CVE-2025-25186

Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. Starting in version 0.3.2 and prior to versions 0.3.8, 0.4.19, and 0.5.6, there is a possibility for denial of service by memory exhaustion in net-imap's response parser. At any time while the client is connected, a malicious server can send can send highly compressed uid-set data which is automatically read by the client's receiver thread. The response parser uses Range#to_a to convert the uid-set data into arrays of integers, with no limitation on the expanded size of the ranges. Versions 0.3.8, 0.4.19, 0.5.6, and higher fix this issue. Additional details for proper configuration of fixed versions and backward compatibility are available in the GitHub Security Advisory.

Summary

There is a possibility for denial of service by memory exhaustion in net-imap's response parser. At any time while the client is connected, a malicious server can send can send highly compressed uid-set data which is automatically read by the client's receiver thread. The response parser uses Range#to_a to convert the uid-set data into arrays of integers, with no limitation on the expanded size of the ranges.

Details

IMAP's uid-set and sequence-set formats can compress ranges of numbers, for example: "1,2,3,4,5" and "1:5" both represent the same set. When Net::IMAP::ResponseParser receives APPENDUID or COPYUID response codes, it expands each uid-set into an array of integers. On a 64 bit system, these arrays will expand to 8 bytes for each number in the set. A malicious IMAP server may send specially crafted APPENDUID or COPYUID responses with very large uid-set ranges.

The Net::IMAP client parses each server response in a separate thread, as soon as each responses is received from the server. This attack works even when the client does not handle the APPENDUID or COPYUID responses.

Malicious inputs:

# 40 bytes expands to ~1.6GB:
"* OK [COPYUID 1 1:99999999 1:99999999]\r\n"

# Worst *valid* input scenario (using uint32 max),
# 44 bytes expands to 64GiB:
"* OK [COPYUID 1 1:4294967295 1:4294967295]\r\n"

# Numbers must be non-zero uint32, but this isn't validated.  Arrays larger than
# UINT32_MAX can be created.  For example, the following would theoretically
# expand to almost 800 exabytes:
"* OK [COPYUID 1 1:99999999999999999999 1:99999999999999999999]\r\n"

Simple way to test this:

require "net/imap"

def test(size)
  input = "A004 OK [COPYUID 1 1:#{size} 1:#{size}] too large?\r\n"
  parser = Net::IMAP::ResponseParser.new
  parser.parse input
end

test(99_999_999)

Fixes

Preferred Fix, minor API changes

Upgrade to v0.4.19, v0.5.6, or higher, and configure:

# globally
Net::IMAP.config.parser_use_deprecated_uidplus_data = false
# per-client
imap = Net::IMAP.new(hostname, ssl: true,
                               parser_use_deprecated_uidplus_data: false)
imap.config.parser_use_deprecated_uidplus_data = false

This replaces UIDPlusData with AppendUIDData and CopyUIDData. These classes store their UIDs as Net::IMAP::SequenceSet objects (not expanded into arrays of integers). Code that does not handle APPENDUID or COPYUID responses will not notice any difference. Code that does handle these responses may need to be updated. See the documentation for UIDPlusData, AppendUIDData and CopyUIDData.

For v0.3.8, this option is not available. For v0.4.19, the default value is true. For v0.5.6, the default value is :up_to_max_size. For v0.6.0, the only allowed value will be false (UIDPlusData will be removed from v0.6).

Mitigation, backward compatible API

Upgrade to v0.3.8, v0.4.19, v0.5.6, or higher.

For backward compatibility, uid-set can still be expanded into an array, but a maximum limit will be applied.

Assign config.parser_max_deprecated_uidplus_data_size to set the maximum UIDPlusData UID set size. When config.parser_use_deprecated_uidplus_data == true, larger sets will raise Net::IMAP::ResponseParseError. When config.parser_use_deprecated_uidplus_data == :up_to_max_size, larger sets will use AppendUIDData or CopyUIDData.

For v0.3,8, this limit is hard-coded to 10,000, and larger sets will always raise Net::IMAP::ResponseParseError. For v0.4.19, the limit defaults to 1000. For v0.5.6, the limit defaults to 100. For v0.6.0, the limit will be ignored (UIDPlusData will be removed from v0.6).

Please Note: unhandled responses

If the client does not add response handlers to…

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. Starting in version 0.3.2 and prior to versions 0.3.8, 0.4.19, and 0.5.6, there is a possibility for denial of service by memory exhaustion in net-imap's response parser. At any time while the client is connected, a malicious server can send can send highly compressed uid-set data which is automatically read by the client's receiver thread. The response parser uses Range#to_a to convert the uid-set data into arrays of integers, with no limitation on the expanded size of the ranges. Versions 0.3.8, 0.4.19, 0.5.6, and higher fix this issue. Additional details for proper configuration of fixed versions and backward compatibility are available in the GitHub Security Advisory.(CVE-2025-25186)

