# Mattermost doesn't filter nil elements from outgoing webhook attachment payloads before processing

Advisory record · `GHSA-5gmf-x7hg-97wf`

- **Identifiers:** `GHSA-5gmf-x7hg-97wf` · `CVE-2026-4915` · `GO-2026-5845`
- **State:** Confirmed
- **Severity:** CVSS 6.5 Medium
- **Critical:** No
- **Weakness (CWE):** [CWE-754](https://cwe.mitre.org/data/definitions/754.html)
- **Tags:** [CWE-754](https://agentprime.io/advisories/tag/cwe-754.md)
- **Published:** 26 May 2026
- **Updated:** 7 Jul 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 26 May 2026, 13:30 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `github.com/mattermost/mattermost-server` (Go): affected `>=11.6.0, <11.6.1 || >=11.5.0, <11.5.4 || >=11.4.0, <11.4.5 || >=10.11.0, <10.11.15`; fixed in `11.6.1`, `11.5.4`, `11.4.5`, `10.11.15`
- `github.com/mattermost/mattermost/server/v8` (Go): affected `<8.0.0-20260407102538-faa7d75b4ea0`; fixed in `8.0.0-20260407102538-faa7d75b4ea0`

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 6.5 Medium, CVSS 3.1: `CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`, from CNA (Mattermost)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0028, the 20th percentile, on 6 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 15:06 UTC: First seen by agentprime
- 31 Oct 2026, 15:06 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-5gmf-x7hg-97wf)

## Sources

- [GitHub Advisory Database: GHSA-5gmf-x7hg-97wf](https://github.com/advisories/GHSA-5gmf-x7hg-97wf)
- [OSV: GHSA-5gmf-x7hg-97wf](https://osv.dev/vulnerability/GHSA-5gmf-x7hg-97wf)
- <https://nvd.nist.gov/vuln/detail/CVE-2026-4915>
- <https://github.com/mattermost/mattermost/commit/faa7d75b4ea041701e97948f8aa1332e3626a39a>
- <https://github.com/mattermost/mattermost>
- <https://mattermost.com/security-updates>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-5gmf-x7hg-97wf>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-5gmf-x7hg-97wf/>. Everything for agents: <https://agentprime.io/llms.txt>.