Medium severity · published May 18, 2026
CVE-2026-5163
Missing authorization check in AI message rewrite endpoint allows access to private thread content
6.5
CVSS
0.2%
EPSS
No
On KEV
Description
Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint.. Mattermost Advisory ID: MMSA-2026-00645
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Confidentiality
High
Integrity
None
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.2% · higher than 10% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
MA
Mattermost · Mattermost
11.5.0 through 11.5.1 · source: vendor
MS
Mattermost Server · Mattermost
11.5.0 before 11.5.2 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-5163
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-5163
Vendor Advisory
https://mattermost.com/security-updates
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