CVE-2026-42258

Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. Prior to versions 0.4.24, 0.5.14, and 0.6.4, symbol arguments to commands are vulnerable to a CRLF Injection / IMAP Command injection via Symbol arguments passed to IMAP commands. This issue has been patched in versions 0.4.24, 0.5.14, and 0.6.4.

Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. Prior to versions 0.4.24, 0.5.14, and 0.6.4, symbol arguments to commands are vulnerable to a CRLF Injection / IMAP Command injection via Symbol arguments passed to IMAP commands. This issue has been patched in versions 0.4.24, 0.5.14, and 0.6.4.

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 #to_sym ever be called on unvetted user-provided input: that will always be a bug in the calling code for the simple reason that user_input_atom is as \user_input_atom.

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 #to_sym 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.

Ruby is a fast and easy interpreted scripting language for object-oriented programming. It has many functions for processing text Files and perform system management tasks (such as Perl).

Security Fix(es):

Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. Prior to versions 0.4.24, 0.5.14, and 0.6.4, 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. This issue has been patched in versions 0.4.24, 0.5.14, and 0.6.4.(CVE-2026-42245)

Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. Prior to versions 0.3.10, 0.4.24, 0.5.14, and 0.6.4, a man-in-the-middle attacker can cause Net::IMAP#starttls to return "successfully", without starting TLS. This issue has been patched in versions 0.3.10, 0.4.24, 0.5.14, and 0.6.4.(CVE-2026-42246)

Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. Prior to versions 0.4.24, 0.5.14, and 0.6.4, 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. This issue has been patched in versions 0.4.24, 0.5.14, and 0.6.4.(CVE-2026-42257)

Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. Prior to versions 0.4.24, 0.5.14, and 0.6.4, symbol arguments to commands are vulnerable to a CRLF Injection / IMAP Command injection via Symbol arguments passed to IMAP commands. This issue has been patched in versions 0.4.24, 0.5.14, and 0.6.4.(CVE-2026-42258)

Ruby is an extensible, interpreted, object-oriented, scripting language. It has features to process text files and to perform system management tasks.

Security Fix(es):

• ruby/net-imap: ruby: Net::IMAP: IMAP Command Injection via Symbol Arguments (CVE-2026-42258)

• net-imap: ruby: Net::IMAP: Information disclosure via man-in-the-middle attack bypassing TLS (CVE-2026-42246)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Ruby is an extensible, interpreted, object-oriented, scripting language. It has features to process text files and to perform system management tasks.

Security Fix(es):

• ruby/net-imap: ruby: Net::IMAP: IMAP Command Injection via Symbol Arguments (CVE-2026-42258)

• net-imap: ruby: Net::IMAP: Information disclosure via man-in-the-middle attack bypassing TLS (CVE-2026-42246)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Ruby is an extensible, interpreted, object-oriented, scripting language. It has features to process text files and to perform system management tasks.

Security Fix(es):

• ruby: net-imap: Net::IMAP: Denial of Service via crafted IMAP responses (CVE-2026-42245)

• ruby/net-imap: ruby: Net::IMAP: IMAP Command Injection via Symbol Arguments (CVE-2026-42258)

• net-imap: ruby: Net::IMAP: Information disclosure via man-in-the-middle attack bypassing TLS (CVE-2026-42246)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Ruby is the interpreted scripting language for quick and easy object-oriented programming. It has many features to process text files and to do system management tasks (as in Perl). It is simple, straight-forward, and extensible.

Security Fix(es):

• ruby: net-imap: Net::IMAP: Denial of Service via crafted IMAP responses (CVE-2026-42245)

• ruby/net-imap: ruby: Net::IMAP: IMAP Command Injection via Symbol Arguments (CVE-2026-42258)

• net-imap: ruby: Net::IMAP: Information disclosure via man-in-the-middle attack bypassing TLS (CVE-2026-42246)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Ruby is an extensible, interpreted, object-oriented, scripting language. It has features to process text files and to perform system management tasks.

