Summary Several Net::IMAP commands accept a raw string argument that is sent to the server without validation or escaping. If this string is derived from user-controlled input, it may contain contain CRLF sequences, which an attacker can use to inject arbitrary IMAP commands.
Details
Net::IMAP's generic argument handling, used by most command arguments, interprets string arguments as an IMAP astring. Depending on the string contents and the connection's UTF-8 support, this encodes strings as either a atom, quoted, or literal. These are safe from command or argument injection.
But the following commands transform specific String arguments to Net::IMAP::RawData, which bypasses normal argument validation and encoding and prints the string directly to the socket: #uidsearch, #search when criteria is a String, it is sent raw #uidfetch, #fetch when attr is a String, it is sent raw when attr is an Array, each String in attr is sent raw #uidstore, #store when attr is a String, it is sent raw #setquota: limit is interpolated with #tos and that string is sent raw
Because these string arguments are sent without any neutralization, they serve as a direct vector for command splitting. Any user controlled data interpolated into these strings can be used to break out of the intended command context.
Using "raw data" arguments for #uidstore, #store, and #setquota I both inappropriate and unnecessary. Net::IMAP's generic argument handling is sufficient to safely validate and encode their arguments. Users of the library probably do not expect arguments to these commands to be sent raw and might not be wary of passing unvalidated input.
The API for search criteria and fetch attributes is intentionally low-level and "close to the wire". It allows developers to use some IMAP extensions without requiring explicit support from the library and allows developers to use complex IMAP grammar without complex argument translation. Even so, basic validation is appropriate and could neutralize command injection.
Although this was explicitly documented for search criteria, it was insufficiently documented for fetch attr. So developers may not have realized that the attr argument to #fetch and #uidfetch is sent as "raw data".
Impact
If a developer passes an unvalidated user-controlled input for one of these method arguments, an attacker can append CRLF sequence followed by a new IMAP command (like DELETE mailbox). Although this does not directly enable data exfiltration, it could be combined with other attack vectors or knowledge of the target system's attributes, e.g.: shared mail folders or the application's installed response handlers.
The SEARCH, STORE, and FETCH commands, and their UID variants are some of the most commonly used features of the library. Applications that build search queries or fetch attributes dynamically based on user input (e.g., mail clients or archival tools) may be at significant risk.
Expected use of Net::IMAP#setquota is much more limited: SETQUOTA is often only usable by users with special administrative privileges. Depending on the server, quota administration might be managed through server configuration rather than via the IMAP protocol SETQUOTA command. It is expected to be uncommonly used in system administration scripts or in interactive sessions, it should be completely controlled by trusted users, and should only use trusted inputs. Calling #setquota with untrusted user input is expected to be a very uncommon use case. Please note however this might be combined with other attacks, for example CSRF, which provide unauthorized access to trusted inputs, and may specifically target users or scripts with administrator privileges.
Mitigation - Update to a patched version of net-imap which: - validates that Net::IMAP::RawData is composed of well-formed IMAP text, literal, and literal8 values, with no unescaped NULL, CR, or LF bytes. - does not use Net::IMAP::RawData for #store, #uidstore, or #setquota. - Prefer to send search criteria as an array of key value pairs. Avoid sending it as an interpolated string. - If an immediate upgrade is not possible: - String inputs to search criteria and fetch attributes can be validated against command injection by checking for \r and \n characters. - Hard-coding the store attr argument is often appropriate. Alternatively, user controlled inputs can be restricted to a small enumerated list which is valid for the calling application. - Use Kernel#Integer to coerce and validate user controlled inputs to #setquota limit.
Summary
Symbol arguments to commands are vulnerable to a CRLF Injection / IMAP Command injection via Symbol arguments passed to IMAP commands.
Details
Symbol arguments represent IMAP "system flags", which are formatted as "atoms" (with no quoting) with a "\" prefix. Vulnerable versions of Net::IMAP sends the symbol name directly to the socket, with no validation.
Because the Symbol input is unvalidated, it could contain invalid flag characters, including SP and CRLF, which could be used to finish the current command and inject new commands.
Although IMAP flag arguments are only valid input for a few IMAP commands, most Net::IMAP commands use generic argument handling, and will allow Symbol (flag) inputs.
Note also that the list of valid symbol inputs should be restricted to an enumerated set of standard RFC defined flag types, which have each been given specific defined semantics. Any user-provided values outside of that list of standard "system flags" needs to use the IMAP keyword syntax, which are sent as atoms, i.e: string inputs. Under no circumstances should #tosym ever be called on unvetted user-provided input: that will always be a bug in the calling code for the simple reason that userinputatom is as \userinputatom.
