Nodemailer before 8.0.4 is vulnerable to SMTP command injection through the unsanitized envelope.size parameter. When an application passes a custom envelope object with a size property containing CRLF characters to sendMail(), the value is concatenated into the SMTP MAIL FROM command (as SIZE=...) without sanitization, allowing injection of arbitrary SMTP commands such as RCPT TO to silently add attacker-controlled recipients. Exploitation requires the application to expose the envelope size to attacker-controlled input, as Nodemailer does not include size in the default auto-constructed envelope.

Nodemailer versions 9.1.0 through 10.0.4 contain a quadratic time complexity vulnerability in the addressparser component when parsing email addresses with RFC 5322 comments. Attackers can craft malicious email headers with comment-separated atoms to consume excessive CPU and block the Node.js event loop for several seconds, causing denial of service.

Nodemailer before 9.1.0 contains a quadratic time complexity vulnerability in the addressparser component that allows remote attackers to cause denial of service by supplying a crafted comma-separated address list. Attackers can send a single email with a large number of addresses to block the Node.js event loop for extended periods, consuming 100% CPU and freezing the process.

nodemailer before 10.0.6 contains a denial of service vulnerability in the addressparser free-text fallback regex pattern that exhibits quadratic backtracking behavior. Attackers can supply crafted email header values with long whitespace-free runs to block the Node.js event loop for tens of seconds, causing service unavailability.

Nodemailer before 8.0.8 disables TLS certificate verification in lib/fetch/index.js through rejectUnauthorized: false, allowing attackers to intercept OAuth2 token requests. Attackers in a machine-in-the-middle position can capture OAuth client secrets, refresh tokens, and access tokens transmitted over compromised HTTPS connections.

Nodemailer before 9.1.0 fails to apply UTS-46 normalization when encoding international domain names, causing the domain resolver to compute a different Punycode A-label than standards-compliant parsers. Attackers can craft recipient addresses with invisible characters or compatibility mappings that pass domain allow-list checks but are delivered to attacker-controlled domains via SMTP.

Nodemailer before 10.0.2 fails to properly flatten deeply nested arrays in recipient fields such as to, cc, and bcc, allowing attackers to cause stack exhaustion. Attackers can supply a deeply nested JSON recipient array that triggers recursive Array.toString() conversion, exhausting the call stack and terminating the Node.js process.

A vulnerability was identified in the email parsing library due to improper handling of specially formatted recipient email addresses. An attacker can exploit this flaw by crafting a recipient address that embeds an external address within quotes. This causes the application to misdirect the email to the attacker's external address instead of the intended internal recipient. This could lead to a significant data leak of sensitive information and allow an attacker to bypass security filters and access controls.

Summary A DoS can occur that immediately halts the system due to the use of an unsafe function.

Details According to RFC 5322, nested group structures (a group inside another group) are not allowed. Therefore, in lib/addressparser/index.js, the email address parser performs flattening when nested groups appear, since such input is likely to be abnormal. (If the address is valid, it is added as-is.) In other words, the parser flattens all nested groups and inserts them into the final group list. However, the code implemented for this flattening process can be exploited by malicious input and triggers DoS

RFC 5322 uses a colon (:) to define a group, and commas (,) are used to separate members within a group. At the following location in lib/addressparser/index.js:

https://github.com/nodemailer/nodemailer/blob/master/lib/addressparser/index.js#L90

there is code that performs this flattening. The issue occurs when the email address parser attempts to process the following kind of malicious address header:

g0: g1: g2: g3: ... gN: victim;

Because no recursion depth limit is enforced, the parser repeatedly invokes itself in the pattern addressparser → handleAddress → addressparser → ... for each nested group. As a result, when an attacker sends a header containing many colons, Nodemailer enters infinite recursion, eventually throwing Maximum call stack size exceeded and causing the process to terminate immediately. Due to the structure of this behavior, no authentication is required, and a single request is enough to shut down the service.