Security Fix(es):

• ruby: net-imap: Net::IMAP: Denial of Service via crafted IMAP responses (CVE-2026-42245)

• ruby/net-imap: ruby: Net::IMAP: IMAP Command Injection via Symbol Arguments (CVE-2026-42258)

• net-imap: ruby: Net::IMAP: Information disclosure via man-in-the-middle attack bypassing TLS (CVE-2026-42246)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Ruby is an extensible, interpreted, object-oriented, scripting language. It has features to process text files and to perform system management tasks.

Security Fix(es):

• ruby: net-imap: Net::IMAP: Denial of Service via crafted IMAP responses (CVE-2026-42245)

• ruby/net-imap: ruby: Net::IMAP: IMAP Command Injection via Symbol Arguments (CVE-2026-42258)

• net-imap: ruby: Net::IMAP: Information disclosure via man-in-the-middle attack bypassing TLS (CVE-2026-42246)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Ruby is an extensible, interpreted, object-oriented, scripting language. It has features to process text files and to perform system management tasks.

Security Fix(es):

• ruby: net-imap: Net::IMAP: Denial of Service via crafted IMAP responses (CVE-2026-42245)

• ruby/net-imap: ruby: Net::IMAP: IMAP Command Injection via Symbol Arguments (CVE-2026-42258)

• net-imap: ruby: Net::IMAP: Information disclosure via man-in-the-middle attack bypassing TLS (CVE-2026-42246)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. Prior to versions 0.4.24, 0.5.14, and 0.6.4, symbol arguments to commands are vulnerable to a CRLF Injection / IMAP Command injection via Symbol arguments passed to IMAP commands. This issue has been patched in versions 0.4.24, 0.5.14, and 0.6.4.

Ruby is an extensible, interpreted, object-oriented, scripting language. It has features to process text files and to perform system management tasks.

Security Fix(es):

• ruby/net-imap: ruby: Net::IMAP: IMAP Command Injection via Symbol Arguments (CVE-2026-42258)
• net-imap: ruby: Net::IMAP: Information disclosure via man-in-the-middle attack bypassing TLS (CVE-2026-42246)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Ruby is an extensible, interpreted, object-oriented, scripting language. It has features to process text files and to perform system management tasks.

Security Fix(es):

• ruby/net-imap: ruby: Net::IMAP: IMAP Command Injection via Symbol Arguments (CVE-2026-42258)
• net-imap: ruby: Net::IMAP: Information disclosure via man-in-the-middle attack bypassing TLS (CVE-2026-42246)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Ruby is an extensible, interpreted, object-oriented, scripting language. It has features to process text files and to perform system management tasks.

Security Fix(es):

• ruby: net-imap: Net::IMAP: Denial of Service via crafted IMAP responses (CVE-2026-42245)
• ruby/net-imap: ruby: Net::IMAP: IMAP Command Injection via Symbol Arguments (CVE-2026-42258)
• net-imap: ruby: Net::IMAP: Information disclosure via man-in-the-middle attack bypassing TLS (CVE-2026-42246)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Ruby is the interpreted scripting language for quick and easy object-oriented programming. It has many features to process text files and to do system management tasks (as in Perl). It is simple, straight-forward, and extensible.

Security Fix(es):

• ruby: net-imap: Net::IMAP: Denial of Service via crafted IMAP responses (CVE-2026-42245)
• ruby/net-imap: ruby: Net::IMAP: IMAP Command Injection via Symbol Arguments (CVE-2026-42258)
• net-imap: ruby: Net::IMAP: Information disclosure via man-in-the-middle attack bypassing TLS (CVE-2026-42246)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Ruby is an extensible, interpreted, object-oriented, scripting language. It has features to process text files and to perform system management tasks.

Security Fix(es):

• ruby: net-imap: Net::IMAP: Denial of Service via crafted IMAP responses (CVE-2026-42245)
• ruby/net-imap: ruby: Net::IMAP: IMAP Command Injection via Symbol Arguments (CVE-2026-42258)
• net-imap: ruby: Net::IMAP: Information disclosure via man-in-the-middle attack bypassing TLS (CVE-2026-42246)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Ruby is an extensible, interpreted, object-oriented, scripting language. It has features to process text files and to perform system management tasks.