For forward compatibility with future IMAP extentions, Net::IMAP, does not restrict flag inputs to an enumerated list. That is the responsibility of the calling application code, which knows which flag semantics are valid for its context.
Impact
If a developer passes user-controlled input as a Symbol to most Net::IMAP commands, an attacker can append CRLF sequence followed by a new IMAP command (like DELETE mailbox).
Mitigation Upgrade to a version of Net::IMAP that validates Symbols are valid as an IMAP flag. User-provided input should never be able to control calling #tosym on string arguments.
For example, do not unsafely serialize and deserialize command arguments (e.g. with YAML or Marshal) in a way that could create unvetted Symbol arguments. For the few IMAP commands which do allow flag arguments, it may be appropriate to hard-code Symbol arguments or restrict them to an enumerated list which is valid for the calling application.
Summary
When authenticating a connection with SCRAM-SHA1 or SCRAM-SHA256, a hostile server can perform a computational denial-of-service attack on the client process by sending a big iteration count value.
Details
A hostile IMAP server can send an arbitrarily large PBKDF2 iteration count in the SCRAM server-first-message, causing the client to perform an expensive OpenSSL::KDF.pbkdf2hmac call. Because the PBKDF2 function is a blocking C extension and holds onto Ruby’s Global VM Lock, it can freeze the entire Ruby VM for the duration of the computation.
OpenSSL enforces an effective maximum by using a 32-bit signed integer for the iteration count, Depending on hardware capabilities and OpenSSL version, this iteration count may be sufficient for to block all Ruby threads in the process for over seven minutes.
This is listed as one of the "Security Considerations", in RFC 7804: A hostile server can perform a computational denial-of-service attack on clients by sending a big iteration count value. In order to defend against that, a client implementation can pick a maximum iteration count that it is willing to use and reject any values that exceed that threshold (in such cases, the client, of course, has to fail the authentication).
Impact
During SCRAM authentication to a hostile server, the entire Ruby VM will be locked for the duration of the computation. Depending on hardware capabilities and OpenSSL version, this may take many minutes.
OpenSSL::KDF.pbkdf2hmac is a blocking C function, so Timeout cannot be used to guard against this. And it retains the Global VM lock, so other ruby threads will also be unable to run.
Mitigation
Upgrade to a patched version of net-imap that adds the maxiterations option to the SASL- authenticators, and call Net::IMAP#authenticate with a maxiterations keyword argument.
NOTE: The default maxiterations is 2³¹ - 1, the maximum signed 32 bit integer, the maximum allowed by OpenSSL. To prevent a denial of service attack, this must be set to a safe value, depending on hardware and version of OpenSSL. It is the user's responsibility to enforce minimum and maximum iteration counts that are appropriate for their security context. Alternatively, avoid SCRAM- mechanisms when authenticating to untrusted servers.
Summary
Net::IMAP::ResponseReader has quadratic time complexity when reading large responses containing many string literals. A hostile server can send responses which are crafted to exhaust the client's CPU for a denial of service attack.
Details
For each literal in a response, ResponseReader rescans the entire growing response buffer. The regular expression that is used to scan the response buffer runs in linear time. With many literals, this becomes O(n²) total work. The regular expression should run in constant time: it is anchored to the end and only the last 23 bytes of the buffer are relevant.
Because the algorithmic complexity is super-linear, this bypasses protection from maxresponsesize: a response can stay well below the default size limit while still causing very large CPU cost.
Net::IMAP::ResponseReader runs continuously in the receiver thread until the connection closes.
Impact
This consumes disproportionate CPU time in the client's receiver thread. A hostile server could use this to exhaust the client's CPU for a denial of service attack.
For a response near the default maxresponsesize, each individual regexp scan could take between 100 to 200ms on common modern hardware, and this may be repeated 200k times per megabyte of response. While the regexp is scanning, it retains the Global VM lock, preventing other threads from running.
Although other threads should not be completely blocked, their run time will be significantly impacted.
Mitigation
Upgrade to a patched version of net-imap that reads responses more efficiently. Do not connect to untrusted IMAP servers. When connecting to untrusted servers, a much smaller maxresponsesize (for example: 8KiB) will limit the impact. Although this is too small for fetching unpaginated message bodies, it should be enough for most other operations.
Summary
A man-in-the-middle attacker can cause Net::IMAP#starttls to return "successfully", without starting TLS.
