attachment.SaveAs(path) performed no authorization check, and Message.Save/Copy and FetchAccount.AccountID/Save performed none either up to 6.3.3 and from 6.3.4 treated a holder with no credential as the server's own event-script host. Because the service does not impersonate the COM caller, Attachments.Add reads any file the service account can read and returns it, Attachment.SaveAs writes attacker-chosen bytes to any path it can write (on a LocalSystem installation, code execution as SYSTEM), Message.Save queues outbound mail from any address past the SMTP checks, and FetchAccount attaches a mail-fetch job to any mailbox. The objects an Application handed out behave the same once a later Authenticate on that Application fails.

Incorrect default permissions in the Windows installer of Progressive Robot hMailServer 6.0.0 through 6.3.5 allow a local authenticated user to read the mail server's data. The installer created the data, log, temp, database and event folders and the hMailServer.INI configuration file with the permissions inherited from the installation folder, by default under Program Files, which give the local Users group read access. Any user who can sign in to the computer could read every stored message, the logs, the built-in database with the accounts' password hashes whenever the service is stopped, and the configuration file, including the database password, which is sealed only with the machine's DPAPI key and can be unsealed by any local account; with an external database that password gives full control of it. The Linux AppImage of 6.3.0 through 6.3.5 likewise created its per-user data folders readable by other local users.

Incorrect default permissions in the Windows installer of Progressive Robot hMailServer 6.0.0 through 6.3.6 allow a local authenticated user to run code as the mail server's high-privileged accounts when the product is installed in a folder whose permissions are inherited from a writable parent, such as the root of a drive. The installer places the server's programs in the Bin folder of the program folder and gives neither the program folder nor Bin any permissions of their own, so they inherit the parent's. Under the default location, Program Files, ordinary users receive only read and execute and the configuration is not affected; but the installer accepts any path without warning, and a program folder at the root of an NTFS volume inherits write access for a broad group - Authenticated Users (Modify) on a Windows client, Users (create files and folders) on Windows Server. A local user without administrator rights can then alter or replace the service program, which runs as LocalSystem or as the service's virtual account, or the programs an administrator runs elevated from Bin during setup and maintenance, or place a library in Bin that those programs load in place of the system copy, gaining code execution with those privileges. The default Program Files installation is not affected.

Incorrect authorization in the COM administration API of Progressive Robot hMailServer 6.3.4 through 6.3.6 allows an account assigned the serveradmin role with a list of domains - a credential an administrator may create meaning to confine it to those domains - to act as the whole-server administrator over COM. Administrator roles are meant to confine such a credential to the domains its list names, and the REST API did not take it for the server administrator, but the COM API's server-administrator check read the administerserver permission as the whole server whatever list the grant carried. So the credential signed in through the Control Panel, or through a script run on the server, as the server administrator of every domain: it could read and change every domain's accounts and mail, rename any domain, map a domain alias onto any, write the server's settings and secrets, and create further server-administrator accounts. The COM API's domain-administrator check, asked with no domain, likewise let an account assigned the domainadmin role with a list of domains that leaves out the account's own domain administer that domain's ordinary accounts over COM. An account holding serveradmin with no list is the server administrator by design and is unaffected, as are the help desk, auditor and custom roles.

Improper link resolution in the Windows installer of Progressive Robot hMailServer 6.0.0 through 6.3.7 allows a local authenticated user to run code as the mail server's service account or as an administrator when the path to the program folder passes through a junction, a symbolic link or a mount point that the user controls or that every user may change, such as a junction made at the root of a drive, which takes the root's permissions, or through a folder that every user may rename. Setup installed the programs through any such link without first checking who controls it, or whether a folder above the program folder may be renamed, and the service is registered to run the programs by that path, so the programs are installed where a link another account controls leads, or that account can later make the path lead to a folder it controls; the service, which runs as LocalSystem or as its virtual account, and the programs administrators run from Bin are then started from there. 6.3.7's fix for CVE-2026-107571 does not cover this: its hardening of the program folder, Bin and ControlPanel does not change a folder reached through a link that the administrators do not alone control, and setup does not refuse such a path: it installs through the link and at most reports afterwards that the folders could not be hardened. The default installation under Program Files is not affected.

