CVE-2026-5740 - Mattermost doesn't properly validate msgpack-encoded WebSocket frames before memory allocation

CVE-2026-5740 + Mattermost doesn’t properly validate msgpack-encoded WebSocket frames before memory allocation + High + 2026-05-26

One-sentence business risk

Broken authentication or authorization can expose tenant data and administrator actions, which buyers treat as a release blocker.

Research notes

  • Root cause: Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to properly validate msgpack-encoded WebSocket frames before memory allocation which allows an unauthenticated remote attacker to crash the server process and ca.
  • Affected versions: go:github.com/mattermost/mattermost-server = 11.6.0; go:github.com/mattermost/mattermost-server >= 11.5.0, < 11.5.4; go:github.com/mattermost/mattermost-server >= 11.4.0, < 11.4.5; go:github.com/mattermost/mattermost-server >= 10.11.0, < 10.11.15; go:github.com/mattermost/mattermost/server/v8 < 8.0.0-20260410202636-17939826efa2.
  • Fixed / safe versions: 10.11.15, 11.4.5, 11.5.4, 11.6.1, 8.0.0-20260410202636-17939826efa2.
  • Public exploit / PoC signal: the consulted NVD/GHSA/vendor sources describe the trigger class; treat it as reproducible until patched.
  • CVSS: use upstream advisory score .

Exact vulnerable code pattern

parse_or_buffer(untrusted_payload)  # unbounded size, time, or concurrency

Fixed / mitigated code pattern

enforce max bytes, frame counts, parser depth, request timeouts, and worker limits

Dependency or runtime update:

go list -m all | grep -i v8 || true
go get github.com/mattermost/mattermost/server/v8@latest
go mod tidy
go test ./...

Step-by-step integration guide

  1. Inventory github.com/mattermost/mattermost/server/v8 across source, lockfiles, SBOMs, container images, CI images, and deployment manifests.
  2. Upgrade to 10.11.15, 11.4.5, 11.5.4, 11.6.1, 8.0.0-20260410202636-17939826efa2 or apply the exact vendor patch referenced below.
  3. Replace the vulnerable dos pattern with the fixed pattern and fail closed on malformed input.
  4. Add a regression test that exercises the advisory trigger and proves the request is rejected, bounded, or authorized.
  5. Deploy through canary, monitor security logs and resource metrics, then remove temporary edge blocks only after all runtimes are fixed.

Alternative mitigations

  • Disable the vulnerable feature path while the package/runtime update rolls out.
  • Add WAF or reverse-proxy rules for traversal tokens, prototype-polluting keys, oversized frames, shell metacharacters, or unauthenticated tool calls matching the trigger class.
  • Restrict internal egress, rotate exposed secrets, and revoke affected tokens/keys if the vulnerability can disclose config, logs, JWT secrets, proxy credentials, or PKI material.
  • For admin/API authorization issues, temporarily require elevated roles and explicit ownership checks at the gateway.

Detection signature

id: cve-2026-5740-mattermost-doesn-t-properly-validate-msgpack-enc
source: GitHub Advisory Database
dependency_or_product: "github.com/mattermost/mattermost/server/v8"
affected: "go:github.com/mattermost/mattermost-server = 11.6.0; go:github.com/mattermost/mattermost-server >= 11.5.0, < 11.5.4; go:github.com/mattermost/mattermost-server >= 11.4.0, < 11.4.5; go:github.com/mattermost/mattermost-server >= 10.11.0, < 10.11.15; go:github.com/mattermost/mattermost/server/v8 < 8.0.0-20260410202636-17939826efa2"
fixed: "10.11.15, 11.4.5, 11.5.4, 11.6.1, 8.0.0-20260410202636-17939826efa2"
signals:
  - "dos"
  - "../"
  - "__proto__"
  - "tools/call"
  - "wp_ajax_nopriv"
  - "pickle.loads"
  - "shell command construction"
action: "upgrade, add regression test, and verify deploy artifact"

Copy-paste skill

You are remediating CVE-2026-5740 (Mattermost doesn't properly validate msgpack-encoded WebSocket frames before memory allocation) in this repository.

Find every affected dependency, runtime, image, service configuration, and reachable code path. Upgrade or patch to 10.11.15, 11.4.5, 11.5.4, 11.6.1, 8.0.0-20260410202636-17939826efa2. Replace the vulnerable dos pattern with a fail-closed implementation, add a regression test for the trigger shape, and update deployment/SBOM artifacts. If this repository does not control the affected runtime, create TRIAGE.md naming the owner, affected version, fixed version, and blocking decision.

Keywords, affected tech stack, and revenue tags

  • Keywords: CVE-2026-5740, dos, github.com/mattermost/mattermost/server/v8, GitHub Advisory Database.
  • Affected tech stack: go.
  • Revenue tags: sellable_to_fintech, enterprise_blocker, high_priority_sla.

References