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CVE-2025-12999 – Cache Poisoning Leading to Malicious Extension Delivery – Eclipse Open VSX 0.6.0 th

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Summary

CVE-2025-12999 is a critical (CVSS 9.1) cache poisoning vulnerability in Eclipse Open VSX, the open-source extension registry used by VS Code-compatible editors. An unauthenticated remote attacker can inject forged X-Forwarded-Host, X-Forwarded-Proto, and X-Forwarded-Prefix headers to poison the registry’s response cache with attacker-controlled URLs, causing every subsequent client to receive malicious extension download, signature, and public-key links.

Technical details

  • Root cause: UrlUtil.getBaseUrl builds absolute URLs (download, icon, asset, API, signature, and public-key URLs) from the client-supplied X-Forwarded-Host, X-Forwarded-Proto, and X-Forwarded-Prefix headers, without verifying the request came from a trusted proxy.
  • The /vscode/gallery/{namespace}/{extension}/latest endpoint and related metadata responses (extension.json, namespace.details.json, sitemap, latest.extension.version.vscode) are cached under keys that do not include the host.
  • Trigger conditions: exploitable when the Open VSX server is reachable directly by clients, or sits behind a reverse proxy that relays rather than overwrites these forwarding headers, and a cache entry for the targeted extension is cold.
  • Attack vector: a single unauthenticated HTTP request carrying a forged header poisons the shared cache entry, which is then served to every client requesting that extension’s metadata for the cache lifetime (default one hour, or cluster-wide when Redis caching is enabled).
  • Impact: victims’ editors resolve download, signature, and public-key URLs to attacker infrastructure; because all three derive from the same poisoned base URL, extension signature verification provides no protection, allowing delivery of a malicious VSIX and arbitrary code execution with the privileges of the extension host.

Affected software

  • Eclipse Open VSX (eclipse-openvsx/openvsx), versions 0.6.0 through 1.1.2
  • Open VSX server container images (GHCR openvsx-server and Docker Hub openvsx-server), versions 0.6.0 through 1.1.2

Severity

CVSS v4.0 Base Score: 9.1 (Critical)
Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:H/SI:H/SA:H

Mitigation and recommended actions

  • Immediate: upgrade Eclipse Open VSX to version 1.2.0 or later, which fixes the vulnerable URL construction.
  • If patching is not immediately possible:
    • Configure the reverse proxy in front of Open VSX to set (overwrite), rather than relay, the X-Forwarded-Host, X-Forwarded-Proto, and X-Forwarded-Prefix headers on incoming requests. Note that nginx’s $host variable reflects the client-supplied Host header and is not a safe substitute.
    • Ensure the Open VSX server is not reachable except through that trusted, header-overwriting proxy.
    • After deploying the fix or mitigation, flush all caches (including Redis, if used), since previously poisoned cache entries persist across configuration changes.

References

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