CVE-2026-16251 — Mattermost local image proxy allows server memory exhaustion via unbounded image response buffering
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
Who exploits it and how urgent it is
A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.
How the score was calculated
- Technical severity (medium)+3
The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).
Triage
Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.
Remediation
Fixes and mitigations documented by the sources — linked to the original advisory.
No fix documented by the sources we poll. That does not mean none exists: check the vendor advisory.
First 24 hours
Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.
- Identify exposed assets running the affected vendor/product/version combinations (Mattermost Mattermost).
- No fix documented in the sources: reduce exposure and monitor the vendor advisory.
- Search available logs for exploit probes matching the vulnerability class (denial of service), errors and authentication anomalies.
Affected scope
Vendor, product and versions as the sources word them.
| Vendor | Product | Versions | Status | Source |
|---|---|---|---|---|
| Mattermost | Mattermost | ≥ 11.10.0 and ≤ 11.10.0≥ 11.7.0 and ≤ 11.7.9≥ 11.9.0 and ≤ 11.9.1≥ 11.8.0 and ≤ 11.8.5 | Vulnerable | CVE Program (cve.org) e CISA ADP |
Attack characteristics
The CVSS vector, decoded. It describes the attack, not your exposure to it.
Every base score collected
Sources score independently and may disagree; each row says who scored it and under which version.
| Score | Version | Severity | Expl. | Impact | Scored by |
|---|---|---|---|---|---|
| 6.5 | CVSS 3.1 | Medium | 2.8 | 3.6 | responsibledisclosure@mattermost.com via NIST National Vulnerability Database |
| 6.5 | CVSS 3.1 | Medium | — | — | Mattermost via CVE Program (cve.org) e CISA ADP |
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H (score shown at the top: responsibledisclosure@mattermost.com) Weakness MITRE
Weakness class according to MITRE’s CWE catalog.
CWE-770 · Allocation of Resources Without Limits or Throttling
Detection
Read off the CVSS vector and the weakness class. Starting points, not tested rules.
- Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
- Track CPU/memory spikes and availability drops correlated with request patterns.
Timeline
What happened to this CVE, newest first.
- 2026
- Affected products added · mattermost/mattermost
- Record published
- CVSS changed · 6.5
- Source status changed · → Received
- Record created in this database
- CVE published
Source activity
Repeated readings are counted rather than listed.
- 1× References added · Oct 16, 2026 – Oct 16, 2026
- 1× Official description updated · Oct 16, 2026 – Oct 16, 2026
References
1 on the record
Elsewhere on this site
Related CVEs (same products)
- Medium CVE-2026-19621 Mattermost leaks interactive-message integration secrets to non-admin users via post-deletion events and thread reads 1h
- Medium CVE-2026-18799 Mattermost Server fails to restrict Slack import account binding, allowing team admins to forge posts as existing users 1h
- Medium CVE-2026-19076 Mattermost fails to reject OAuth application sessions in account type switch, allowing account takeover via bearer token 1h
- Medium CVE-2026-19600 Mattermost logs plaintext authentication tokens in debug-level server logs on failed auth attempts 1h
- High CVE-2026-16453 Mattermost SAML RelayState forgery allows unauthorized team membership 1h
- Medium CVE-2026-18223 Mattermost invitation tokens not bound to invited email address allowing unauthorized team and private channel access 1h
Not in any source we poll
Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.
- No FIRST EPSS score.
- No fixed version, patch or mitigation documented.
- No CISA SSVC assessment.
- No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
- No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
- No exploit packet captures, log samples, or incident case IDs supplied.
Answered from this record
Text assembled only from the fields above — no additional claims.
What should defenders know first?
CVE-2026-16251 is "Mattermost local image proxy allows server memory exhaustion via unbounded image response buffering", rated Medium. It affects Mattermost (Mattermost); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 6.5 describes technical severity. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.
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