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CVE-2026-55094 – Unauthenticated Remote Code Execution – Taskcluster < 100.3.0

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Summary

CVE-2026-55094 is an unauthenticated remote code execution (RCE) vulnerability in Taskcluster, the task execution framework that underpins Mozilla’s CI and release build infrastructure. The flaw resides in the web-server component’s GraphQL endpoint, where the filter argument is passed unvalidated to the sift npm library, whose $where operator evaluates attacker-controlled text as JavaScript. Any Taskcluster deployment that exposes the GraphQL endpoint to an anonymous role is remotely exploitable with no credentials or user interaction, earning a CVSS v4.0 HIGH severity rating of 8.7.

Technical details

  • Root cause: The GraphQL filter argument (used, for example, by the expandScopes query) is passed directly into the sift library without sanitization. sift‘s $where operator evaluates the supplied string as executable JavaScript within the Node.js server process.
  • Trigger conditions: The Taskcluster deployment must expose the GraphQL endpoint to an "anonymous" role and use the vulnerable sift-based filtering logic in web-server. No authentication token or session is required to reach the vulnerable code path.
  • Attack vector: Network — a single unauthenticated HTTP POST request to /graphql containing a crafted filter argument with a $where clause is sufficient to trigger code execution.
  • Impact: Successful exploitation allows arbitrary server-side JavaScript execution within the Taskcluster web-server process. Depending on deployment configuration, this can expose backend credentials and secrets accessible to that process (e.g., database credentials, deployment tokens, OAuth client secrets, and other runtime configuration), leading to full compromise of the Taskcluster instance.
  • Weaknesses involved: CWE-20 (Improper Input Validation), CWE-94 (Code Injection), CWE-95 (Eval Injection), CWE-250 (Execution with Unnecessary Privileges), and CWE-306 (Missing Authentication for a Critical Function).

Affected software

  • Taskcluster (taskcluster/taskcluster) — all versions prior to 100.3.0

Severity

  • CVSS v4.0 Base Score: 8.7 (High)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade Taskcluster to version 100.3.0 or later, which removes the vulnerable sift-based GraphQL filtering logic entirely.
  • If immediate patching is not possible:
    • Restrict or disable anonymous/unauthenticated access to the GraphQL endpoint at the network or reverse-proxy layer until the upgrade can be applied.
    • Audit web-server deployment configuration to ensure the GraphQL endpoint is not reachable by unauthenticated clients.
    • After upgrading, rotate any credentials and secrets accessible to the web-server process (database credentials, deployment access tokens, OAuth client secrets, session secrets, and encryption keys) as a precaution, given the potential for prior exposure.

References

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