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openmls has improper tag validation

High severity GitHub Reviewed Published Feb 4, 2026 in openmls/openmls • Updated Feb 4, 2026

Package

cargo openmls (Rust)

Affected versions

< 0.7.2

Patched versions

0.7.2

Description

Membership and confirmation tags may not be checked correctly due to a missing length check. Any tag that is shorter than the expected tag, but matches up to its length, as well as any empty tag is considered valid.

Impact

The vulnerability affects a secondary authentication guarantee that MLS provides in certain scenarios. The primary authentication guarantee for all messages comes from the signature on MLS messages. This guarantee is not affected by the vulnerability.
The secondary authentication attests to the group membership of the message author. For MLS private messages, it is implied in the AEAD. For MLS public messages, it is expressed as the ‘membership tag’, a MAC whose key is derived from the private group state only known to group members.

In addition, for public Commit messages, the ‘confirmation tag’ works in a similar manner. Its purpose is to help members who processed the Commit message to ascertain that they now have the same view on the group as the creator of the Commit message for both the private and public group state.

The vulnerability lets an attacker create MLS messages with a truncated tag that are considered valid nonetheless.

The vulnerability does not affect the primary authentication guarantees of MLS, but breaks post-compromise security (PCS) of the MLS authentication guarantees. As a consequence, an adversary that has compromised a member’s signature key can create valid-looking proposals even after the affected member has successfully updated its key material. However, this is only possible in applications where the following conditions are met:

  • The application uses public MLS messages (i.e. it has not restricted the wire format type to private MLS messages only), and
  • the application supports proposals by reference (aka standalone proposals).

Note that, in deployments that allow external Commits, an attacker in possession of a member’s signature key can insert itself into the group without having to forge a membership tag.

Patches

There are two ways to mitigate the issue:

  • Upgrade to openmls v0.7.2: This minor release includes a fix for the issue and bumps the libcrux dependencies. This release does not contain any breaking changes from v0.7.1.
  • Upgrade to openmls v0.8.0: This release contains the fix, as well as other improvements. The list of changes is in CHANGELOG.md. Some of the changes are breaking API changes.

References

@franziskuskiefer franziskuskiefer published to openmls/openmls Feb 4, 2026
Published to the GitHub Advisory Database Feb 4, 2026
Reviewed Feb 4, 2026
Last updated Feb 4, 2026

Severity

High

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 High
User interaction None
Vulnerable System Impact Metrics
Confidentiality Low
Integrity High
Availability Low
Subsequent System Impact Metrics
Confidentiality Low
Integrity High
Availability Low

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:H/UI:N/VC:L/VI:H/VA:L/SC:L/SI:H/SA:L

EPSS score

Weaknesses

Improper Check for Unusual or Exceptional Conditions

The product does not check or incorrectly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the product. Learn more on MITRE.

CVE ID

No known CVE

GHSA ID

GHSA-8x3w-qj7j-gqhf

Source code

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