Details
When using Net::IMAP#starttls to upgrade a plaintext connection to use TLS, a man-in-the-middle attacker can inject a tagged OK response with an easily predictable tag. By sending the response before the client finishes sending the command, the command completes "successfully" before the response handler is registered. This allows #starttls to return without error, but the response handler is never invoked, the TLS connection is never established, and the socket remains unencrypted.
This allows man-in-the-middle attackers to perform a STARTTLS stripping attack, unless the client code explicitly checks Net::IMAP#tlsverified?.
Impact
TLS bypass, leading to cleartext transmission of sensitive information.
Mitigation
Upgrade to a patched version of net-imap that raises an exception whenever #starttls does not establish TLS. Connect to an implicit TLS port, rather than use STARTTLS with a cleartext port. This is strongly recommended anyway: RFC 8314: Cleartext Considered Obsolete: Use of Transport Layer Security (TLS) for Email Submission and Access NO STARTTLS: Why TLS is better without STARTTLS, A Security Analysis of STARTTLS in the Email Context Explicitly verify Net::IMAP#tlsverified? is true, before using the connection after #starttls.
Summary
There is a possibility for denial of service by memory exhaustion when net-imap reads server responses. At any time while the client is connected, a malicious server can send can send a "literal" byte count, which is automatically read by the client's receiver thread. The response reader immediately allocates memory for the number of bytes indicated by the server response.
This should not be an issue when securely connecting to trusted IMAP servers that are well-behaved. It can affect insecure connections and buggy, untrusted, or compromised servers (for example, connecting to a user supplied hostname).
Details
The IMAP protocol allows "literal" strings to be sent in responses, prefixed with their size in curly braces (e.g. {1234567890}\r\n). When Net::IMAP receives a response containing a literal string, it calls IO#read with that size. When called with a size, IO#read immediately allocates memory to buffer the entire string before processing continues. The server does not need to send any more data. There is no limit on the size of literals that will be accepted.
Fix Upgrade Users should upgrade to net-imap 0.5.7 or later. A configurable maxresponsesize limit has been added to Net::IMAP's response reader. The maxresponsesize limit has also been backported to net-imap 0.2.5, 0.3.9, and 0.4.20.
To set a global value for maxresponsesize, users must upgrade to net-imap ~> 0.4.20, or > 0.5.7.
Configuration
To avoid backward compatibility issues for secure connections to trusted well-behaved servers, the default maxresponsesize for net-imap 0.5.7 is very high (512MiB), and the default maxresponsesize for net-imap ~> 0.4.20, ~> 0.3.9, and 0.2.5 is nil (unlimited).
When connecting to untrusted servers or using insecure connections, a much lower maxresponsesize should be used. ruby Set the global maxresponsesize (only ~> v0.4.20, > 0.5.7) Net::IMAP.config.maxresponsesize = 256 << 10 # 256 KiB
Set when creating the connection imap = Net::IMAP.new(hostname, ssl: true, maxresponsesize: 16 << 10) # 16 KiB
Set after creating the connection imap.maxresponsesize = 256 << 20 # 256 KiB flush currently waiting read, to ensure the new setting is loaded imap.noop
Please Note: maxresponsesize only limits the size per response. It does not prevent a flood of individual responses and it does not limit how many unhandled responses may be stored on the responses hash. Users are responsible for adding response handlers to prune excessive unhandled responses.
Compatibility with lower maxresponsesize
A lower maxresponsesize may cause a few commands which legitimately return very large responses to raise an exception and close the connection. The maxresponsesize could be temporarily set to a higher value, but paginated or limited versions of commands should be used whenever possible. For example, to fetch message bodies:
ruby imap.maxresponsesize = 256 << 20 # 256 KiB imap.noop # flush currently waiting read
fetch a message in 252KiB chunks size = imap.uidfetch(uid, "RFC822.SIZE").first.rfc822size limit = 252 << 10 message = ((0..size) % limit).eachwithobject("") {|offset, str| str << imap.uidfetch(uid, "BODY.PEEK[]<#{offset}.#{limit}>").first.message(offset:) }
imap.maxresponsesize = 16 << 20 # 16 KiB imap.noop # flush currently waiting read
References
PR to introduce maxresponsesize: https://github.com/ruby/net-imap/pull/444 Specific commit: 0ae8576c1 - lib/net/imap/responsereader.rb Backport to 0.4: https://github.com/ruby/net-imap/pull/445 Backport to 0.3: https://github.com/ruby/net-imap/pull/446 Backport to 0.2: https://github.com/ruby/net-imap/pull/447