CVE-2025-12999: Eclipse Open VSX · CWE-345
Affects Eclipse Open VSXSource CVE Database V5
Not known to be exploited · CVSS 9.1 Critical.
Data as of 5 Nov 2026, 00:01 UTC
- EPSS
- 0.3% chance of exploitation in 30 days – higher than 26% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:H/SI:H/SA:HExploitation evidence
Disclosed: evidence found.
CVE published 21 Sep 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors
A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 5 Nov 2026.
What to do
Affected software
| Product | Affected | Fixed in |
|---|---|---|
| Eclipse FoundationEclipse Open VSX | Affected
| Fixed in– |
GitHubeclipse-openvsx/openvsxpkg:github/eclipse-openvsx/openvsxOther threats | Affected
| Fixed in– |
Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.
AI-suggested
Workarounds include configuring the reverse proxy to set (not relay) the X-Forwarded-Host, X-Forwarded-Proto, and X-Forwarded-Prefix headers with safe values (not using nginx's $host variable, which reflects the client's Host header). Additionally, ensure the Open VSX server is only reachable through a trusted proxy that enforces these header settings. After applying these changes, flush the server caches to remove any poisoned entries, as they persist for the cache lifetime.
Description
CVE-2025-12999 is a critical vulnerability in Eclipse Open VSX affecting versions from 0.6.0 up to and including 1.1.2. The vulnerability arises because the application builds absolute URLs in responses from untrusted X-Forwarded-* headers without verifying the sender is a trusted proxy. These responses are cached and served to other clients, allowing an unauthenticated attacker to poison the cache with malicious URLs. This can lead to downstream VS Code-compatible editors fetching and installing malicious VSIX packages. Exploitability depends on deployment topology, particularly how proxies handle these headers.