The problematic code section is as follows: js if (isGroup) { ... if (data.group.length) { let parsedGroup = addressparser(data.group.join(',')); // <- boom! parsedGroup.forEach(member => { if (member.group) { groupMembers = groupMembers.concat(member.group); } else { groupMembers.push(member); } }); } } data.group is expected to contain members separated by commas, but in the attacker’s payload the group contains colon (:) tokens. Because of this, the parser repeatedly triggers recursive calls for each colon, proportional to their number.

PoC

const nodemailer = require('nodemailer');

function buildDeepGroup(depth) { let parts = []; for (let i = 0; i < depth; i++) { parts.push(g${i}:); } return parts.join(' ') + ' user;'; }

const DEPTH = 3000; // <- control depth const toHeader = buildDeepGroup(DEPTH); console.log('to header length:', toHeader.length);

const transporter = nodemailer.createTransport({ streamTransport: true, buffer: true, newline: 'unix' });

console.log('parsing start');

transporter.sendMail( { from: 'test', to: toHeader, subject: 'test', text: 'test' }, (err, info) => { if (err) { console.error('error:', err); } else { console.log('finished :', info && info.envelope); } } ); As a result, when the colon is repeated beyond a certain threshold, the Node.js process terminates immediately.

Impact The attacker can achieve the following:

1. Force an immediate crash of any server/service that uses Nodemailer 2. Kill the backend process with a single web request 3. In environments using PM2/Forever, trigger a continuous restart loop, causing severe resource exhaustion”

nodemailer before 9.0.1 fails to apply disableFileAccess and disableUrlAccess flags to message-level raw option, allowing authenticated attackers to read arbitrary files or perform server-side request forgery by supplying path or href properties. Attackers can exploit this by crafting raw messages with file paths or URLs that bypass the intended sandbox, with fetched content delivered in the outgoing message to attacker-controlled recipients.

nodemailer before 6.9.9 contains a regular expression denial of service vulnerability in email parsing when attachDataUrls parameter is set or processing embedded file attachments. Attackers can send specially crafted emails with malicious data URLs or embedded attachments to cause the event loop to hang and deny service.

Nodemailer is a Node.js email-sending library. In versions >= 9.1.0 and < 10.0.9, the address parser (src/addressparser) mishandles addresses whose local-part is a quoted string and that are followed by RFC 5322 comments, allowing trailing comment-separated domain atoms to be retained in the normalized address. For example, the input "user"@example.com(x)evil.com is parsed to the address value 'user@example.com evil.com', which contains additional attacker-controlled domain text separated by a literal space. This parsed value is used without further strict recipient validation when the message envelope is built (envelope.to in src/mime-node), so a malformed/ambiguous recipient address can be accepted and placed in the SMTP envelope. Whether this results in delivery to an unintended recipient on real SMTP servers has not been confirmed. The issue is a variant of the RFC 5322 comment parsing problem addressed in GHSA-cc9r-2j5m-2m83, affecting the separate quoted-local-part code path. Version 10.0.9 contains a fix.

Nodemailer (npm package nodemailer) versions 9.1.0 and earlier do not honor the disableFileAccess and disableUrlAccess sandbox options when message content is resolved through the public plugin API MailMessage.resolveContent() using the documented legacy three-argument signature resolveContent(data, key, callback). Because shared.resolveContent() normalizes the missing options argument to an empty object, the message-level flags copied into mail.data by the MailMessage constructor are discarded, and resolveContentValue() skips both access-control checks, reaching nmfetch(url) or fs.createReadStream(path). As a result, plugin or application code that resolves untrusted message content (html, text, attachment path or href) via this API can be induced to read arbitrary local files or issue outbound HTTP(S) requests (server-side request forgery), bypassing the sandbox the application enabled. The internal paths used by transporter.sendMail() (resolveAll(), convertDataImages(), and the MIME streaming path) are not affected. Fixed in version 9.1.1.