Missing network timeouts in the Linux builds of Progressive Robot hMailServer 6.3.0 through 6.3.5 allow a remote attacker to hold server threads indefinitely and so stop outbound mail delivery (denial of service). The server set its socket timeouts in the form Windows takes, which Linux refuses, and its HTTPS clients read without a deadline, so a peer that accepts a connection and then sends nothing held the waiting thread for as long as the connection stayed open. The MTA-STS policy fetch, enabled by default, is made during outbound delivery to mta-sts.<recipient domain>, so anyone who can make the server deliver mail to a domain they control - for example as the envelope sender of a message that bounces - can hold delivery threads until outbound delivery stops. The same flaw affects the DANE TLSA query, the OAuth2 token request, the ACME client, and the ManageSieve and metrics listeners, which a silent client stops from serving anyone else. Windows builds are not affected.

Missing Host header validation and missing throttling of failed administrator sign-ins in the REST API listener of Progressive Robot hMailServer 6.0.0 through 6.3.5 allow a remote attacker to brute-force the server administrator's password through the administrator's own browser by DNS rebinding. The listener, which is off by default and bound to the loopback when enabled, answered requests whatever their Host header named, and a failed sign-in with the administrator's password from the loopback was neither auto-banned nor delayed. A web page whose host name the attacker rebinds to 127.0.0.1, opened in a browser on the server, can therefore send authenticated requests to the listener, read the answers and try administrator passwords at full speed until one is accepted, giving the attacker full administrative control of the mail server.

Inefficient algorithmic complexity in the JSON reader of Progressive Robot hMailServer allows a remote unauthenticated attacker to make the mail services unavailable. Reading a JSON object kept the first of each duplicated member name by searching the members already read, so an object of N distinct member names cost O(N^2): 1 MB took 8.9 seconds and 4 MB took 300 seconds against the affected code. A remote attacker reaches the reader without an account by mailing a crafted TLS-RPT report (up to 16 MB after decompression) to a hosted domain's published report mailbox, which is read on a delivery thread; a few such reports hold every delivery thread (ten by default), so the server delivers no mail, local or outbound, for over an hour per report. A signed-in account reaches the same 16 MB body through the webmail's own REST routes, holding the REST API's own worker threads. Where no report mailbox is configured, the unauthenticated REST sign-in routes that read a JSON body are capped at 64 KB and are not affected.

Inefficient algorithmic complexity and a non-terminating loop in the MIME processing of received messages in Progressive Robot hMailServer 6.0.0 through 6.3.5 allow a remote unauthenticated attacker to make the mail services unavailable by sending a message. Removing a MIME header parameter whose value is empty and directly followed by a semicolon (for example a Content-Disposition with 'filename=a.bat; filename=;') entered a loop that never terminates, holding a worker thread at full load until the server is restarted; this is reached when the attachment blocker renames a blocked attachment or a filename is set over the REST API. Separately, decoding a header field that holds many RFC 2047 encoded words of an encoding other than base64 or quoted-printable, removing a parameter with many RFC 2231 continuations, and deleting many header fields of one name each took time growing with the square of the message, on the small thread pools that serve IMAP, SMTP and POP3 connections, delivery and the REST API.

Inefficient algorithmic complexity in the bounce and complaint processing of Progressive Robot hMailServer 6.3.4 and 6.3.5 allows a remote unauthenticated attacker to stop mail delivery by sending messages, when bounce processing or complaint processing is enabled or a mailing list is managed by the server (none is by default). The readers of incoming delivery status notifications (RFC 3464) and abuse feedback reports (RFC 5965) removed the blank lines at the start of the returned headers part two bytes at a time, copying the rest of the part each time, so their work grew with the square of the number of blank lines. A message shaped like such a report, whose headers part begins with a very large number of blank lines within the reader's 2 MB limit, keeps a delivery thread busy for over a minute while it is delivered, and a few such messages a minute keep every delivery thread busy.

