Medium File handling

CVE-2026-6957 — Path traversal in Mattermost Legal Hold plugin via unsanitized file name from federated peer allows arbitrary file write.

Mattermost Plugins versions <=1.1.5 fail to sanitize filenames received from federated peers before using them to construct export destination paths, which allows an administrator of a remote federated Mattermost server to write files to arbitrary locations within the target server's filestore via a malicious filename delivered through the shared-channel attachment sync protocol. Mattermost Advisory ID: MMSA-2026-00659

Published May 27, 2026 · Updated Oct 13, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP · CNA: Mattermost

Low

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 0.5% (30-day exploitation probability)+6
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

0.46%

ahead of 38% of scored CVEs · Oct 14, 2026

Affects

Mattermost

Legal Hold and 1 more

CVSS base

4.9

Medium · 3 source(s)

Prioritization reference: 8.0 provisional

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

can an attacker script the kill-chain steps

Technical impact

Total

partial · total control of the vulnerable component

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 Legal Hold, 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 (file handling), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
Mattermost	Legal Hold	≤ 1.1.5	Vulnerable	NIST National Vulnerability Database
Mattermost	Mattermost	≤ 1.1.5	Vulnerable	CVE Program (cve.org) e CISA ADP

Attack characteristics

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

Attack vector

Network

Attack complexity

Low

Privileges required

High

User interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

High

Availability

None

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
8.0	CVSS 3.1	High	1.3	6.0	responsibledisclosure@mattermost.com via NIST National Vulnerability Database
8.0	CVSS 3.1	High	—	—	Mattermost via CVE Program (cve.org) e CISA ADP
4.9	CVSS 3.1	Medium	1.2	3.6	nvd@nist.gov via NIST National Vulnerability Database

CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:H/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 8.0 (responsibledisclosure@mattermost.com via NIST National Vulnerability Database) as a conservative provisional reference — not a definitive official score.

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-22 · Path Traversal

Other CVEs with this weakness →

EPSS over time FIRST

Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 0.46%

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 file creation in web directories and paths with traversal sequences (../).

web/proxy/WAF logsnetwork IDS/IPSfile integrity (FIM)endpoint telemetry (EDR)

Timeline

What happened to this CVE, newest first.

2026
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 · 4.9
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 27, 2026 · Mattermost

Source activity

Repeated readings are counted rather than listed.

1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

1 on the record

mattermost.com/security-updates
Vendor Advisory, vendor-advisory, NIST National Vulnerability Database

Elsewhere on this site

Mattermost
every CVE for this vendor

File handling
same class

CWE-22
other pages naming this weakness

Related CVEs (same products)

Medium CVE-2026-96260 Mattermost server missing request body size limit on plugin routes allows denial of service by an authenticated user 23d
Medium CVE-2026-96259 Mattermost server-side request forgery via OAuth endpoints configurable by a System Administrator 23d
Medium CVE-2026-95666 Unbounded post ID array in the bulk reactions endpoint allows denial of service 23d
Low CVE-2026-75588 Mattermost Desktop App plugin popout scheme validation bypass 28d
Low CVE-2026-12284 Mattermost Desktop App Missing IPC Sender Validation in Calls Leave Handler 28d
Medium 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 fixed version, patch or mitigation documented.
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-2026-6957 is "Path traversal in Mattermost Legal Hold plugin via unsanitized file name from federated peer allows arbitrary file write.", rated Medium. It affects Legal Hold (Mattermost); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 4.9 describes technical severity. EPSS 0.46% 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.

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