CVE-2025-12999 Eclipse Foundation CVE debrief

Who should care

Administrators and users of Open VSX extension manager, as well as developers of VS Code-compatible editors, should assess their exposure and take necessary actions to mitigate the vulnerability.

Why it matters

CVE-2025-12999 is a critical vulnerability in Open VSX that allows an unauthenticated remote attacker to poison the cache with malicious URLs, potentially leading to the installation of malicious VSIX extensions. Defenders should assess their exposure, prioritize remediation, and verify affected versions and exploitation.

• An unauthenticated remote attacker can poison the cache with attacker-controlled download, signature, and public-key URLs
• Downstream VS Code-compatible editors may fetch and install malicious VSIX extensions
• The vulnerability depends on deployment topology, and exploitability varies depending on proxy configuration
• Verification of affected versions, exploitation, and remediation is required

Technical summary

The Open VSX extension manager is vulnerable to cache poisoning due to the insecure use of X-Forwarded-Host, X-Forwarded-Proto, and X-Forwarded-Prefix request headers in the UrlUtil.getBaseUrl function. This allows an unauthenticated remote attacker to forge URLs in cached responses, potentially leading to the installation of malicious VSIX extensions in downstream VS Code-compatible editors. The exploitability depends on the deployment topology, particularly proxy configuration. An attacker can poison the cache with attacker-controlled download, signature, and public-key URLs, which can cause downstream VS Code-compatible editors to fetch and install malicious VSIX extensions.

Defensive priority

High

Recommended defensive actions

• Configure the reverse proxy to set rather than relay X-Forwarded-Host, X-Forwarded-Proto, and X-Forwarded-Prefix headers
• Ensure the server is not reachable except through the proxy
• Flush the caches after configuration changes
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented

Evidence notes

The vulnerability is caused by the insecure use of X-Forwarded-Host, X-Forwarded-Proto, and X-Forwarded-Prefix request headers in the UrlUtil.getBaseUrl function, which allows an attacker to forge URLs in cached responses. The exploitability depends on the deployment topology, and workarounds include configuring the reverse proxy to set rather than relay these headers or ensuring the server is not reachable except through that proxy.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.