Incoming Webhook User Access Bypass in Mattermost

Executive Summary

This vulnerability affects certain versions of Mattermost (11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19) where the system fails to verify that an incoming webhook user has permission to access the target team or channel.

As a result, a requester who has webhook management permissions can create posts or direct messages that appear to be sent by another user by crafting specific incoming webhook configurations and payloads.

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Detection Guidance

Detecting this vulnerability requires checking the Mattermost server version and verifying incoming webhook configurations. The vulnerability affects specific versions of Mattermost (11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19).

• Check the Mattermost server version by running the following command on the server: 'mattermost version' or by navigating to 'System Console > About > Mattermost Version' in the web interface.
• Review incoming webhook configurations for any unauthorized or misconfigured webhooks. This can be done via the Mattermost API or by inspecting the database for webhook entries.
• Audit logs for unusual webhook activity, such as posts or direct messages attributed to users who did not create them. Look for entries in the server logs related to webhook execution.

There are no specific network-level commands to detect this vulnerability, as it is a logic flaw in the application's webhook validation process.

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Impact Analysis

This vulnerability can lead to impersonation within the Mattermost platform, allowing an attacker with webhook management permissions to post messages or direct messages as if they were another user.

Such unauthorized actions can cause misinformation, confusion, or misuse of trust within teams or channels.

The CVSS score indicates a moderate impact with a base score of 4.9, primarily affecting integrity without impacting confidentiality or availability.

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Compliance Impact

This vulnerability could impact compliance with standards and regulations like GDPR and HIPAA due to its potential to allow unauthorized posting or direct messaging attributed to another user. Under GDPR, this could lead to violations of data integrity and confidentiality principles, as it may enable unauthorized access or manipulation of user data. Specifically, it could result in unauthorized disclosure of personal data, which is a breach of Article 5(1)(f) (integrity and confidentiality) and Article 32 (security of processing).

For HIPAA, if the affected Mattermost instance is used to handle protected health information (PHI), this vulnerability could lead to unauthorized access or disclosure of PHI. This would violate the HIPAA Security Rule, which requires covered entities to implement safeguards to protect the confidentiality, integrity, and availability of PHI. Unauthorized posts or messages attributed to another user could also result in a breach notification requirement under the HIPAA Breach Notification Rule.

• GDPR: Potential violation of data integrity and confidentiality (Article 5(1)(f)) and security of processing (Article 32).
• HIPAA: Potential violation of the Security Rule and Breach Notification Rule if PHI is involved.

Organizations using Mattermost in regulated environments should assess the risk of non-compliance and apply the necessary patches or mitigations to address this vulnerability.

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Mitigation Strategies

To mitigate this vulnerability, follow these immediate steps:

• Upgrade Mattermost to the latest patched version as soon as possible. The fix is available in versions released after the advisory (MMSA-2026-00683).
• If upgrading is not immediately possible, restrict webhook management permissions to trusted administrators only. Remove webhook creation or modification permissions from non-administrative users.
• Review and validate all existing incoming webhook configurations to ensure they are correctly assigned to users with appropriate access to the target teams or channels.
• Monitor webhook activity for suspicious behavior, such as unexpected posts or direct messages attributed to other users.
• Consider disabling incoming webhooks temporarily if they are not critical to operations until the system can be upgraded.

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