Security Fix(es):

• ruby: net-imap: Net::IMAP: Denial of Service via crafted IMAP responses (CVE-2026-42245)
• ruby/net-imap: ruby: Net::IMAP: IMAP Command Injection via Symbol Arguments (CVE-2026-42258)
• net-imap: ruby: Net::IMAP: Information disclosure via man-in-the-middle attack bypassing TLS (CVE-2026-42246)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Ruby is an extensible, interpreted, object-oriented, scripting language. It has features to process text files and to perform system management tasks.

Security Fix(es):

• ruby: net-imap: Net::IMAP: Denial of Service via crafted IMAP responses (CVE-2026-42245)
• ruby/net-imap: ruby: Net::IMAP: IMAP Command Injection via Symbol Arguments (CVE-2026-42258)
• net-imap: ruby: Net::IMAP: Information disclosure via man-in-the-middle attack bypassing TLS (CVE-2026-42246)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

• CVE-2026-42258: fix Net::IMAP CRLF / IMAP command injection: strictly validate symbol (flag) arguments, send STORE attr as an atom, and validate the

• CVE-2026-42246: net-imap: fix STARTTLS stripping (TLS downgrade by MITM)
• CVE-2026-42258: net-imap: validate Symbol flag args (CRLF injection)

• CVE-2026-42246: fix Net::IMAP#starttls returning success without TLS on a man-in-the-middle error response
• CVE-2026-42258: fix IMAP command injection via Symbol arguments

• CVE-2026-42246: fix Net::IMAP#starttls returning success without TLS on a man-in-the-middle error response
• CVE-2026-42258: fix IMAP command injection via Symbol arguments

• CVE-2026-42246: fix Net::IMAP#starttls returning success without TLS on a man-in-the-middle error response
• CVE-2026-42258: fix IMAP command injection via Symbol arguments

This update for ruby3.4 fixes the following issues:

• CVE-2025-58767: denial of service when parsing XML containing multiple XML declarations (bsc#1250016).
• CVE-2026-27820: insufficient checks in zstream_buffer_ungets can lead to a buffer overflow (bsc#1259239).
• CVE-2026-41316: erb: @_init deserialization guard bypass via def_module / def_method / def_class (bsc#1262441).
• CVE-2026-42258: ruby: Net:IMAP: IMAP Command Injection via Symbol Arguments (bsc#1268011).
• CVE-2026-47240: Net:IMAP: Command Injection via non-synchronizing literal in "raw" argument (bsc#1268337).
• CVE-2026-47241: Net:IMAP: Denial of Service via incomplete raw argument validation (bsc#1268338).
• CVE-2026-47242: Net:IMAP: Command Injection via ID and ENABLE command arguments (bsc#1268339).
• CVE-2025-61594: rubygem-uri: URI concatenation with the plus operator can cause credential disclosure (bsc#1255833).

Changes for ruby3.4:

• Update to 3.4.10.

• Update to 3.4.9:

• Bug #21715: Miscompilation on x86-64-v2 due to undefined behavior in search_nonascii in string.c - Ruby - Ruby Issue Tracking System

• Bug #21787: IO::Buffer Integer Overflow in Range Validation Leads to Out-of-Bounds Memory Access - Ruby - Ruby Issue

• Bug #21757: Splatted args array is mutated when passing unexpected kwargs - Ruby - Ruby Issue Tracking System

• Bug #21326: Instruction generation differences between parse.y and prism for def a(x, ...); b(...); end - Ruby - Ruby Issue

• Bug #21814: 0.pow(2,-9999999999999999990) should be zero - Ruby

• Ruby Issue Tracking System

• Bug #21819: A Data object should be frozen even if it has no members - Ruby - Ruby Issue Tracking System