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. Starting in version 0.3.2 and prior to versions 0.3.8, 0.4.19, and 0.5.6, there is a possibility for denial of service by memory exhaustion in net-imap's response parser. At any time while the client is connected, a malicious server can send can send highly compressed uid-set data which is automatically read by the client's receiver thread. The response parser uses Range#to_a to convert the uid-set data into arrays of integers, with no limitation on the expanded size of the ranges. Versions 0.3.8, 0.4.19, 0.5.6, and higher fix this issue. Additional details for proper configuration of fixed versions and backward compatibility are available in the GitHub Security Advisory.(CVE-2025-25186)

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):

• net-imap: Net::IMAP vulnerable to possible DoS by memory exhaustion (CVE-2025-25186)

• CGI: Denial of Service in CGI::Cookie.parse (CVE-2025-27219)

• uri: userinfo leakage in URI#join, URI#merge and URI#+ (CVE-2025-27221)

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):

• net-imap: Net::IMAP vulnerable to possible DoS by memory exhaustion (CVE-2025-25186)

• CGI: Denial of Service in CGI::Cookie.parse (CVE-2025-27219)

• uri: userinfo leakage in URI#join, URI#merge and URI#+ (CVE-2025-27221)

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):

• net-imap: Net::IMAP vulnerable to possible DoS by memory exhaustion (CVE-2025-25186)

• CGI: Denial of Service in CGI::Cookie.parse (CVE-2025-27219)

• uri: userinfo leakage in URI#join, URI#merge and URI#+ (CVE-2025-27221)

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. Starting in version 0.3.2 and prior to versions 0.3.8, 0.4.19, and 0.5.6, there is a possibility for denial of service by memory exhaustion in net-imap's response parser. At any time while the client is connected, a malicious server can send can send highly compressed uid-set data which is automatically read by the client's receiver thread. The response parser uses Range#to_a to convert the uid-set data into arrays of integers, with no limitation on the expanded size of the ranges. Versions 0.3.8, 0.4.19, 0.5.6, and higher fix this issue. Additional details for proper configuration of fixed versions and backward compatibility are available in the GitHub Security Advisory.

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):

• net-imap: Net::IMAP vulnerable to possible DoS by memory exhaustion (CVE-2025-25186)
• CGI: Denial of Service in CGI::Cookie.parse (CVE-2025-27219)
• uri: userinfo leakage in URI#join, URI#merge and URI#+ (CVE-2025-27221)

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):

• net-imap: Net::IMAP vulnerable to possible DoS by memory exhaustion (CVE-2025-25186)
• CGI: Denial of Service in CGI::Cookie.parse (CVE-2025-27219)
• uri: userinfo leakage in URI#join, URI#merge and URI#+ (CVE-2025-27221)

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. Starting in version 0.3.2 and prior to versions 0.3.8, 0.4.19, and 0.5.6, there is a possibility for denial of service by memory exhaustion in net-imap's response parser. At any time while the client is connected, a malicious server can send can send highly compressed uid-set data which is automatically read by the client's receiver thread. The response parser uses Range#to_a to convert the uid-set data into arrays of integers, with no limitation on the expanded size of the ranges. Versions 0.3.8, 0.4.19, 0.5.6, and higher fix this issue. Additional details for proper configuration of fixed versions and backward compatibility are available in the GitHub Security Advisory.

Net::IMAP vulnerable to possible DoS by memory exhaustion. (CVE-2025-25186) In the CGI gem before 0.4.2 for Ruby, the CGI::Cookie.parse method in the CGI library contains a potential Denial of Service (DoS) vulnerability. The method does not impose any limit on the length of the raw cookie value it processes. This oversight can lead to excessive resource consumption when parsing extremely large cookies. (CVE-2025-27219) In the CGI gem before 0.4.2 for Ruby, a Regular Expression Denial of Service (ReDoS) vulnerability exists in the Util#escapeElement method. (CVE-2025-27220) In the URI gem before 1.0.3 for Ruby, the URI handling methods (URI.join, URI#merge, URI#+) have an inadvertent leakage of authentication credentials because userinfo is retained even after changing the host. (CVE-2025-27221)

It was discovered that Ruby incorrectly handled parsing of an XML document that has specific XML characters in an attribute value using REXML gem. An attacker could use this issue to cause Ruby to crash, resulting in a denial of service. This issue only affected in Ubuntu 22.04 LTS, Ubuntu 24.04 LTS, and Ubuntu 24.10. (CVE-2024-35176, CVE-2024-39908, CVE-2024-41123, CVE-2024-43398)

It was discovered that Ruby incorrectly handled expanding ranges in the net-imap response parser. If a user or automated system were tricked into connecting to a malicious IMAP server, a remote attacker could possibly use this issue to consume memory, leading to a denial of service. This issue only affected Ubuntu 24.04 LTS, and Ubuntu 24.10. (CVE-2025-25186)

It was discovered that the Ruby CGI gem incorrectly handled parsing certain cookies. A remote attacker could possibly use this issue to consume resources, leading to a denial of service. (CVE-2025-27219)

It was discovered that the Ruby CGI gem incorrectly handled parsing certain regular expressions. A remote attacker could possibly use this issue to consume resources, leading to a denial of service. (CVE-2025-27220)

It was discovered that the Ruby URI gem incorrectly handled certain URI handling methods. A remote attacker could possibly use this issue to leak authentication credentials. (CVE-2025-27221)