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CVE-2026-19603

Private channel ID disclosure via user status endpoints

MEDIUM
4.3
CVSS Score
Published: October 16, 2026 at 07:02 AM UTC
Updated: October 29, 2026 at 06:08 PM UTC
2 products affected across 1 vendorMattermost ServerMattermost

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Severity
MEDIUM4.3
Exploit prediction
EPSS 0.2%
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 sanitize the active channel field returned by the user status API endpoints, which allows an authenticated user to learn the channel ID of a private channel another user is actively viewing via the status endpoints, without being a member of that channel. Mattermost Advisory ID: MMSA-2026-00765

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
Mattermost· Mattermost
90%
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

4.3MEDIUMMattermost
Attack Vector
NETWORK
Complexity
LOW
Privileges
LOW
User Interaction
NONE
Scope
UNCHANGED
Confidentiality
LOW
Integrity
NONE
Availability
NONE
Vector?

Quick Info

StatusANALYZED
CWE
CWE-201

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