Inefficient algorithmic complexity in the inbound DKIM and ARC signature verification of Progressive Robot hMailServer 6.0.0 through 6.3.5 allows a remote unauthenticated attacker to make the mail services unavailable by sending a message. Building the canonical header and choosing the header fields named in a signature's h= tag took time growing with the square of the message's header: the 'simple' canonicalisation prepended each continuation line of a folded field to the lines already gathered, and both canonicalisations searched the gathered fields from the bottom for each h= name and erased the match from the middle of the list. A message whose header holds very many fields, or a field folded over very many lines, with a DKIM-Signature the attacker signs for a domain they control, keeps a worker thread busy for tens of seconds per signature; up to ten signatures are evaluated per message by each of the DKIM and DMARC tests, on the threads that serve delivery and SMTP.

Use of a less trusted source in the DKIM signing of Progressive Robot hMailServer 6.0.0 through 6.3.6 allows a remote unauthenticated attacker to have mail they write signed with the DKIM key of a domain the server hosts, so that it passes DKIM and DMARC for that domain at other receivers. With DKIM signing enabled for a hosted domain, the server signed every message on its first delivery pass as the domain named in its From header, before local delivery, whatever the message's origin and without checking that its sender had authenticated. A sender anywhere could therefore send the server, as ordinary inbound mail, a message whose From is any address at such a domain, and the server signed it with that domain's key. Every copy the server then sent on to an address outside carried a valid signature aligned with the forged From, whatever the domain's DMARC policy: an account's forward, a mail rule's forward or copy, a Sieve redirect, an alias or distribution list with an outside member, the mirror copy, a managed-list or journal copy, or a message released from quarantine. A signed copy can also be re-sent to other recipients while the domain's key is unchanged. Exploitation needs DKIM signing to be enabled for the hosted domain (it is off until configured) and, for the forgery to leave the server, a forward, alias, list or rule that carries the domain's mail to an outside address. The signer likewise signed a message submitted over authenticated SMTP as whatever hosted domain its From header named, a header authenticated SMTP does not hold to the submitting account, so that an account of one hosted domain could have mail written in the name of another hosted domain signed with that domain's key; this mattered where the server holds signed-in senders to their own envelope addresses (SmtpAuthenticatedSenderCheck, off by default), as the signature was then the remaining way for such mail to pass the other domain's DMARC policy.

Unchecked input for a loop condition in Progressive Robot hMailServer 6.0.0 through 6.3.6 allows a remote unauthenticated attacker to stop the server carrying mail. The server bounds the mail it generates in response to a message - a rule forward, reply or copy, an account forward, a Sieve redirect, vacation or notify, a bounce, an out-of-office reply - by a counter it carries in the message's X-hMailServer-LoopCount header, and makes no more once the counter reaches the configured rule loop limit (5 by default); that is what ends a forwarding or auto-reply cycle in a bounded number of hops. X-hMailServer-LoopCount is an ordinary header that is not stripped from mail arriving from outside, and the server read it with a signed conversion and no validation, so a count below zero passed every test against the limit: a sender who sets X-hMailServer-LoopCount to a negative value, or on the Linux builds to a value above the 32-bit signed range that the conversion wraps to a negative one, lifts the bound, and a message that enters a generated-mail cycle then loops up to about two thousand million times, each hop a queued message and a delivery, filling the queue and disk and stopping mail service. The Sieve notify action (from 6.2.22) consulted no limit at all, and its increment of the count could overflow a signed integer; and an account could reset the counter on its own mail through a Sieve editheader of the header (from 6.2.22) or a rule Set header value action (all versions) ahead of a forward or redirect.

Incorrect authorization in the REST administration API of Progressive Robot hMailServer 6.3.4 through 6.3.6 allows a remote authenticated user whose credential holds the manageroles permission in named domains - a full-scope API key issued for those domains, or an account assigned the serveradmin role with a list of domains - to obtain the server administrator's authority. An account's admin level gives it authority, and a role assigned to it replaces that authority, which is how an administrator narrows an account whose admin level is the server administrator's. The route that takes a role away (DELETE /api/v1/role-assignments/{address}) checked only that the credential takes in what the account held at the time, not what it would hold afterwards, so such a credential could take a narrowing role away from an account in its domains whose admin level is the server administrator's, returning it to the server administrator's authority, and, having set that account's password while it held the narrower role, sign in as the server administrator.

