# CVE-2026-2456: Denial of Service via Unbounded Memory Allocation in Integration Actions > An authenticated attacker can cause server memory exhaustion and denial of service by configuring a malicious integration server to return large responses. - **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:N/A:H`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** A user has to click or open something - **Fix available:** Yes - **Published:** 16 Mar 2026 - **Updated:** 16 Mar 2026 - **Also known as:** GHSA-34g8-9fpp-46ch, GO-2026-4726 ## What is CVE-2026-2456? CVE-2026-2456 is a medium-severity denial of service vulnerability in Mattermost Mattermost Server, published on 16 Mar 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link. ### Official 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 Weakness: CWE-789 (Memory Allocation with Excessive Size Value) ## Which versions are affected by CVE-2026-2456? 14 open source packages ship an affected version, including github.com/mattermost/mattermost-server, github.com/mattermost/mattermost-server/v5 and github.com/mattermost/mattermost-server/v6. It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `github.com/mattermost/mattermost-server` | Go | All versions | 5.3.2-0.20260127165411-fe3052073dc6 | | `github.com/mattermost/mattermost-server` | Go | All versions | 5.3.2-0.20260127165411-fe3052073dc6 | | `github.com/mattermost/mattermost-server` | Go | From 10.11.0-rc1 | 10.11.11 | | `github.com/mattermost/mattermost-server` | Go | From 10.11.0-rc1 | 10.11.11 | | `github.com/mattermost/mattermost-server` | Go | From 10.11.0-rc1+incompatible | 10.11.11+incompatible, 11.2.3+incompatible, 11.3.1+incompatible | | `github.com/mattermost/mattermost-server` | Go | From 11.2.0-rc1 | 11.2.3 | | `github.com/mattermost/mattermost-server` | Go | From 11.2.0-rc1 | 11.2.3 | | `github.com/mattermost/mattermost-server` | Go | From 11.3.0-rc1 | 11.3.1 | | `github.com/mattermost/mattermost-server` | Go | From 11.3.0-rc1 | 11.3.1 | | `github.com/mattermost/mattermost-server/v5` | Go | All versions | None | | `github.com/mattermost/mattermost-server/v6` | Go | All versions | None | | `github.com/mattermost/mattermost/server/v8` | Go | All versions | 8.0.0-20260127165411-fe3052073dc6 | | `github.com/mattermost/mattermost/server/v8` | Go | All versions | 8.0.0-20260127165411-fe3052073dc6 | | `github.com/mattermost/mattermost/server/v8` | Go | All versions | 8.0.0-20260127165411-fe3052073dc6 | Products: Mattermost Mattermost Server. ## How do I fix CVE-2026-2456? Upgrade github.com/mattermost/mattermost-server to a fixed version for your release line: 5.3.2-0.20260127165411-fe3052073dc6, 10.11.11, 10.11.11+incompatible or 11.2.3+incompatible. ## Is CVE-2026-2456 being exploited? There is no known exploitation of CVE-2026-2456 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-2026-2456 and fix it if it is affected. What it is: CVE-2026-2456 is a medium-severity denial of service vulnerability in Mattermost Mattermost Server, published on 16 Mar 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link. Exploitation: There is no known exploitation of CVE-2026-2456 in the wild and no public exploit code. Affected packages and fixed versions: - github.com/mattermost/mattermost-server (Go): affected all versions; fixed in 5.3.2-0.20260127165411-fe3052073dc6 - github.com/mattermost/mattermost-server (Go): affected all versions; fixed in 5.3.2-0.20260127165411-fe3052073dc6 - github.com/mattermost/mattermost-server (Go): affected from 10.11.0-rc1; fixed in 10.11.11 - github.com/mattermost/mattermost-server (Go): affected from 10.11.0-rc1; fixed in 10.11.11 - github.com/mattermost/mattermost-server (Go): affected from 10.11.0-rc1+incompatible; fixed in 10.11.11+incompatible, 11.2.3+incompatible, 11.3.1+incompatible - github.com/mattermost/mattermost-server (Go): affected from 11.2.0-rc1; fixed in 11.2.3 - github.com/mattermost/mattermost-server (Go): affected from 11.2.0-rc1; fixed in 11.2.3 - github.com/mattermost/mattermost-server (Go): affected from 11.3.0-rc1; fixed in 11.3.1 - github.com/mattermost/mattermost-server (Go): affected from 11.3.0-rc1; fixed in 11.3.1 - 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 - github.com/mattermost/mattermost/server/v8 (Go): affected all versions; fixed in 8.0.0-20260127165411-fe3052073dc6 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-2026-2456/ (Markdown: https://insights.sbomify.com/CVE-2026-2456/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-2456 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-2456 - OSV: https://osv.dev/vulnerability/CVE-2026-2456 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-2456/ Summaries are generated and may contain mistakes; the official description above is the record.