CVE-2026-16049 in GitLab Plugin
Summary
by MITRE • 08/17/2026
Mattermost Plugins versions <=11.8 10.20.11 11.5.7.0 _The Mattermost GitLab plugin fails to verify channel permissions when processing API requests with a caller-supplied_ {{post_id}}_, and fails to validate the_ {{web_url}} _parameter against the configured GitLab instance, which allows an authenticated attacker to inject bot-authored messages containing arbitrary URLs into channels they do not have access to via the_ {{createIssue}} _and_ {{attachCommentToIssue}} _API endpoints._ Mattermost Advisory ID: MMSA-2026-00673
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Analysis
by VulDB Data Team • 08/17/2026
The vulnerability identified in Mattermost Plugins versions 11.8, 10.20.11, and 11.5.7.0 represents a critical authorization bypass within the GitLab integration plugin. This flaw stems from insufficient server-side validation of user permissions when processing API requests that involve post identifiers and external web URLs. Specifically, the application fails to verify whether the authenticated caller possesses the necessary channel-level permissions before executing actions associated with a specified {{post_id}}. Additionally, the system does not adequately validate the {{web_url}} parameter against the configured GitLab instance settings, allowing for arbitrary URL injection. This combination of failures creates an environment where authentication alone is sufficient to bypass access controls that should restrict interaction based on channel membership or role-based permissions.
From a technical perspective, the core issue lies in the logic governing the {{createIssue}} and {{attachCommentToIssue}} API endpoints. When these endpoints are invoked with a caller-supplied post ID, the backend processes the request without cross-referencing it against the current user's channel access list. This oversight allows an attacker who has valid credentials for any Mattermost account to target posts in channels where they have no membership or viewing rights. Furthermore, the lack of strict validation on the {{web_url}} parameter enables the injection of arbitrary links into bot-authored messages. The plugin treats these inputs as trusted data without performing rigorous checks against a whitelist of allowed domains or verifying that the URL belongs to the intended GitLab instance, thereby facilitating phishing attacks and malicious link distribution within restricted communication spaces.
The operational impact of this vulnerability is significant for organizations relying on Mattermost for secure internal communications. An authenticated attacker can exploit this flaw to inject bot-authored messages containing arbitrary URLs into channels they do not have access to. This capability effectively bypasses the intended isolation between different teams or project groups within a Mattermost instance. The injected links could lead users to malicious websites, facilitating credential harvesting, malware distribution, or social engineering attacks against colleagues who trust the source of the message due to its appearance as an automated bot post. This undermines the integrity and confidentiality expectations associated with private channels, potentially exposing sensitive discussions to unauthorized parties through indirect means such as phishing payloads embedded in seemingly legitimate system notifications.
This vulnerability aligns closely with CWE-269 Improper Privilege Assignment, as it allows a user to perform actions restricted by their assigned privileges. It also relates to CWE-74 Injection and CWE-807 Reliance on Untrusted Inputs in Security Decision Making, given the failure to validate external inputs like post IDs and URLs against expected constraints. In terms of MITRE ATT&CK framework mapping, this behavior corresponds to T1534 Internal Spearphishing, where an attacker uses internal systems or trusted channels to deliver malicious content. The exploitation path involves leveraging valid credentials (T1078 Valid Accounts) to bypass access controls and inject malicious payloads into restricted areas of the network environment.
To mitigate this risk, organizations should immediately upgrade Mattermost Plugins to versions greater than 11.8, 10.20.11, or 11.5.7.0, depending on their specific deployment version, as these updates contain patches for MMSA-2026-00673. Until the upgrade is performed, administrators should restrict API access to trusted IP ranges if possible and monitor logs for unusual patterns of post ID usage or URL injection attempts. It is also advisable to review channel permissions regularly to ensure that least privilege principles are strictly enforced across all integrations. Security teams should remain vigilant for any signs of unauthorized bot activity in restricted channels, as this may indicate active exploitation of the flaw before a patch can be applied.