Inefficient algorithmic complexity in the JSON reader of Progressive Robot hMailServer 6.3.6 and 6.3.7 allows a remote attacker to hold the server's mail delivery threads or its REST API's worker threads, making mail delivery, the REST API and the webmail unavailable. 6.3.6 fixed CVE-2026-107574, in which reading a JSON object searched the member names already read for each new name, by keeping the names in a hash set; the set uses the standard library's hash, which takes no secret, so member names chosen to fall in one bucket of it still cost time growing with the square of their number. Against the released reader, one such object of 16 MB took about 5 seconds to read in the Linux build, and one of under 6 MB about 23 seconds in the Windows build, where objects of the same sizes with other names took under a tenth of a second. A remote unauthenticated attacker reaches the reader by mailing such an object as a TLS-RPT report, up to 16 MB once decompressed, to a hosted domain's report mailbox, where an administrator has set one; it is read on a mail delivery thread (ten by default), and those threads deliver all mail, local and outbound, so a stream of such reports stops all delivery for as long as it continues. It is also reached without an account through the reply to a password-protected message, with the message's link alone, where the REST API is on (it is off by default) and a public base URL for such links is set; and by any account through the webmail's routes granted 16 MB, and by an application's API key through the send API. Each such request holds one of the REST API's four worker threads, in the process that also serves SMTP, IMAP and POP3.

Allocation of resources without limits in the JSON reader of Progressive Robot hMailServer 6.3.2 through 6.3.7 allows a remote attacker to exhaust the server's memory and hold its threads, making mail delivery, the REST API and the webmail unavailable. The reader built a tree of values far larger than the text it read, copying each value into every level above it, and set no bound on the number of values, so a single JSON document of many small values, within the 16 MB the server reads, raised the service's memory by about 2 GB and held one thread for seconds to minutes. From 6.3.4 a remote unauthenticated attacker reaches the reader by mailing such a document as a TLS-RPT report, up to 16 MB once decompressed, to a hosted domain's report mailbox, where an administrator has set one; it is read on a mail delivery thread (ten by default), and those threads deliver all mail, local and outbound. From 6.3.4 it is also reached without an account through the reply to a password-protected message, with the message's link alone, where the REST API is on (it is off by default) and a public base URL for such links is set; and by any account, from 6.3.2 through the webmail's send and draft routes and from 6.3.4 through its other routes granted 16 MB and the send API; the webmail's send, draft and settings routes parsed the body again for each member read. The REST API serves every request on four worker threads, in the process that also serves SMTP, IMAP and POP3, so a few such documents at once make it unavailable and can exhaust the machine's memory and end the whole service. This is a separate defect from CVE-2026-107574, the reader's handling of duplicate member names, fixed in 6.3.6.

Progressive Robot hMailServer 6.2.25 through 6.3.7 on Windows, with a PostgreSQL database, the connection's sslmode require, verify-ca or verify-full, and a root certificate file named by PostgreSQLSslRootCert (from 6.2.25) or, with TlsMode=verify-full and neither PostgreSQL key set, by TlsCaFile (from 6.3.4), hands the file's name to libpq as bytes in the system's ANSI code page (up to 6.3.5 as Latin-1 bytes, which on code page 1252 are the code page's own for U+00A0 to U+00FF), and libpq looks for the file by those bytes and then hands the same bytes to OpenSSL, whose file opening on Windows reads a name as UTF-8 first and opens the file at that reading when one is there. When every byte of the name outside ASCII also reads as UTF-8 - for example U+00C3 U+00A9, the bytes C3 A9 on code page 1252, which are U+00E9 in UTF-8 - the database server's certificate was checked against the file at that other name whenever one was there, which allows a local attacker who can create that file but cannot change the configured one, as any user can by default beside a file an administrator keeps under ProgramData, or beside a folder directly under the system drive's root whose permissions the administrator restricted, and who can intercept the connection to the database server or stand in for it, to have hMailServer trust a certificate authority of the attacker's own and read and modify the traffic between hMailServer and its database, the database password included; the file alone, holding no authority that issued the database server's certificate, stops hMailServer from connecting to its database. The same applies in 6.3.4 through 6.3.7 to TlsCaFile when an Oracle MySQL client library has been put in place of the bundled MariaDB Connector/C, since that client opens the file through OpenSSL in the same way; the bundled MariaDB Connector/C opens the file in the code page and is not affected, and neither are the Linux builds. The configuration is not the default: by default no root certificate file is named.

