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CVE-2026-89040 – Unauthenticated Path Traversal to Root RCE – Tencent Mass Service Engine in Cluster

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Summary

CVE-2026-89040 is a critical path traversal vulnerability (CWE-22) in Tencent’s Mass Service Engine in Cluster (MSEC), a backend DevOps/service framework used to manage RPC, load balancing, monitoring, and remote deployment across clustered server fleets. A remote, unauthenticated attacker can submit a crafted POST request containing directory traversal sequences (../) to write a file outside the intended restricted directory, ultimately dropping a webshell and executing arbitrary code with root privileges on the target host. The flaw carries a CVSS v3.1 base score of 9.8 (Critical) and requires no authentication or user interaction.

Technical details

  • Root cause: MSEC fails to properly sanitize or restrict pathname input supplied in a POST request body/parameter, allowing directory traversal sequences (../) to escape the intended upload/restricted directory (CWE-22: Improper Limitation of a Pathname to a Restricted Directory).
  • Trigger conditions: An attacker sends a specially crafted POST request containing traversal sequences to a network-exposed MSEC service endpoint; no valid credentials or prior session are required.
  • Attack vector: Network-based (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N) — the vulnerability can be exploited directly over the network against any reachable MSEC deployment.
  • Impact: Successful exploitation allows the attacker to write an arbitrary file (e.g., a webshell) to a location it can subsequently execute, resulting in full compromise of confidentiality, integrity, and availability with root-level code execution on the underlying host.

Affected software

  • Tencent Mass Service Engine in Cluster (MSEC) — all released versions are listed as affected in the CVE record; no version-specific fix boundary has been published.

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  • A CVSS v4.0 score of 9.3 (Critical) is also recorded for this vulnerability.

Mitigation and recommended actions

  • Immediate: At the time of publication, the CVE record does not identify a patched MSEC release. Organizations running MSEC should check Tencent/MSEC’s official channels for an updated build that fixes the path traversal flaw and apply it as soon as it becomes available.
  • If no patch is available:
    • Restrict network access to MSEC management/service endpoints to trusted internal networks only; do not expose MSEC interfaces directly to the internet.
    • Place a web application firewall (WAF) or reverse proxy in front of MSEC endpoints to detect and block requests containing directory traversal sequences (../) or unexpected file-write parameters.
    • Monitor MSEC hosts for unexpected file writes outside expected upload directories and for newly created executable/script files (potential webshells).
    • Run MSEC services with the least privilege necessary rather than root, and apply file-system permissions/segmentation to limit the blast radius of a successful write.
    • Given MSEC’s underlying GitHub repository has been archived, evaluate migration to an actively maintained alternative if no vendor fix is forthcoming.

References

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