Eclipse Open VSX Security Vulnerability

CVE Details

• Protection status: Protected by Atomicorp rules · About protection statuses
• CVE: CVE-2025-12999
• Name: Eclipse Open VSX Security Vulnerability
• Severity: Critical
• CVSS: 9.1 (v4.0) (Critical)
• CVSS version: 4.0
• CVSS source: eclipse CNA
• CVSS 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
  • Attack vector: Network
  • Attack complexity: Low
  • Attack requirements: Present
  • Privileges required: None
  • User interaction: None
  • Vulnerable-system confidentiality: None
  • Vulnerable-system integrity: High
  • Vulnerable-system availability: Low
  • Subsequent-system confidentiality: High
  • Subsequent-system integrity: High
  • Subsequent-system availability: High
  • Safety: Not defined
• CWE: N/A
• Vendor: Eclipse Foundation
• Vulnerable product: Eclipse Open VSX
• CVE published: 2026-09-21
• CVE last modified: 2026-09-21
• EPSS: 0.0040; 34.5th percentile as of 2026-09-21 (FIRST (opens in a new tab) )
• Latest Update: 2026-09-22
• Selected Rule IDs observed: 337109 , 337110 , 340165 , 344360 , 347009 , 390722 , 398001 , 398008 , 398021 , 398022
• Matching rules: 10; rules with blocking actions: 8
• Would-block Rule IDs: 337109 , 337110 , 340165 , 344360 , 347009 , 390722 , 398021 , 398022
• Request surfaces inspected: JSON request data, Request URI, Request argument names, Request arguments, Request cookies, SOAP request data, XML request data

Vulnerability Description

UrlUtil.getBaseUrl builds the absolute URLs in a response — download links, icons, asset and API URLs — from the X-Forwarded-Host, X-Forwarded-Proto and X-Forwarded-Prefix request headers, with no check on whether the sender was a trusted proxy, falling back to the client-supplied Host header.

Those responses are cached under keys that do not include the host (extension.json since 0.6.0, namespace.details.json since 0.9.0, sitemap since 0.14.5, latest.extension.version.vscode since 0.34.2). A single request carrying a forged header therefore places attacker-chosen URLs into an entry served to every other client for the lifetime of that entry — one hour by default, and cluster-wide where ovsx.redis.enabled is set.

The VSIX download URL, its signature URL and the public key URL are all derived from the same base URL, so extension signing does not limit the impact: an attacker who poisons an entry supplies the package, the signature over it, and the key used to verify it.

Exploitability depends on deployment topology. A server reachable directly by clients, or fronted by a proxy that relays the client’s X-Forwarded-Host rather than overwriting it, is exploitable by an unauthenticated remote attacker. A proxy that overwrites the header is not.

An unauthenticated attacker can poison Open VSX’s per-extension metadata cache with attacker-controlled download, signature, and public-key URLs by supplying a crafted X-Forwarded-Host header, causing downstream VS Code-compatible editors to fetch and install a malicious VSIX.

Workarounds (unpatched versions)

1. Configure the reverse proxy to set rather than relay X-Forwarded-Host, X-Forwarded-Proto and X-Forwarded-Prefix — note that nginx’s $host is the client’s Host header and is not a safe value.

2. Ensure the server is not reachable except through that proxy.

3. Flush the caches afterwards; poisoned entries survive the configuration change.

Recommended Actions

1. Maintain the observed Atomicorp protections. Keep your Atomicorp commercial rules current and confirm that the selected observed rules (337109 , 337110 , 340165 , 344360 , 347009 , 390722 , 398021 , 398022 ) have not been locally disabled. Atomicorp commercial rules are enabled by default. If you use Atomic WAF, review the applicable policy and rule settings.

2. Apply the vendor’s remediation. WAF protection is defense in depth and does not replace upgrading or following the affected vendor’s guidance.

   • Affected versions: Eclipse Foundation Eclipse Open VSX versions through 1.1.2.
   • No specific CNA/vendor solution or workaround is included in the current CVE Record (opens in a new tab) . Review the vendor’s security advisories and supported-release guidance before making changes.

3. Validate after changes. Confirm rule updates are current, review WAF events for the affected application, and test application behavior after vendor upgrades, mitigations, or local policy changes.

Research Notes

Atomicorp WAF protections are designed around attack techniques and exploit behaviors, allowing existing protections to apply across multiple vulnerabilities without requiring a unique rule for every CVE.

Public Vulnerability References

• NVD (opens in a new tab)
• CVE Record (opens in a new tab)
• CVE authority reference: https://github.com/eclipse-openvsx/openvsx/pull/2196 (opens in a new tab)
• CVE authority reference: https://github.com/eclipse-openvsx/openvsx/security/advisories/GHSA-f55q-5m46-rmxc (opens in a new tab)
• CVE authority reference: https://gitlab.eclipse.org/security/cve-assignment/-/work_items/75 (opens in a new tab)

Test or Research Source

• Source: Atomicorp AI Research Framework
• Tags: cve, derived, ssrf, waf-probe

Research Note Context

This research note documents exploit-technique analysis associated with this vulnerability and references Atomicorp WAF protections observed during engineering review, testing, or validation activities.

Atomicorp security controls are generally designed to identify and mitigate attack techniques and exploit behaviors rather than individual CVE identifiers. As a result, protections developed for one vulnerability may also apply to related attack variants and future vulnerabilities using similar techniques.

Learn More

Atomicorp develops commercially supported security technologies for web applications, servers, endpoints, workloads, and legacy enterprise platforms.

Organizations interested in protecting systems against exploit techniques associated with vulnerabilities such as this one can learn more about:

• Atomic ModSecurity Rules: commercially maintained WAF rules for ModSecurity deployments, including protections for web attacks, bots, scanners, malware, exploitation attempts, and emerging vulnerability classes.
• Atomic WAF: an enterprise web application firewall with Atomicorp rules, centralized management, reporting, authentication controls, and operational tooling.
• Atomic ModSecurity Integrator: a special licensing option for MSPs, MSSPs, OEMs, VARs, reverse proxy providers, and other integrators who want to incorporate Atomicorp ModSecurity rule technology into their own products or services.
• Atomic OSSEC: an enterprise OSSEC-based intrusion detection, compliance, and security monitoring platform.
• Atomic Protector: workload protection for Linux, web, cloud, data center, and hybrid environments.

For additional information, product evaluations, or technical discussions, visit https://www.atomicorp.com/ or https://modsecurity.io/ (opens in a new tab) .