• Bug #21811: Fix underflow in Array#pack - Ruby - Ruby Issue

• Bug #21838: Rails seeing degradation (20% slowdown) related to Revision 079ef92b "Implement global allocatable slots and empty pages" (from Sep 5 2024) - Ruby - Ruby Issue Tracking System

• Bug #21931: GC Crash in String#% (backport 726205b354d1068147719fb42e1de743f1838ef1) - Ruby - Ruby Issue

• Bug #21860: Process.fork: the child may deadlock on th->interrupt_lock in threadptr_interrupt_exec_cleanup - Ruby - Ruby Issue Tracking System

• Bug #21873: UnboundMethod#== returns false for methods from included/extended modules - Ruby - Ruby Issue Tracking System

• Update to 3.4.8:

  • Bug #21629: Ruby-3.4.7 prints -Wdefault-const-init-field-unsafe warnings on clang / llvm 21 - Ruby - Ruby Issue Tracking System

• Bug #21626: Backport WASI setjmp handler memory leak fixes - Ruby - Ruby Issue Tracking System

• Bug #21631: Backport openssl gem bugfix releases - Ruby - Ruby Issue Tracking System

• Bug #21632: Backport REXML CVE-2025-58767 fix (bsc#1250016) -

• Bug #21644: Stack consistency error for the newrange INSN peephole optimization with chilled string - Ruby - Ruby Issue

• Bug #21668: Improve performance of UnicodeNormalize.canonical_ordering_one - Ruby - Ruby Issue

• Bug #21638: Ractor-local $DEBUG is not marked - Ruby - Ruby

• Bug #21652: Marshal#dump documentation out-of-date/unclear regarding Data class - Ruby - Ruby Issue Tracking System

• Bug #13671: Regexp with lookbehind and case-insensitivity raises RegexpError only on strings with certain characters -

• Bug #21625: Allow IO#wait_readable together with IO#ungetc even in text mode - Ruby - Ruby Issue Tracking System

• Bug #21671: Rails CI raises Assertion Failed: rbimpl_rstring_getmem:RB_TYPE_P(str, RUBY_T_STRING): actual type: 26 with "-DENABLE_PATH_CHECK=0 -DRUBY_DEBUG=1" enabled -

• Update next stable version to 4.0 from 3.5 by hsbt . Pull Request #15146

• Bug #21679: Segfault when ruby calls pthread_detach in rb_getnameinfo - Ruby - Ruby Issue Tracking System

• Bug #21694: Crash when looking up super method from BasicObject

• Ruby - Ruby Issue Tracking System

• Bug #21707: Destructuring assignment of SimpleDelegator wrapped array bug with YJIT - Ruby - Ruby Issue Tracking System

• Bug #21265: Crash when proc from…

This update for ruby3.4 fixes the following issues

• CVE-2026-42258: Net:IMAP: Command Injection via Symbol Arguments (bsc#1268011).
• CVE-2026-47240: Net:IMAP: Command Injection via non-synchronizing literal in 'raw' argument (bsc#1268337).
• CVE-2026-47241: Net:IMAP: Denial of Service via incomplete raw argument validation (bsc#1268338).
• CVE-2026-47242: Net:IMAP: Command Injection via ID and ENABLE command arguments (bsc#1268339).
• CVE-2025-61594: merging URIs using the + operator could expose sensitive user credentials (bsc#1270034).

Changes for ruby3.4:

• Update to 3.4.10:

  • bundling net-imap 0.5.15.

It was discovered that the Net::IMAP client in Ruby did not properly sanitize Symbol arguments passed to IMAP commands. A remote attacker controlling a malicious IMAP server, or able to influence command arguments, could use this to inject arbitrary IMAP commands via CRLF sequences. (CVE-2026-42258)

It was discovered that the Zlib::GzipReader in Ruby did not correctly ensure sufficient buffer capacity in the zstream_buffer_ungets function. An attacker could use this to craft a gzip stream that, when processed, could cause a buffer overflow, resulting in memory corruption and possibly arbitrary code execution. (CVE-2026-27820)