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Denial of Service via incomplete raw argument validation

Low
nevans published GHSA-c4fp-cxrr-mj66 Jun 9, 2026

Package

bundler net-imap (rubygems)

Affected versions

>=0.6.0, <= 0.6.4
<= 0.5.14

Patched versions

0.6.4.1
0.5.15

Description

Summary

Several Net::IMAP commands accept a raw string argument which is only validated to prevent CRLF injection and then sent verbatim. If this string is derived from user-controlled input, an attacker can force the next command to be absorbed as a continuation of the first command. This will cause the first command to eventually fail, but also prevents it from returning until another command is sent (from another thread). That other command will not return until the connection is closed.

Details

Net::IMAP::RawData was hardened in v0.6.4, v0.5.14, and v0.4.24 to reject string arguments that would smuggle an invalid literal-continuation marker onto the wire (CVE-2026-42257, GHSA-hm49-wcqc-g2xg). But the trailing-marker check uses an incorrect regex which does not match {0} or {0+}, so an attacker-controlled seach criteria or fetch attr string ending in {0} or {0+} passes validation and is sent verbatim. Since these arguments are sent as the last argument in the command, they will be followed by CRLF. Although the CRLF was intended to end the command, the server will interpret it as part of a literal prefix. This consumes the next command the client puts on the socket as additional arguments to the current command.

This affects the following command's arguments:

  • criteria for #search and #uid_search
  • search_keys for #sort, #thread, #uid_sort, and #uid_thread
  • attr for #fetch and #uid_fetch

The command which contained the attacker's raw data will not be able to complete until the next command is issued. If commands are only sent from single thread, the first command will hang until the connection times out (most likely by the server closing the connection).

If a second command is sent (from another thread), this would allow the server to respond to the first command. This combined command will be invalid:

  • The {0}\r\n literal prohibits other arguments (such as a quoted string) from spanning both commands
  • It will be sent without the space delimiter which is required between arguments.
  • The second command's tag will not be a valid argument to any of the vulnerable commands.

So the server should respond to the first command with a BAD response, which will raise a BadResponseError.

But, since the server never saw a second command, the second command will never receive a tagged response and the thread that sent it will hang until the connection is closed.

Impact

This will result in unexpected crashes and timeouts, which could be used to create a simple denial of service attack. This attack will present very similarly to common network issues or server issues which also result in commands hanging or unexpectedly raising exceptions. By itself, this does not allow command injection. But the confusion caused by these errors could lead to other downstream issues, especially in a multi-threaded environment.

Mitigation

Update to a patched version of net-imap which validates that RawData arguments may not end with literal continuation markers.
If net-imap cannot be upgraded:

  • Validate that user input to the affected command arguments does not end with "}".
  • Use of Timeout or other standard strategies for slow connections and misbehaving servers will also mitigate the effects of this.

Extra caution is required when issuing commands from multiple threads. While net-imap does have rudimentary support for issuing commands from multiple threads, the user is responsible for synchronizing that commands are issued in a logically coherent order, and for ensuring that commands are only pipelined when it is safe to do so. Practically, this means that many commands cannot be safely pipelined together, and user code will often need to wait for state changing commands to successfully complete before issuing commands that rely on that state change.

Severity

Low

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required Low
User interaction Passive
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability Low
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:P/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

CVE ID

CVE-2026-47241

Weaknesses

Improper Neutralization of Trailing Special Elements

The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes trailing special elements that could be interpreted in unexpected ways when they are sent to a downstream component. Learn more on MITRE.

Collapse of Data into Unsafe Value

The product filters data in a way that causes it to be reduced or collapsed into an unsafe value that violates an expected security property. Learn more on MITRE.

Overly Restrictive Regular Expression

A regular expression is overly restrictive, which prevents dangerous values from being detected. Learn more on MITRE.

Credits
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