CVE-2026-10819: Mattermost Animated GIF DoS Vulnerability

CVE-2026-10819 Overview

CVE-2026-10819 is a denial of service vulnerability in Mattermost that stems from missing frame count limits and unenforced file size caps on animated GIF uploads. An authenticated attacker can upload a crafted animated GIF as a custom emoji to exhaust server resources. The flaw is tracked as Mattermost Advisory ID MMSA-2026-00695 and maps to CWE-409: Improper Handling of Highly Compressed Data (Data Amplification).

Critical Impact
An authenticated user can trigger a denial of service on the Mattermost server by uploading a crafted animated GIF as a custom emoji, degrading availability for all users.

Affected Products

Mattermost 11.6.x versions up to and including 11.6.5
Mattermost 11.8.x versions up to and including 11.8.1 and 11.7.x versions up to and including 11.7.4
Mattermost 10.11.x versions up to and including 10.11.20

Discovery Timeline

2026-07-27 - CVE-2026-10819 published to NVD
2026-07-27 - Last updated in NVD database

Technical Details for CVE-2026-10819

Vulnerability Analysis

Mattermost supports uploading custom emojis, including animated GIF images. The vulnerable versions do not validate the number of frames within an uploaded animated GIF and do not enforce the configured file size cap on this upload path. An authenticated attacker abuses this gap to submit a small GIF that expands into a large number of decoded frames during server-side processing.

The result is a data amplification condition. The server consumes disproportionate memory and CPU while decoding and storing the emoji, degrading responsiveness for legitimate users. Because the flaw affects an authenticated feature, any user with permission to upload custom emojis can trigger the condition.

Root Cause

The root cause is missing input validation on animated GIF uploads used as custom emojis. Two controls are absent: a hard cap on the number of frames the server will decode, and enforcement of the file size limit specific to this upload flow. This aligns with CWE-409, where highly compressed or highly repetitive input expands into resource-heavy structures once processed.

Attack Vector

Exploitation requires network access to the Mattermost instance and valid low-privilege credentials. The attacker crafts an animated GIF with an abusive frame count or oversized dimensions, then submits it through the custom emoji upload endpoint. No user interaction is required. Successful exploitation impacts availability only. Confidentiality and integrity are not affected.

See the Mattermost Security Updates advisory for vendor-supplied technical details.

Detection Methods for CVE-2026-10819

Indicators of Compromise

Custom emoji upload requests containing animated GIFs with unusually high frame counts or file sizes near or above the configured cap.
Sudden spikes in Mattermost server memory or CPU utilization correlated with /api/v4/emoji POST requests.
Repeated emoji upload attempts from a single authenticated account within a short interval.

Detection Strategies

Inspect reverse proxy and Mattermost access logs for POST requests to the custom emoji endpoint that carry Content-Type: image/gif payloads.
Correlate emoji upload events with process resource metrics to identify uploads that precede resource exhaustion.
Flag accounts that submit multiple large or high-frame-count GIFs within short windows for review.

Monitoring Recommendations

Alert on Mattermost server memory pressure, out-of-memory kills, or restart events on hosts running affected versions.
Track counts and sizes of custom emoji uploads per user and per hour to establish a baseline and detect deviations.
Forward Mattermost application logs and host telemetry to a centralized analytics platform for correlation and retention.

How to Mitigate CVE-2026-10819

Immediate Actions Required

Upgrade Mattermost to a fixed release above the affected version ranges: beyond 11.6.5, 11.7.4, 11.8.1, or 10.11.20.
Restrict custom emoji creation permissions to trusted roles until patching is complete.
Review recent custom emoji uploads for oversized or high-frame-count animated GIFs and remove offending entries.

Patch Information

Mattermost has released fixed versions addressing the missing frame count and file size validation on animated GIF uploads. Refer to the Mattermost Security Updates page for the specific fixed builds and upgrade guidance corresponding to advisory MMSA-2026-00695.

Workarounds

Disable custom emoji creation via the System Console until affected servers are upgraded.
Place Mattermost behind a reverse proxy that enforces strict client_max_body_size limits on emoji upload paths.
Apply rate limiting on the custom emoji upload endpoint to slow abusive submissions from a single account.

bash

# Example nginx reverse proxy hardening for the emoji upload path
location /api/v4/emoji {
    client_max_body_size 1m;
    limit_req zone=emoji burst=5 nodelay;
    proxy_pass http://mattermost_backend;
}