CVE-2025-13352: 
vulnerability analysis and mitigation

CVE-2025-13352 is a bot identity validation bypass vulnerability in the Mattermost GitHub plugin that allows authenticated attackers to hijack the GitHub reaction feature, tricking users into adding reactions to arbitrary GitHub objects via crafted notification posts. It affects Mattermost versions 10.11.x ≤ 10.11.6 and Mattermost GitHub plugin versions ≤ 2.4.0. The vulnerability was published on December 17, 2025, with patches released the same day. It carries a CVSS v3.1 base score of 3.0 (Low) (GitHub Advisory, Mattermost Security).

The root cause is classified as CWE-1287 (Improper Validation of Specified Type of Input): the Mattermost GitHub plugin fails to validate the identity of the plugin bot during the reaction forwarding process, meaning it does not confirm that a notification post originates from the legitimate GitHub plugin bot before processing reaction events. An authenticated, low-privilege attacker can craft a malicious notification post that mimics a legitimate GitHub plugin notification, causing the reaction forwarding mechanism to associate user reactions with arbitrary GitHub objects (e.g., issues, pull requests, or comments) chosen by the attacker. Exploitation requires network access, high attack complexity, and user interaction (a victim must react to the crafted post) (GitHub Advisory).

Successful exploitation allows an attacker to manipulate GitHub repository interactions by causing users to unknowingly add reactions to arbitrary GitHub objects, potentially skewing engagement metrics or misleading repository maintainers about community sentiment. The impact is limited to integrity (low), with no confidentiality or availability impact. There is no evidence of lateral movement potential or sensitive data exposure associated with this vulnerability (GitHub Advisory, Mattermost Security).

1. Reconnaissance: Identify a Mattermost instance running server version 10.11.0–10.11.6 with the GitHub plugin version ≤ 2.4.0 installed and active.
2. Authenticate: Log in to the Mattermost instance with any low-privilege user account.
3. Craft malicious notification post: Create a post in a channel that mimics the format of a legitimate GitHub plugin notification post, referencing an arbitrary GitHub object (e.g., a specific issue or pull request URL) that the attacker wants to manipulate.
4. Trigger reaction forwarding: Because the plugin does not validate the bot identity of the notification post, when a victim user adds a reaction (e.g., 👍) to the crafted post, the plugin's reaction forwarding mechanism processes it as if it were a legitimate GitHub notification reaction.
5. Achieve objective: The victim's reaction is forwarded to the arbitrary GitHub object specified by the attacker, adding an unauthorized reaction to that object on GitHub without the victim's knowledge or intent (GitHub Advisory).

• Logs: Mattermost server logs showing reaction events being forwarded to GitHub objects from posts not originating from the official GitHub plugin bot user account; unexpected GitHub API calls for reaction creation associated with non-plugin-bot post IDs.
• Network: Outbound GitHub API requests (api.github.com/repos/.../reactions) triggered by user reactions on posts that were not created by the GitHub plugin bot.
• Application Behavior: Users reporting unexpected GitHub reaction notifications on issues or pull requests they did not intentionally react to; notification posts in Mattermost channels that appear to be from the GitHub plugin but were authored by a regular user account.

Update Mattermost server to version 10.11.7 (specifically ≥ 10.11.7-0.20251106103514-3b05384dd014) or 11.1.0 and later, and update the Mattermost GitHub plugin to version ≥ 1.0.1-0.20250829075715-0deffcfc6bee. No configuration-based workaround has been published; upgrading both the server and plugin is the recommended remediation. Additionally, administrators should implement strict channel access controls to limit who can post in channels where the GitHub plugin is active, and monitor for suspicious reaction activity (GitHub Advisory, Mattermost Security).

The vulnerability received limited public attention given its low severity score. It was noted in automated vulnerability tracking feeds including VulnDB, CVEFeed, and CIRCL's vulnerability lookup service shortly after disclosure. No significant researcher commentary, vendor blog posts, or media coverage beyond the standard advisory and aggregator entries has been observed (GitHub Advisory).