CVE-2026-26246: Mattermost Server DoS Vulnerability

CVE-2026-26246 Overview

CVE-2026-26246 is a memory exhaustion vulnerability affecting Mattermost Server that allows authenticated attackers to cause denial of service conditions. The vulnerability exists in the image processing functionality where PSD (Photoshop Document) files are handled without proper memory allocation bounds checking. An attacker can upload a specially crafted PSD file that triggers unbounded memory allocation, leading to server resource exhaustion and service disruption.

Critical Impact

Authenticated attackers can exhaust server memory resources by uploading malicious PSD files, causing denial of service for all Mattermost users on the affected instance.

Affected Products

• Mattermost Server versions 11.3.x <= 11.3.0
• Mattermost Server versions 11.2.x <= 11.2.2
• Mattermost Server versions 10.11.x <= 10.11.10

Discovery Timeline

• 2026-03-16 - CVE-2026-26246 published to NVD
• 2026-03-18 - Last updated in NVD database

Technical Details for CVE-2026-26246

Vulnerability Analysis

This vulnerability is classified as CWE-789 (Memory Allocation with Excessive Size Value), a weakness where a software component allocates memory based on an untrusted size value without properly validating that the size is reasonable. In Mattermost Server, the PSD image processing functionality fails to implement proper bounds checking when parsing dimension and layer information from uploaded PSD files.

When a user uploads an image file, Mattermost processes it to generate thumbnails and previews. PSD files can contain metadata specifying extremely large image dimensions or layer counts. Without proper validation, the server attempts to allocate memory buffers sized according to these malicious values, potentially requesting gigabytes of memory for what appears to be a small file.

The attack requires authentication to the Mattermost platform, limiting the attack surface to registered users. However, in many enterprise deployments, any employee or authorized user could potentially exploit this vulnerability to disrupt team communications.

Root Cause

The root cause lies in insufficient validation of PSD file header values before memory allocation. PSD files contain structured metadata including canvas dimensions, layer counts, and channel information. The vulnerable code path trusts these values and attempts to pre-allocate buffers accordingly without checking against reasonable maximum limits or available system resources.

Attack Vector

The attack is network-based and requires low privileges (authenticated user access) with no user interaction needed from other parties. An authenticated attacker uploads a maliciously crafted PSD file through any channel, direct message, or file upload endpoint. The file appears small on disk but contains header values indicating massive image dimensions.

Upon processing, the server's image handler reads the PSD header and attempts to allocate memory proportional to the claimed dimensions. A crafted PSD claiming dimensions of 30,000 x 30,000 pixels with multiple 32-bit channels could trigger allocation requests of several gigabytes. Repeated uploads can rapidly exhaust available server memory, causing the Mattermost service to crash or become unresponsive.

Detection Methods for CVE-2026-26246

Indicators of Compromise

• Unusual spikes in server memory consumption coinciding with file upload activities
• PSD file uploads with abnormally small file sizes relative to claimed image dimensions
• Multiple PSD file uploads from the same user in rapid succession
• Server process crashes or out-of-memory (OOM) killer activity in system logs

Detection Strategies

• Monitor server memory utilization patterns and alert on sudden consumption spikes during file processing
• Implement file upload logging to track PSD file uploads and correlate with resource usage anomalies
• Configure application performance monitoring (APM) to detect memory allocation anomalies in image processing threads
• Review Mattermost application logs for image processing errors or memory-related exceptions

Monitoring Recommendations

• Enable detailed logging for file upload and processing operations in Mattermost
• Set up memory threshold alerts that trigger before OOM conditions occur
• Monitor the Mattermost process for unusual resource consumption patterns
• Implement rate limiting alerts for file uploads per user to detect potential exploitation attempts

How to Mitigate CVE-2026-26246

Immediate Actions Required

• Upgrade Mattermost Server to the latest patched version immediately
• Review recent PSD file uploads for suspicious activity patterns
• Consider temporarily blocking PSD file uploads until patches are applied
• Implement memory limits for the Mattermost server process using cgroups or container resource constraints

Patch Information

Mattermost has released security updates addressing this vulnerability. Organizations should upgrade to patched versions as documented in the Mattermost Security Updates advisory (MMSA-2026-00572). Ensure you are running a version newer than 11.3.0 for the 11.3.x branch, newer than 11.2.2 for the 11.2.x branch, or newer than 10.11.10 for the 10.11.x branch.

Workarounds

• Restrict file upload permissions to trusted users only through Mattermost system console settings
• Block PSD file uploads at the reverse proxy or web application firewall level by filtering .psd extensions
• Implement container memory limits to prevent a single service from exhausting system resources
• Configure operating system-level memory limits for the Mattermost process using ulimit or systemd resource controls

# Example: Configure memory limits for Mattermost using systemd
# Edit /etc/systemd/system/mattermost.service.d/limits.conf

[Service]
MemoryMax=4G
MemoryHigh=3G