Nodemailer versions 5.0.0 through 10.0.1 use a process-global DNS cache that is keyed only by the DNS host, while each cache entry also stores the caller-specific TLS servername. When two direct TLS/SMTPS transports (secure: true) resolve the same non-IP host with different tls.servername values, the first transport's servername is returned on the cache hit and overwrites the second transport's explicitly configured value, so Nodemailer sends the wrong SNI value and validates the peer certificate against the wrong identity. In multi-tenant services or SNI-routed SMTP gateways, an attacker who can prime the cache can cause a victim transport to connect to the attacker's TLS virtual host and accept the attacker's certificate even with rejectUnauthorized: true, disclosing the victim's SMTP credentials. Fixed in 10.0.2.

Nodemailer before 8.0.9 fails to enforce disableFileAccess and disableUrlAccess options during message normalization in jsonTransport. Attackers can read local files or fetch URLs by supplying path or href values in message content fields, bypassing intended access controls.

Nodemailer before 8.0.9 fails to sanitize carriage return and line feed characters in list comment fields, allowing attackers to inject arbitrary message headers. An attacker with control over list..comment parameters can inject CRLF sequences to create additional headers in generated RFC822 messages, altering mail client behavior and message semantics.

Summary A DoS can occur that immediately halts the system due to the use of an unsafe function.

Details According to RFC 5322, nested group structures (a group inside another group) are not allowed. Therefore, in lib/addressparser/index.js, the email address parser performs flattening when nested groups appear, since such input is likely to be abnormal. (If the address is valid, it is added as-is.) In other words, the parser flattens all nested groups and inserts them into the final group list. However, the code implemented for this flattening process can be exploited by malicious input and triggers DoS

RFC 5322 uses a colon (:) to define a group, and commas (,) are used to separate members within a group. At the following location in lib/addressparser/index.js:

https://github.com/nodemailer/nodemailer/blob/master/lib/addressparser/index.js#L90

there is code that performs this flattening. The issue occurs when the email address parser attempts to process the following kind of malicious address header:

g0: g1: g2: g3: ... gN: victim;

Because no recursion depth limit is enforced, the parser repeatedly invokes itself in the pattern addressparser → handleAddress → addressparser → ... for each nested group. As a result, when an attacker sends a header containing many colons, Nodemailer enters infinite recursion, eventually throwing Maximum call stack size exceeded and causing the process to terminate immediately. Due to the structure of this behavior, no authentication is required, and a single request is enough to shut down the service.

The problematic code section is as follows: js if (isGroup) { ... if (data.group.length) { let parsedGroup = addressparser(data.group.join(',')); // <- boom! parsedGroup.forEach(member => { if (member.group) { groupMembers = groupMembers.concat(member.group); } else { groupMembers.push(member); } }); } } data.group is expected to contain members separated by commas, but in the attacker’s payload the group contains colon (:) tokens. Because of this, the parser repeatedly triggers recursive calls for each colon, proportional to their number.

PoC

const nodemailer = require('nodemailer');

function buildDeepGroup(depth) { let parts = []; for (let i = 0; i < depth; i++) { parts.push(g${i}:); } return parts.join(' ') + ' user;'; }

const DEPTH = 3000; // <- control depth const toHeader = buildDeepGroup(DEPTH); console.log('to header length:', toHeader.length);

const transporter = nodemailer.createTransport({ streamTransport: true, buffer: true, newline: 'unix' });

console.log('parsing start');

transporter.sendMail( { from: 'test', to: toHeader, subject: 'test', text: 'test' }, (err, info) => { if (err) { console.error('error:', err); } else { console.log('finished :', info && info.envelope); } } ); As a result, when the colon is repeated beyond a certain threshold, the Node.js process terminates immediately.

Impact The attacker can achieve the following:

1. Force an immediate crash of any server/service that uses Nodemailer 2. Kill the backend process with a single web request 3. In environments using PM2/Forever, trigger a continuous restart loop, causing severe resource exhaustion”