Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. Prior to 0.6.5 and 0.5.15, several Net::IMAP commands accept a "raw data" argument that is sent verbatim after validation to prevent command injection. However, if a server does not support non-synchronizing literals, it may still be possible to inject arbitrary IMAP commands inside non-synchronizing literals. A server without support for non-synchronizing literals may interpret the "+}\r\n" as the end of a malformed command line and respond with a tagged BAD. In that case, the contents of the literal will be interpreted as one or more new pipelined commands, allowing a CRLF command injection attack to succeed. This affects criteria for #search and #uid_search; search_keys for #sort, #thread, #uid_sort, and #uid_thread; and attr for #fetch and #uid_fetch. This vulnerability is fixed in 0.6.5 and 0.5.15.
Net::IMAP: Command Injection via non-synchronizing literal in "raw" argument
Problem type
- CWE-77: Improper Neutralization of Special Elements used in a Command ('Command Injection')
- CWE-93: Improper Neutralization of CRLF Sequences ('CRLF Injection')
Affected products
ruby
>= 0.6.0, < 0.6.4.1 - AFFECTED
< 0.5.15 - AFFECTED
References
GitHub Security Advisories
GHSA-8p34-64r3-mwg8
Net::IMAP: Command Injection via non-synchronizing literal in "raw" argument
https://github.com/advisories/GHSA-8p34-64r3-mwg8Several Net::IMAP commands accept a "raw data" argument that is sent verbatim after validation to prevent command injection. However, if a server does not support non-synchronizing literals, it may still be possible to inject arbitrary IMAP commands inside non-synchronizing literals.
Details
Raw data arguments support embedded literal values, both synchronizing and non-synchronizing. Non-synchronizing literals can only be safely sent when the server advertises any of the LITERAL+, LITERAL-, or IMAP4rev2 capabilities. But raw data arguments do not verify server support for non-synchronizing literals prior to sending.
Servers without support for non-synchronizing literals could handle them in several different ways: If a server sees a "}\r\n" byte sequence but can't parse the literal bytesize, it may cautiously decide to close the connection, blocking any command injection attacks. However, a server without support for non-synchronizing literals may instead interpret the "+}\r\n" as the end of a malformed command line and respond with a tagged BAD. In that case, the contents of the literal will be interpreted as one or more new pipelined commands, allowing a CRLF command injection attack to succeed.
This affects the following commands' string arguments:
criteriafor#searchand#uid_searchsearch_keysfor#sort,#thread,#uid_sort, and#uid_threadattrfor#fetchand#uid_fetch
Prior to net-imap v0.6.4, v0.5.14, and v0.4.24, raw data arguments were not validated in any way, so they were also vulnerable to this attack. See CVE-2026-42257 (GHSA-hm49-wcqc-g2xg).
Impact
Fortunately, LITERAL- is supported by most modern IMAP servers. Even without support for non-synchronizing literals, cautious servers may handle invalid literal bytesize by closing the connection . However, servers which handle a non-synchronizing literal just like any other malformed command will enable this vulnerability.
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.
Mitigation
Update to a version of net-imap which validates server support for non-synchronizing literals before sending them.
If upgrading net-imap is not possible:
- Explicitly validate user-controlled inputs to prevent embedded non-synchronizing literals unless the server supports them.
- For a simpler, more cautious approach: all embedded literals can be unconditionally prohibited, by checking that string inputs do not contain any CR or LF bytes.
- Verify that the server advertises any of the
LITERAL+,LITERAL-, orIMAP4rev2capabilities before using untrusted string inputs for the affected "raw data" arguments.
JSON source
https://cveawg.mitre.org/api/cve/CVE-2026-47240Click to expand
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