# CVE-2025-12999: UrlUtil.getBaseUrl builds the absolute URLs in a response — download links, icons, asset and API URLs — from the X-Forwarded-Host, X-Forward - **Severity:** Critical 9.1 (`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`) - **Exploited in the wild:** Not known - **Fix available:** No - **Published:** 21 Sep 2026 - **Updated:** 30 Sep 2026 - **Also known as:** GHSA-f55q-5m46-rmxc ## What is CVE-2025-12999? CVE-2025-12999 is a critical vulnerability in Eclipse Open VSX, published on 21 Sep 2026. ### Official 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. Weakness: CWE-345 (Insufficient Verification of Data Authenticity) ## Which versions are affected by CVE-2025-12999? ## How do I fix CVE-2025-12999? No fix has been published for CVE-2025-12999 yet. Limit exposure of the affected software until one is available. ## Is CVE-2025-12999 being exploited? It is not confirmed as exploited in the wild, but public exploit code for CVE-2025-12999 exists. ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2025-12999 and fix it if it is affected. What it is: CVE-2025-12999 is a critical vulnerability in Eclipse Open VSX, published on 21 Sep 2026. Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2025-12999 exists. Affected product: Eclipse Open VSX. No package-level version data is available, so check the vendor advisory linked from the reference page. Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2025-12999/ (Markdown: https://insights.sbomify.com/CVE-2025-12999/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2025-12999 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-12999 - OSV: https://osv.dev/vulnerability/CVE-2025-12999 --- From sbomify Insights: https://insights.sbomify.com/CVE-2025-12999/ Summaries are generated and may contain mistakes; the official description above is the record.