# CVE-2026-5163: Missing authorization check in AI message rewrite endpoint allows access to private thread content > An authenticated attacker can read private thread content in Mattermost by bypassing channel membership checks on the AI message rewrite endpoint. - **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N`) - **Exploited in the wild:** Not known - **Login needed:** Any signed-in user - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 18 May 2026 - **Updated:** 18 May 2026 - **Also known as:** GHSA-8r89-8w26-cq32, GO-2026-5263 ## What is CVE-2026-5163? CVE-2026-5163 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 18 May 2026. Any signed-in user can exploit it over the network, with no action from the victim. ### Official 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 Weakness: CWE-862 (Missing Authorization) ## Which versions are affected by CVE-2026-5163? 10 open source packages ship an affected version, including github.com/mattermost/mattermost/server/v8, github.com/mattermost/mattermost-server and github.com/mattermost/mattermost-server/v5. It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `github.com/mattermost/mattermost/server/v8` | Go | All versions | 8.0.0-20260401090745-f4d1abe7e8f5 | | `github.com/mattermost/mattermost/server/v8` | Go | All versions | 8.0.0-20260401090745-f4d1abe7e8f5 | | `github.com/mattermost/mattermost/server/v8` | Go | All versions | 8.0.0-20260401090745-f4d1abe7e8f5 | | `github.com/mattermost/mattermost/server/v8` | Go | From 11.5.0 | 11.5.2 | | `github.com/mattermost/mattermost/server/v8` | Go | From 11.5.0 | 11.5.2 | | `github.com/mattermost/mattermost-server` | Go | All versions | 5.3.2-0.20260401090745-f4d1abe7e8f5 | | `github.com/mattermost/mattermost-server` | Go | All versions | 5.3.2-0.20260401090745-f4d1abe7e8f5 | | `github.com/mattermost/mattermost-server` | Go | From 11.5.0+incompatible | 11.5.2+incompatible | | `github.com/mattermost/mattermost-server/v5` | Go | All versions | None | | `github.com/mattermost/mattermost-server/v6` | Go | All versions | None | Products: Mattermost Mattermost Server. ## How do I fix CVE-2026-5163? Upgrade github.com/mattermost/mattermost/server/v8 to a fixed version for your release line: 8.0.0-20260401090745-f4d1abe7e8f5 or 11.5.2. ## Is CVE-2026-5163 being exploited? There is no known exploitation of CVE-2026-5163 in the wild and no public exploit code. ## Other vulnerabilities in Mattermost Mattermost Server - [CVE-2025-12419](https://insights.sbomify.com/CVE-2025-12419/): Account takeover on OAuth/OpenID-enabled servers (critical 9.9) - [CVE-2025-12421](https://insights.sbomify.com/CVE-2025-12421/): Account Takeover via Code Exchange Endpoint (critical 9.9) - [CVE-2025-4981](https://insights.sbomify.com/CVE-2025-4981/): Path Traversal Leading to RCE by Any Authenticated Mattermost User (critical 9.9) - [CVE-2025-20051](https://insights.sbomify.com/CVE-2025-20051/): Arbitrary file read via block duplication in Mattermost Boards (critical 9.9) - [CVE-2025-25279](https://insights.sbomify.com/CVE-2025-25279/): Arbitrary file read in Mattermost Boards via import & export board archive (critical 9.9) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2026-5163 and fix it if it is affected. What it is: CVE-2026-5163 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 18 May 2026. Any signed-in user can exploit it over the network, with no action from the victim. Exploitation: There is no known exploitation of CVE-2026-5163 in the wild and no public exploit code. Affected packages and fixed versions: - github.com/mattermost/mattermost/server/v8 (Go): affected all versions; fixed in 8.0.0-20260401090745-f4d1abe7e8f5 - github.com/mattermost/mattermost/server/v8 (Go): affected all versions; fixed in 8.0.0-20260401090745-f4d1abe7e8f5 - github.com/mattermost/mattermost/server/v8 (Go): affected all versions; fixed in 8.0.0-20260401090745-f4d1abe7e8f5 - github.com/mattermost/mattermost/server/v8 (Go): affected from 11.5.0; fixed in 11.5.2 - github.com/mattermost/mattermost/server/v8 (Go): affected from 11.5.0; fixed in 11.5.2 - github.com/mattermost/mattermost-server (Go): affected all versions; fixed in 5.3.2-0.20260401090745-f4d1abe7e8f5 - github.com/mattermost/mattermost-server (Go): affected all versions; fixed in 5.3.2-0.20260401090745-f4d1abe7e8f5 - github.com/mattermost/mattermost-server (Go): affected from 11.5.0+incompatible; fixed in 11.5.2+incompatible - github.com/mattermost/mattermost-server/v5 (Go): affected all versions; no fix listed - github.com/mattermost/mattermost-server/v6 (Go): affected all versions; no fix listed Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2026-5163/ (Markdown: https://insights.sbomify.com/CVE-2026-5163/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-5163 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-5163 - OSV: https://osv.dev/vulnerability/CVE-2026-5163 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-5163/ Summaries are generated and may contain mistakes; the official description above is the record.