CVE-2026-25780: Mattermost Server DoS Vulnerability

CVE-2026-25780 Overview

CVE-2026-25780 is a memory exhaustion vulnerability in Mattermost Server that stems from improper memory allocation bounds when processing DOC files. An authenticated attacker can exploit this flaw by uploading a specially crafted DOC file, causing the server to allocate excessive memory and ultimately leading to denial of service conditions.

Critical Impact
Authenticated users can cause server-wide denial of service by uploading malicious DOC files, potentially disrupting communication for all users in the organization.

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-2026-25780 published to NVD
2026-03-18 - Last updated in NVD database

Technical Details for CVE-2026-25780

Vulnerability Analysis

This vulnerability is classified under CWE-789 (Memory Allocation with Excessive Size Value), which occurs when a program allocates memory based on untrusted input without proper size validation. In the context of Mattermost Server, the DOC file parsing functionality fails to implement appropriate bounds checking on memory allocation requests.

When a user uploads a DOC file, the server processes the document to extract content or generate previews. The vulnerable code path accepts size parameters from the DOC file structure without validating whether the requested allocation is reasonable. An attacker can craft a malicious DOC file that specifies extremely large memory allocation values, causing the server to attempt allocating gigabytes of memory.

The attack requires authentication, meaning the attacker needs valid credentials to access the Mattermost instance. However, once authenticated, they can repeatedly upload malicious files to exhaust server memory, causing the application to become unresponsive or crash entirely.

Root Cause

The root cause of this vulnerability lies in the insufficient input validation within the DOC file processing module. The code responsible for parsing document structures reads size values directly from the file without implementing upper bounds or sanity checks. This allows an attacker to specify arbitrary memory allocation sizes that the server blindly attempts to fulfill.

The vulnerability specifically affects the memory allocation routines that handle document content extraction. Without proper bounds checking, the server trusts the size parameters embedded in the DOC file format, which can be manipulated by an attacker to request excessive memory allocations.

Attack Vector

The attack is executed over the network by an authenticated user uploading a specially crafted DOC file through Mattermost's file sharing functionality. The attacker constructs a DOC file with manipulated size fields in its internal structure, triggering unbounded memory allocation when the server processes the upload.

The vulnerability can be exploited through the standard file upload interface, making it accessible to any authenticated user. The malicious DOC file contains embedded size values that, when parsed by the server, cause memory allocation requests far exceeding reasonable limits for document processing.

Detection Methods for CVE-2026-25780

Indicators of Compromise

Unusual memory consumption spikes on the Mattermost Server process
Multiple DOC file uploads from a single user in a short time period
Server crashes or out-of-memory (OOM) errors in system logs
Abnormally large DOC files being uploaded to channels

Detection Strategies

Monitor server memory usage for sudden spikes correlating with file upload events
Implement file upload logging to track DOC file submissions and their sizes
Configure alerting for out-of-memory conditions on the Mattermost Server
Review application logs for errors related to document processing failures

Monitoring Recommendations

Enable detailed logging for file upload operations in Mattermost
Set up memory usage thresholds with automated alerting
Monitor for repeated upload attempts from the same authenticated user
Track server process restarts that may indicate crash recovery from memory exhaustion

How to Mitigate CVE-2026-25780

Immediate Actions Required

Update Mattermost Server to the latest patched version immediately
Temporarily restrict DOC file uploads if patching is delayed
Monitor server memory usage closely until patches are applied
Review recent DOC file uploads for potentially malicious activity

Patch Information

Mattermost has released security updates to address this vulnerability. Organizations should upgrade to versions beyond 11.3.0, 11.2.2, and 10.11.10 respectively for their deployment branch. Detailed patch information is available in the Mattermost Security Updates advisory (MMSA-2026-00581).

Workarounds

Disable DOC file uploads at the server configuration level until patches can be applied
Implement file size limits for uploads to reduce the attack surface
Use a reverse proxy or web application firewall to filter DOC file uploads
Restrict file upload permissions to trusted users only

bash

# Configuration example - Restrict file extensions in Mattermost config.json
# Add to the FileSettings section to block DOC uploads temporarily
"RestrictedFileExtensions": ".doc,.docx"

# Alternative: Set maximum file size to limit potential impact
"MaxFileSize": 52428800