# CVE-2026-16251: Mattermost Server memory exhaustion in remote image proxy

Source: https://junglewise.ai/threats/cve-2026-16251-mattermost-versions-11-10-x-11-10-0-11-7-x-11-7-9-11-9-x-11-9-1  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-16. Updated: 2026-10-16  
License: CC BY 4.0 with a link back

CVE-2026-16251 · Severity: medium · CVSS 6.5 · Published 2026-10-16

Mattermost Server, a team communication and collaboration platform, fails to limit the size of remote image responses when fetching images through its local proxy. An attacker can send a specially crafted oversized image to exhaust the server's memory, causing a denial of service that impacts all users on the affected instance.

Technologies: [Mattermost Server](https://junglewise.ai/threats/technologies/server-public). Vendors: [Mattermost](https://junglewise.ai/threats/vendors/mattermost).

## Quick answers

**Is CVE-2026-16251 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-16251 being exploited in the wild.

**How severe is CVE-2026-16251?** Medium severity, with a CVSS score of 6.5, published 16 October 2026.

**Which products and versions are affected?** Mattermost Server 11.7.x through 11.10.0 (specifically: 11.7.0-11.7.9, 11.8.0-11.8.5, 11.9.0-11.9.1, 11.10.0).

**Is there a fix for CVE-2026-16251?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-16251 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-16251?** Check whether you run Mattermost Server at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

Mattermost's local image proxy does not properly validate or limit the size of buffered remote image responses in memory. An attacker can exploit this by serving an oversized image response to a remote image fetch request, triggering unbounded memory allocation on the server. The vulnerability affects versions 11.7.x through 11.10.x with no authentication or user interaction required beyond a network request.

## Affected products

- Mattermost Server 11.7.x through 11.10.0 (specifically: 11.7.0-11.7.9, 11.8.0-11.8.5, 11.9.0-11.9.1, 11.10.0)

## Timeline

- 2026-10-16: disclosed

## References

- [https://mattermost.com/security-updates](https://mattermost.com/security-updates)

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