---
id: "CVE-2026-16251"
kind: "vulnerability"
aliases: ["CVE-2026-16251"]
title: "Mattermost: unbounded image proxy buffering lets remote servers exhaust memory"
severity: "medium"
cvss: 6.5
epss: null
exploited: false
status: "open"
category: "other"
products: []
ecosystems: []
tags: ["mattermost", "other", "self-hosted"]
not_affected_if: ["managed-hosting", "unaffected-version"]
published_at: "2026-10-16T08:17:12+00:00"
first_seen_at: "2026-10-23T21:57:12+00:00"
updated_at: "2026-10-25T03:57:57+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-16251", "first_seen_at": "2026-10-23T21:57:12+00:00"}]
---

# Mattermost: unbounded image proxy buffering lets remote servers exhaust memory

Mattermost's local image proxy buffers remote image responses in memory without a size limit during direct fetches. A remote attacker who serves a crafted, oversized image can exhaust server memory and cause a denial of service.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the Mattermost server version with `mattermost version` or the System Console and compare it to the affected ranges: 11.10.0 and earlier in 11.10.x, 11.7.9 and earlier in 11.7.x, 11.9.1 and earlier in 11.9.x, 11.8.5 and earlier in 11.8.x.
- Confirm whether the local image proxy direct fetch path is in use (unknown from the sources whether it is enabled by default).

## Fix

- Upgrade Mattermost to a fixed release in the 11.10.x, 11.9.x, 11.8.x or 11.7.x line beyond the affected ranges listed above (exact fixed versions: unknown from the sources).
- Review the MMSA-2026-00726 advisory from Mattermost for the patched version numbers.

## Timeline

- 2026-10-23T21:57:12+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-16251

## From the sources

Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to limit the size of remote image responses buffered in memory during local image proxy direct fetches, which allows a remote attacker to cause memory exhaustion on the server via a crafted oversized image response.. Mattermost Advisory ID: MMSA-2026-00726

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_