CVE Intelligence
Medium SSRF

CVE-2026-6333 — SSRF via Host Header Spoofing in Custom Slash Commands

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to validate the Host header when constructing response URLs for custom slash commands which allows an authenticated attacker to redirect slash command responses to an attacker-controlled server via a spoofed Host header.. Mattermost Advisory ID: MMSA-2026-00582

Published May 18, 2026 · Updated Oct 14, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP, GitHub Security Advisories · CNA: Mattermost

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.
How the score was calculated
  • EPSS 0.2% (30-day exploitation probability)+2
  • 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.

Exploitation
Not confirmed
Not in CISA KEV — an absence of record, not proof it is unexploited.
EPSS FIRST
0.22%
ahead of 12% of scored CVEs · Oct 14, 2026
Affects
Mattermost
Mattermost and 3 more
CVSS base
5.0
Medium · 4 source(s)

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation
None
none · proof-of-concept · active
Automatable
No
can an attacker script the kill-chain steps
Technical impact
Partial
partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

  • Go Github.com/mattermost/mattermost/server/v8: fixed in version 11.5.2 — github.com (GitHub Security Advisories)
  • Go Github.com/mattermost/mattermost/server/v8: fixed in version 10.11.14 — github.com (GitHub Security Advisories)
  • Go Github.com/mattermost/mattermost/server/v8: fixed in version 8.0.0-20260325160634-e738016c5920 — github.com (GitHub Security Advisories)
  • Go Github.com/mattermost/mattermost-server: fixed in version 5.3.2-0.20260325160634-e738016c5920 — github.com (GitHub Security Advisories)

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.

  1. Identify exposed assets running the affected vendor/product/version combinations (Mattermost Mattermost, Mattermost Mattermost Server, Go Github.com/mattermost/mattermost-server…).
  2. Apply the fixed version or the patch documented under Remediation.
  3. Search available logs for exploit probes matching the vulnerability class (ssrf), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

VendorProductVersionsStatusSource
Mattermost Mattermost ≥ 11.5.0 and ≤ 11.5.1≥ 10.11.0 and ≤ 10.11.13 Vulnerable CVE Program (cve.org) e CISA ADP
Mattermost Mattermost Server ≥ 10.11.0 and < 10.11.14≥ 11.5.0 and < 11.5.2 Vulnerable NIST National Vulnerability Database
Go Github.com/mattermost/mattermost-server < 5.3.2-0.20260325160634-e738016c5920 Vulnerable GitHub Security Advisories
Go Github.com/mattermost/mattermost/server/v8 ≥ 11.5.0 and < 11.5.2≥ 10.11.0 and < 10.11.14< 8.0.0-20260325160634-e738016c5920 Vulnerable GitHub Security Advisories

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Changed
Confidentiality
None
Integrity
Low
Availability
None

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

ScoreVersionSeverityExpl.ImpactScored by
5.0 CVSS 3.1 Medium 3.1 1.4 nvd@nist.gov via NIST National Vulnerability Database
3.5 CVSS 3.1 Low 1.8 1.4 responsibledisclosure@mattermost.com via NIST National Vulnerability Database
3.5 CVSS 3.1 Low — — GitHub, Inc. via GitHub Security Advisories
3.5 CVSS 3.1 Low — — Mattermost via CVE Program (cve.org) e CISA ADP
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:L/A:N (score shown at the top: nvd@nist.gov)
Scoring entities (NIST, CNA, ADP) score independently; each score is shown with its origin. For prioritization we use 5.0 (NVD (NIST)) as a conservative provisional reference — not a definitive official score.

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-918 · Server-Side Request Forgery (SSRF)

Other CVEs with this weakness →

EPSS over time FIRST

Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 0.22%

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).
  • Monitor outbound requests from the server to internal IPs and cloud metadata endpoints.
web/proxy/WAF logsnetwork IDS/IPSegress traffic

Timeline

What happened to this CVE, newest first.

  • 2026
  • Fix documented
    Oct 14, 2026 · GitHub Security Advisories
  • Affected products added · go/github.com/mattermost/mattermost/server/v8, go/github.com/mattermost/mattermost-server
    Oct 14, 2026 · GitHub Security Advisories
  • Affected products added · mattermost/mattermost
    Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
  • SSVC assessment updated
    Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
  • Record published
    Oct 13, 2026
  • CVSS changed · 5.0
    Oct 13, 2026 · NIST National Vulnerability Database
  • Source status changed · → Analyzed
    Oct 13, 2026 · NIST National Vulnerability Database
  • Record created in this database
    Oct 13, 2026 · NIST National Vulnerability Database
  • CVE published
    May 18, 2026 · Mattermost

Source activity

Repeated readings are counted rather than listed.

  • 1× References added · Oct 14, 2026 – Oct 14, 2026
  • 1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

4 on the record

Elsewhere on this site

Mattermost
every CVE for this vendor
SSRF
same class
CWE-918
other pages naming this weakness

Related CVEs (same products)

  • CVE-2026-96260 Mattermost server missing request body size limit on plugin routes allows denial of service by an authenticated user 23d
  • CVE-2026-96259 Mattermost server-side request forgery via OAuth endpoints configurable by a System Administrator 23d
  • CVE-2026-95666 Unbounded post ID array in the bulk reactions endpoint allows denial of service 23d
  • CVE-2026-75588 Mattermost Desktop App plugin popout scheme validation bypass 28d
  • CVE-2026-12284 Mattermost Desktop App Missing IPC Sender Validation in Calls Leave Handler 28d
  • CVE-2026-75025 Mattermost Desktop local network access from server-rendered content 29d

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 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-2026-6333 is "SSRF via Host Header Spoofing in Custom Slash Commands", 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 5.0 describes technical severity. EPSS 0.22% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

Comments

Moderated technical discussion. Read the community guidelines.

No comments yet.

Sign in or create an account to comment and suggest corrections.