CVE-2026-2456
MEDIUMCVE-2026-2456 is a medium-severity (CVSS 5.3) CWE-789 vulnerability in github.com/mattermost/mattermost/server/v8. O3 Security confirms whether CVE-2026-2456 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Denial of Service via Unbounded Memory Allocation in Integration Actions
Real-World Exposure
github.com/mattermost/mattermost/server/v8🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-serverReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 Mattermost fails to limit the size of responses from integration action endpoints, which allows an authenticated attacker to cause server memory exhaustion and denial of service via a malicious integration server that returns an arbitrarily large response when a user clicks an interactive message button.. Mattermost Advisory ID: MMSA-2026-00571
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/mattermost/mattermost/server/v8 | all versions | 8.0.0-20260127165411-fe3052073dc6 |
| 🐹Go | github.com/mattermost/mattermost-server | all versions | 5.3.2-0.20260127165411-fe3052073dc6 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 10.11.0-rc1&&< 10.11.11 | 10.11.11 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 11.2.0-rc1&&< 11.2.3 | 11.2.3 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 11.3.0-rc1&&< 11.3.1 | 11.3.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/mattermost/mattermost/server/v8. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update github.com/mattermost/mattermost/server/v8 to 8.0.0-20260127165411-fe3052073dc6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-2456 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-2456 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-2456. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-2456 in your dependencies?
O3 detects CVE-2026-2456 across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.