Progressive Robot hMailServer 6.0.0-B1 through 6.3.7 on Windows hands OpenSSL the names of its own TLS files - the listeners' certificates, chains, private keys and Diffie-Hellman parameters, a port's client-certificate CA file (from 6.2.19), the object store's CA file (from 6.3.4) and the ACME client's keys and certificates - as Latin-1 bytes, which on code page 1252 are the code page's own for U+00A0 to U+00FF, and OpenSSL's file opening on Windows reads a name as UTF-8 first and opens the file at that reading when it is there, or creates it there when its folder is. When every byte of a configured name outside ASCII also reads as UTF-8 - for example U+00C3 U+00A9, the bytes C3 A9, which are U+00E9 in UTF-8 - the server used the file at that other name whenever one was there, which allows a local attacker who can create that file or its folder but cannot change the configured one, as any user can by default beside a file an administrator keeps under ProgramData, to have a port that logs on by client certificate (from 6.2.25) accept certificates from a certificate authority of the attacker's own and so sign in over the network as any mailbox without its password; to have the server trust such an authority for its object store and, intercepting that connection, read and modify the messages it stores; to receive the private keys the ACME client writes, the key of a certificate issued for the server's names among them; or to keep TLS ports from starting. The configuration is not the default: by default these files have names in ASCII, client certificates are off on every port, and the object store and ACME are off. The Linux builds are not affected.

Progressive Robot hMailServer 6.0.0 through 6.3.5 does not enforce TLS for outbound SMTP delivery to a mail exchanger whose DNSSEC-validated TLSA records contain no DANE-EE (usage 3) record, contrary to RFC 7672 section 2.2. The server used only DANE-EE records and treated a validated TLSA record set consisting of DANE-TA (usage 2) or otherwise unusable records as if no records were published, so delivery to such a host fell back to opportunistic TLS. An attacker with an active position on the network path between the server and the recipient's mail exchanger can suppress or break the STARTTLS negotiation and cause messages to be delivered in cleartext, where they can be read and modified.

Progressive Robot hMailServer 6.0.0 through 6.3.5 fails open when applying DANE (RFC 7672) to outbound SMTP delivery. The server's validating DNSSEC resolver treated a TLSA or MX lookup that did not complete (no answer, SERVFAIL, a malformed reply), an answer without the requested records and without an NSEC/NSEC3 proof of their absence, and an answer whose records carried no applicable RRSIG as if the recipient domain were unsigned, and from 6.2.19 it also delivered to mail exchangers taken from an unvalidated MX lookup that the DNSSEC-validated MX record set did not name. An attacker who can drop, forge or strip DNS answers on the path to the server's resolver, at the resolver, or between the resolver and the recipient domain's name servers, and who holds an active position on the SMTP path, can thereby disable DANE for a DNSSEC-signed recipient domain and cause messages to be delivered in cleartext or to a host of the attacker's choosing with an arbitrary certificate, where they can be read and modified.

Progressive Robot hMailServer 6.3.4 through 6.3.7, with a MySQL or MariaDB database and TlsMode=verify-full, does not hand the database client the CA file that TlsCaFile names when the file's name contains a character outside ASCII on Linux, or above U+00FF on Windows, which allows an attacker who can intercept the connection to the database server to read and modify the traffic between hMailServer and its database, by presenting, with MariaDB Connector/C 3.4.2 to 3.4.10 as the client, a certificate from any authority, its own included, for any name, and otherwise a certificate for the database server's name issued by any certificate authority in the system's trust store. The server converted the name for the client with the C runtime's conversion in the C locale, which gives an empty string for such a name, and then set no CA file at all, without an error, while certificate verification stayed on, so the client - MariaDB Connector/C, bundled on Windows and the distribution's on Linux - verified the certificate of a database server over the network against the system's trust store instead of the configured CA; Connector/C 3.4.2 to 3.4.10 (bundled on Windows in 6.3.4 to 6.3.6) went further and, with no CA file, took a certificate within its dates whose issuer it could not find, for any name, with neither its chain nor its name checked and no password proof, when the server offered first a sign-in method that hashes the password, which an attacker in the middle chooses (MariaDB's CONC-853, fixed in Connector/C 3.4.11), and Connector/C 3.4 accepted from a MariaDB 11.4 or later server a certificate the store did not vouch for when that server proved it held the account's password; on a connection it treats as local (127.0.0.1, ::1, a Unix socket, a named pipe, and localhost on Windows), Connector/C 3.4.1 and later checked neither the certificate's issuer nor its name, so that a local attacker able to stand in for the database server could present any certificate within its dates of its own making. The settings line of hmailserver --check-database still named the configured file, and the application log said the certificate had been checked. With MariaDB Connector/C 3.4.1 to 3.4.9 as the client - bundled on Windows in 6.3.4 to 6.3.6, and the client of Linux distributions that have not patched it - the missing CA file also met the precondition of CVE-2026-107819, so that a certificate issued for any other name by a trusted authority was enough to obtain the database password. On Linux the file was found only when the server ran on glibc under a locale whose character set holds the name's characters (a UTF-8 locale, or a legacy 8-bit one the system has, in whose bytes the file is then named); with no locale, LCALL=C or a locale the system lacks, and on Alpine (musl), the connection failed instead. The configuration is not the default: TlsMode defaults to prefer, which checks no certificate.

