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CVE-2026-96259

Mattermost server-side request forgery via OAuth endpoints configurable by a System Administrator

MEDIUM
5.5
CVSS Score
Published: September 22, 2026 at 08:34 PM UTC
Updated: October 7, 2026 at 04:41 PM UTC
2 products affected across 1 vendorMattermost ServerMattermost

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Severity
MEDIUM5.5
Exploit prediction
EPSS 0.3%
SSVC
No known exploit

Description

Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 fail to apply the internal-connection filter to OAuth endpoint requests, which allows a System Administrator to make the server issue requests to internal network addresses and read the responses via the configured OAuth token and userinfo endpoints.. Mattermost Advisory ID: MMSA-2026-00776

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Mattermost Server· Mattermost
95%
11.10.0 ≤ v < 11.10.211.7.0 ≤ v < 11.7.1111.8.0 ≤ v < 11.8.611.9.0 ≤ v < 11.9.2
Affected range stops below 11.10.2, so it looks safe — our inference, not a vendor statement
Affected range stops below 11.7.11, so it looks safe — our inference, not a vendor statement
Affected range stops below 11.8.6, so it looks safe — our inference, not a vendor statement
Affected range stops below 11.9.2, so it looks safe — our inference, not a vendor statement
Mattermost· Mattermost
60%
11.10.0 ≤ v ≤ 11.10.111.7.0 ≤ v ≤ 11.7.1011.8.0 ≤ v ≤ 11.8.511.9.0 ≤ v ≤ 11.9.111.10.011.7.011.8.011.9.0
Vendor states this is fixed in 11.10.2 (unaffected)
Vendor states this is fixed in 11.11.0 (unaffected)
Vendor states this is fixed in 11.7.11 (unaffected)
Vendor states this is fixed in 11.8.6 (unaffected)
Vendor states this is fixed in 11.9.2 (unaffected)

Remediation

No guide yet

No remediation guidance has been written for this CVE yet. What the record does show:

Patch and advisory references

References (1)

Vendor Advisory
Source: cverecord
https://mattermost.com/security-updates

SSVC — Action Prioritization

SSVCNo known exploitNot automatablePartial impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

5.5MEDIUMMattermost
Attack Vector
NETWORK
Complexity
LOW
Privileges
HIGH
User Interaction
NONE
Scope
CHANGED
Confidentiality
LOW
Integrity
LOW
Availability
NONE
Vector?

Quick Info

StatusANALYZED
CWE
CWE-918

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