CVE-2026-42245

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.

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.

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 max_response_size: 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 max_response_size, 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 max_response_size (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.

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: 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, 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.

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.