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CVE-2026-4858: Mattermost Server Path Traversal (CVSS 8)

CVE-2026-4858 is a high severity path traversal in Mattermost Server with a CVSS score of 8. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.

Last updated: Jul 23, 2026

Quick answer

Direct answer

CVE-2026-4858 is a high severity path traversal in Mattermost Server with a CVSS score of 8. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.

Risk scores

CVSS base score
8HIGH
EPSS probability
0.40%
Higher than 32 percent of all CVEs
CISA KEV
Not listed
CVSS metricWhat it means
Attack vector (AV:N)Network: reachable remotely
Attack complexity (AC:H)High complexity
Privileges required (PR:L)Low privileges required
User interaction (UI:R)A user must take an action
Scope (S:C)Scope changed: impact spreads beyond the component
Confidentiality (C:H)High confidentiality impact
Integrity (I:H)High integrity impact
Availability (A:H)High availability impact

CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:C/C:H/I:H/A:H

About Mattermost Server

CVE-2026-4858 affects Mattermost Server, a product from Mattermost. Before acting, confirm where Mattermost Server runs in your estate, who owns it and whether it is reachable from the internet, because exposure decides real risk more than the score alone.

What the flaw is

In plain terms: Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to check integration URL for path traversal which allows an malicious authenticated user to call an arbitrary API via system admin Mattermost auth token using via path traversal in integration action URL.. Mattermost Advisory ID: MMSA-2026-00640

The weakness class is Path Traversal. An attacker can reach files outside the intended folder by tampering with a file path.

Based on its CVSS vector, the flaw can be reached over the network, needs some level of access, and relies on a user taking an action. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.

Affected versions and timeline

FieldDetail
ProductMattermost Server
Affected versionsMattermost Server from 10.11.0 before 10.11.15; Mattermost Server from 11.4.0 before 11.4.5; Mattermost Server from 11.5.0 before 11.5.4; Mattermost Server 11.6.0
PublishedMay 21, 2026
Last modifiedJul 23, 2026
Added to CISA KEVNot listed
Assigning CNAresponsibledisclosure@mattermost.com

How to detect it

  • Inventory every instance of Mattermost Server and record the running version against the affected range above.
  • Check external attack surface scans for internet facing instances of the component.
  • Review logs on affected systems for unusual requests, errors or new processes around the vulnerable function.
  • Confirm with an authenticated vulnerability scan once your scanner has a check for this CVE.

How to fix it

  • Apply the fixed version of Mattermost Server named in the vendor advisory.
  • If patching must wait, restrict network access to the affected service and put it behind authentication or a web application firewall rule.
  • Rescan after the change to prove the exposure is gone, then close the ticket.

CTEM action plan

CTEM stageWhat to do for this CVE
ScopeAdd every business service that depends on Mattermost Server to the current CTEM cycle.
DiscoverFind all instances, including shadow and cloud deployments, and record versions and exposure.
PrioritizeRank using EPSS and CVSS together with asset exposure and business impact.
ValidateConfirm the flaw is reachable and exploitable in your environment and that compensating controls work.
MobilizeAssign the fix to the asset owner with a due date matched to the priority, then verify closure.

CWE References

Technical References

Frequently asked questions

CVE-2026-4858 is a high severity path traversal in Mattermost Server with a CVSS score of 8. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.

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