Incorrect default permissions in the Windows installer of Progressive Robot hMailServer 6.3.7 allow a local authenticated user to run code as the administrator who runs setup, when the server is installed into a program folder that is already there and that accounts other than the administrators could write, such as a folder made beforehand at the root of a drive, or when such an installation is upgraded or repaired. 6.3.7 fixed CVE-2026-107571 by giving the program folder, Bin and ControlPanel permissions that only administrators may change, but for a folder that is already there it does so only at the end of setup: setup first copies the programs into Bin, which every user may still write, and runs programs from Bin with the administrator's elevated rights, registering the service among them, before that hardening, so that a library another account places in Bin meanwhile is loaded in place of the system's. A handle an account opened on such a folder while it could write it keeps that access after the hardening. The default installation under Program Files, whose folders give ordinary users read and execute only, is not affected.

Unverified ownership in the Windows installer of Progressive Robot hMailServer 6.3.7 allows a local authenticated user to run code as the mail server's service account or as an administrator when setup installs into, or upgrades, a program folder that accounts other than the administrators could write before, such as one made beforehand at the root of a drive. 6.3.7 fixed CVE-2026-107571 by giving the program folder, Bin and ControlPanel permissions that only administrators may change, but setup refused nothing that was already in them, or in the folders that hold the event script, the add-ons and the database scripts: a program, a library, a script or a settings file that another account owns, or whose own permissions let another account change it, is kept with that owner and those permissions, which the hardening of the folders does not take away. Setup then runs programs from Bin with the administrator's elevated rights, and the service, which runs as LocalSystem or as its virtual account, runs them and the event script, so that a library such an account placed in Bin is loaded in place of the system's; the account can still change the file after the installation. The default installation under Program Files is not affected.

Progressive Robot hMailServer 6.3.4 through 6.3.7, with a MySQL or MariaDB database, TlsMode=verify-full and no TlsCaFile, does not verify the database server's certificate's issuer or name when the connection is one that MariaDB Connector/C treats as local - Server= set to 127.0.0.1, ::1 or localhost (on Linux a Unix socket), to . on Windows (a named pipe), or to a comma-separated list of servers that holds such an element - which allows a local attacker who can stand in for the database server, for example by listening on its port while it is stopped or restarting, to receive the database sign-in, the password included, and to read and modify the traffic between hMailServer and its database. hMailServer applied verify-full by turning the client's certificate verification on without a CA file, but MariaDB Connector/C 3.4.1 and later, on a connection they treat as local and with no CA file, accept any certificate within its validity period that is self-signed or sent with its own root, for any name (Connector/C 3.4.2 to 3.4.10 also one whose issuer they cannot find), so the client accepted the certificate an impostor made, asked no proof of the password and allowed a switch to an authentication method that sends the password unhashed, while hmailserver --check-database said the certificate had been checked against the system's trust store and the application log that it had been checked. The Windows builds bundle Connector/C 3.4.9 (6.3.4 to 6.3.6) and 3.4.11 (6.3.7); on Linux the distribution's client is used, and Connector/C 3.3 and 3.4.0 and Oracle's MySQL client are not affected. The configuration is not the default: TlsMode defaults to prefer, which checks no certificate.