CVE-2026-93850 in Mattermost
Summary
by MITRE • 10/26/2026
Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 fail to bind the volume of text extracted from an uploaded PDF which allows an authenticated user to exhaust server memory and cause a denial of service via a crafted PDF whose compressed content stream expands into a very large amount of text.. Mattermost Advisory ID: MMSA-2026-00769
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Analysis
by VulDB Data Team • 10/26/2026
The vulnerability identified in Mattermost versions 11.9.x through 11.9.1, 11.8.x through 11.8.5, and 11.7.x through 11.7.10, as well as version 11.10.1, represents a critical resource exhaustion flaw rooted in the improper handling of PDF file uploads. This issue is classified under CWE-400, which denotes uncontrolled resource consumption, specifically manifesting here as an excessive allocation of memory resources during the processing of uploaded documents. The core technical failure lies within the application's PDF parsing logic, where the system fails to adequately bind or limit the volume of text extracted from a document before attempting to process it further. When a user uploads a maliciously crafted PDF file containing compressed content streams that are designed to decompress into an exceptionally large amount of textual data, the server attempts to allocate memory proportional to this expanded size without sufficient safeguards. This lack of input validation and resource limiting allows an attacker to trigger a disproportionate consumption of system resources relative to the actual size of the uploaded file on disk.
From an operational perspective, this vulnerability enables authenticated users to execute a denial-of-service attack against the Mattermost instance with minimal effort. By uploading such a crafted PDF, an adversary can cause the server's memory usage to spike dramatically, potentially leading to application crashes or severe performance degradation for all connected users. This type of attack is particularly dangerous because it requires only authentication, which many threat actors may already possess through compromised credentials or social engineering tactics. The impact extends beyond simple downtime; it affects the availability and reliability of communication channels that organizations rely on for daily operations. In enterprise environments where Mattermost serves as a primary collaboration tool, such instability can disrupt workflows, delay critical communications, and erode trust in the platform's stability.
This vulnerability aligns with several entries in the MITRE ATT&CK framework, particularly those related to resource exhaustion and denial of service via application layer attacks. It also reflects common pitfalls in handling untrusted input from file uploads, a category often associated with CWE-20 improper input validation. The failure to implement strict limits on decompressed content size is a known anti-pattern in secure software development, where developers must assume that any uploaded file could be malicious and design parsers to handle worst-case scenarios gracefully rather than catastrophically.
To mitigate this risk, organizations running affected versions of Mattermost should immediately upgrade to the latest patched release as outlined in Advisory MMSA-2026-00769. Until an upgrade is feasible, administrators can implement network-level controls such as web application firewalls or reverse proxy configurations that limit the maximum size and complexity of uploaded files. Additionally, deploying file upload scanners that inspect PDF structures for signs of compression bombs or excessive text density before they reach the application server can provide a layer of defense in depth. Regular security audits focusing on input validation practices and resource management policies are essential to prevent similar vulnerabilities from being introduced into custom integrations or future updates.