CVE-2026-12882 in Mattermost
Summary
by MITRE • 09/14/2026
Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to parse Markdown autolinks with unmatched trailing closing parentheses in linear time, which allows an authenticated user with permission to create posts to cause excessive server CPU consumption and degrade availability for other users via specially crafted post or message attachment content. Mattermost Advisory ID: MMSA-2026-00703
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Analysis
by VulDB Data Team • 09/14/2026
The vulnerability identified in Mattermost versions 11.9.x through 11.8.4, and earlier branches including 11.7.x up to 11.7.7 and 10.11.x up to 10.11.22 represents a significant performance regression rooted in the Markdown parsing engine's handling of autolinks. Specifically, the flaw occurs when processing URLs or links that contain unmatched trailing closing parentheses within their text representation. In standard Markdown syntax, an autolink is typically defined by angle brackets surrounding a URL, such as <http://example.com>. However, users often include additional context in parentheses after the link, for example: <http://example.com> (some note). The parser's algorithm fails to efficiently resolve cases where these trailing closing parentheses are not properly matched with opening ones. Instead of utilizing an efficient linear-time parsing strategy that would quickly identify and discard or correctly format such malformed structures, the engine enters a state of catastrophic backtracking. This behavior transforms what should be a simple text processing task into a computationally expensive operation whose complexity grows exponentially relative to the length of the input string containing these specific patterns.
This technical flaw directly impacts the operational stability and availability of Mattermost instances that are running affected versions. Because the vulnerability allows an authenticated user with permission to create posts or message attachments to trigger this excessive CPU consumption, it constitutes a local denial-of-service vector. An attacker does not need elevated privileges beyond standard posting rights to exploit this issue. By crafting a post containing a specifically designed string of unmatched parentheses following an autolink pattern, the malicious actor can force the Mattermost server's Markdown parser to consume disproportionate amounts of central processing power. This resource exhaustion degrades performance for all other users connected to the instance, leading to increased latency in message delivery, delayed page loads, and potentially causing the application or underlying host system to become unresponsive if multiple such requests are processed concurrently.
From a security classification perspective, this vulnerability aligns with CWE-400, which describes Uncontrolled Resource Consumption. The specific mechanism of exploiting input parsing logic to cause performance degradation is also characteristic of certain denial-of-service attack patterns found in the MITRE ATT&CK framework, particularly those involving resource exhaustion through application-layer inputs. While it does not involve privilege escalation or data exfiltration, its impact on availability makes it a critical concern for organizations relying on Mattermost for real-time communication and collaboration. The fact that it requires authentication distinguishes it from remote unauthenticated attacks but still poses a severe risk to internal teams who may inadvertently trigger the issue through malformed links shared by malicious insiders or compromised accounts with posting privileges.
To mitigate this vulnerability, administrators must upgrade their Mattermost instances immediately to versions 11.9.0 and later for the 11.9 branch, version 11.8.5 or later for the 11.8 branch, version 11.7.8 or later for the 11.7 branch, and version 10.11.23 or later for the 10.11 branch. These updated releases contain patches that correct the Markdown parser's logic to handle unmatched parentheses efficiently without triggering exponential time complexity. In addition to upgrading software, organizations should review their incident response procedures to include monitoring for unusual spikes in CPU usage associated with post creation activities. Implementing rate limiting on message posting endpoints can also serve as a temporary compensating control to mitigate the impact of such attacks until all systems are patched. Regular security audits and adherence to secure coding practices that prioritize efficient input parsing algorithms will help prevent similar resource exhaustion vulnerabilities in future software updates.