# CVE-2026-2456: server DoS — Fix Available 🐹 🐹 Go Not in CISA KEV MEDIUM severity # CVE-2026-2456 — server MEDIUM [Fix: mattermost/mattermost@fe30520](https://github.com/mattermost/mattermost/commit/fe3052073dc67e3c920baf9fe7efd44ac1d8124c) CVE-2026-2456 is a medium-severity (CVSS 5.3) CWE-789 vulnerability in github.com/mattermost/mattermost/server/v8. A fix is available for github.com/mattermost/mattermost/server/v8 — see the affected versions and patch details below. Denial of Service via Unbounded Memory Allocation in Integration Actions Also known as[GHSA-34g8-9fpp-46ch](/vulnerability/GHSA-34g8-9fpp-46ch)GO-2026-4726 Published Mar 16, 2026 Updated Nov 7, 2026 Affected 5 pkgs Patched 5 / 5 Exploits None indexed Exploitation data as of Nov 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS) ## Exploitation Status ### No confirmed exploitation observed yet - CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment. Exploitation and automatability from CISA’s SSVC triage for CVE-2026-2456. ## EPSS Exploitation Probability [via FIRST.org ↗](https://www.first.org/epss/) 0.2%probability of exploitation in next 30 days Lower Risk+0.00% Lower risk than most CVEs7th percentile — riskier than 7% of all scored CVEsHighest risk 0.00%0.22%0.45%0.67%0.0%0.2%May 26Sep 26Nov 26 Probability of exploitation in the next 30 days, from [FIRST.org EPSS](https://www.first.org/epss/). ## How urgent is this, really ## Where this sits among everything scored ## Real-World Exposure 5 pkgs affected 🐹`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-server` Real-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 5 total 5 fixed Ecosystem Package Vulnerable range Fix 🐹Go `github.com/mattermost/mattermost/server/v8` all versions 8.0.0-20260127165411-fe3052073dc6`go get github.com/mattermost/mattermost/server/v8@v8.0.0-20260127165411-fe3052073dc6` 🐹Go `github.com/mattermost/mattermost-server` all versions 5.3.2-0.20260127165411-fe3052073dc6`go get github.com/mattermost/mattermost-server@v5.3.2-0.20260127165411-fe3052073dc6` 🐹Go `github.com/mattermost/mattermost-server` ≥ 10.11.0-rc1&&< 10.11.11 10.11.11`go get github.com/mattermost/mattermost-server@v10.11.11` 🐹Go `github.com/mattermost/mattermost-server` ≥ 11.2.0-rc1&&< 11.2.3 11.2.3`go get github.com/mattermost/mattermost-server@v11.2.3` 🐹Go `github.com/mattermost/mattermost-server` ≥ 11.3.0-rc1&&< 11.3.1 11.3.1`go get github.com/mattermost/mattermost-server@v11.3.1` ## Affected Products 1 product · 3 configurations Application `mattermost server`mattermost ≥ 11.3.0 && < 11.3.1 range ## Detection & mitigation playbook Open-source dependency 1. ### Detect Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/mattermost/mattermost/server/v8, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version. 2. ### 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. 3. ### Workarounds Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service. ## Frequently Asked Questions 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 O3 Security · Impact-Aware SCA ### Is CVE-2026-2456 in your dependencies? Find it across Go, including transitive dependencies. [Scan my dependencies](/book-demo) [How O3 SCA works](/impact-aware-sca)