Which versions are affected?

Affected product and version evidence is listed in the product table above. Current public advisories should be checked for exact version scope before patch targeting is finalized.

Is there active exploitation?

Active exploitation is not confirmed from current sources for CVE-2025-12999. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

No. CVE-2025-12999 is not listed in CISA KEV in the supplied enrichment data. KEV status should be monitored because it can change after initial disclosure or after exploitation becomes publicly confirmed.

Is there a public exploit?

4 public PoC repositories reported; 0 marked weaponized in current dataset. Public exploit or PoC availability should raise triage priority, especially for exposed or business-critical systems.

How can defenders detect it?

Start with web/proxy/WAF logs, application logs, network IDS/IPS where available. Look for vulnerability-class behavior, product errors, suspicious authentication events, exploit probes, and abnormal process activity based only on evidence that applies to the affected product.

How should teams prioritize it?

Prioritize CVE-2025-12999 using CVSS 9.1, EPSS 0%, KEV status, public PoC status, and asset exposure. Internet-facing, privileged, or business-critical deployments should be handled first.

Are workarounds available?

The supplied mitigation guidance says to apply vendor patches and restrict external exposure until fixed. Fixed-version evidence is listed in the mitigation section above.

Which threat actors are linked publicly?

No public threat actor linkage is confirmed from current sources. Do not attribute exploitation to a threat actor unless the source evidence explicitly supports that linkage.

What should be done in the next 24 hours?

Identify exposed assets running affected vendor/product/version combinations. Prioritize based on EPSS, PoC availability, and external exposure. Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class. Open remediation tickets with affected assets, owners, evidence status, and detection coverage so decisions are traceable.

criticalpublic exploit

CVE-2025-12999

CVE-2025-12999 — UrlUtil.getBaseUrl builds the absolute URLs in a response — download links, icons, asset and API URLs — from the X-Forwarded-Host…

Q: What is CVE-2025-12999?

CVE-2025-12999 is UrlUtil.getBaseUrl builds the absolute URLs in a response — download links, icons, asset and API URLs — from the X-Forwarded-Host…. The supplied enrichment data classifies it as Unclassified mapped to Not reported.

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.

Published Sep 21, 2026Updated Sep 21, 2026Sources: NVD, Eclipse Foundation (CNA), GitHub, SOCRadar CTI

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Exploit code

public exploit, none observed

EPSS

ahead of 0% of scored CVEs

Affects

Eclipse Foundation

Eclipse Open VSX

CVSS base

9.1

critical

Remediation

The vendor's own words where we have them.

Upgrade Eclipse Open VSX to a fixed release. Apply vendor patches per advisory and restrict external exposure of the affected component until patched.

First 24 hours

Ordered from the record's own fields — exposure first, because you cannot patch what you have not found.

Identify exposed assets running affected vendor/product/version combinations.
Prioritize based on EPSS, PoC availability, and external exposure.
Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class.

Affected scope

Vendor, product and version as the advisories word them.

Vendor	Product	Versions	Status
Eclipse Foundation	Eclipse Open VSX	0.6.0 to <= 1.1.2	Vulnerable

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack Complexity

Low

Attack Vector

Network

Privileges Req

None

User Interaction

None

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Source
9.1	CVSS 4.0	critical	—	—	Eclipse Foundation (CNA)

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

Public exploit

Capability, not use: code existing is a different claim from anyone running it.

Repositories

Detection

Read off the CVSS vector and the weakness class. Starting points, not rules we have tested.

Prioritize edge telemetry for network-reachable Eclipse Open VSX.
Monitor for scanner or exploit-pattern traffic after 4 public PoC repositories were reported.

web/proxy/WAF logsapplication logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first — with the readings a source repeats on a schedule counted underneath rather than listed.

2026
Added · CVSS 3.1 9.1 (AV:N)
Sep 21, 2026 · NVD
Initial · CVE published
Sep 21, 2026 · NVD
CVE published by MITRE.
Sep 21, 2026 · SOCRadar CTI

References

3 on the record

github.com/eclipse-openvsx/openvsx/pull/2196
Exploit, Third Party Advisory
github.com/eclipse-openvsx/openvsx/security/advisories/GHSA-f55q-5m46-rmxc
Exploit, Third Party Advisory
gitlab.eclipse.org/security/cve-assignment/-/work_items/75
Vendor Advisory

Elsewhere on this site

Eclipse Foundationevery CVE for this vendor
Unclassifiedsame class

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record1

What should defenders know first?

CVE-2025-12999 is UrlUtil.getBaseUrl builds the absolute URLs in a response — download links, icons, asset and API URLs — from the X-Forwarded-Host…, a critical vulnerability affecting Eclipse Open VSX from Eclipse Foundation. The current evidence does not list it in CISA KEV, and the exploit status is: Active exploitation is not confirmed from current sources for CVE-2025-12999. Public exploit evidence is: 4 public PoC repositories reported; 0 marked weaponized in current dataset. The affected-version evidence is listed in the key facts and affected products tables. Defenders should first verify whether exposed or business-critical assets run those versions, then apply vendor patches or mitigations, restrict reachable attack surface, and preserve logs for detection review. CVSS 9.1 describes technical severity, while EPSS 0% helps estimate near-term exploit likelihood; neither replaces asset context. Unknown fields should remain explicit in tickets, and threat actor, IOC, victimology, or payload claims should not be added unless a cited source supports them. Monitor CISA KEV, vendor advisories, NVD changes, public PoC repositories, and internal telemetry for update triggers.