How severe is GHSA-g8mr-fgfg-5qpc?

GHSA-g8mr-fgfg-5qpc has a CVSS score of 4.7/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by GHSA-g8mr-fgfg-5qpc?

GHSA-g8mr-fgfg-5qpc affects the following packages: koa (npm), koa (npm). Ecosystems affected: npm.

How do I fix GHSA-g8mr-fgfg-5qpc?

Update koa to 3.0.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-g8mr-fgfg-5qpc is resolved across your whole dependency graph.

How do I detect GHSA-g8mr-fgfg-5qpc in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for koa. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate GHSA-g8mr-fgfg-5qpc if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against GHSA-g8mr-fgfg-5qpc?

O3 pinpoints whether GHSA-g8mr-fgfg-5qpc is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is GHSA-g8mr-fgfg-5qpc actively exploited in the wild?

No public exploit code has been indexed for GHSA-g8mr-fgfg-5qpc yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for GHSA-g8mr-fgfg-5qpc?

GHSA-g8mr-fgfg-5qpc has an EPSS (Exploit Prediction Scoring System) score of 0.3%, placing it in the 19th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is GHSA-g8mr-fgfg-5qpc?

GHSA-g8mr-fgfg-5qpc is classified as Open Redirect (CWE-601). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was GHSA-g8mr-fgfg-5qpc published, and has it been updated?

GHSA-g8mr-fgfg-5qpc was published on October 21, 2025 and was last updated on February 3, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦 npm

GHSA-g8mr-fgfg-5qpc

MEDIUM

Koa Vulnerable to Open Redirect via Trailing Double-Slash (//) in back Redirect Logic

Also known asCVE-2025-62595

Published

Oct 21, 2025

Updated

Feb 3, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

EPSS Exploitation Probability

via FIRST.org ↗

0.3%probability of exploitation in next 30 days

Lower Risk19th percentile+0.26%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Blast Radius

2 pkgs affected

Weekly download volume for affected packages — a proxy for how broadly this vulnerability is deployed.

koanpm

7.9Mdownloads / week

Description

Summary:

A bypass was discovered in the Koa.js framework affecting its back redirect functionality. In certain circumstances, an attacker can manipulate the Referer header to force a user’s browser to navigate to an external, potentially malicious website. This occurs because the implementation incorrectly treats some specially crafted URLs as safe relative paths. Exploiting this vulnerability could allow attackers to perform phishing, social engineering, or other redirect-based attacks on users of affected applications.

This vulnerability affects the code referenced in GitHub Advisory GHSA-jgmv-j7ww-jx2x (which is tracked as CVE‑2025‑54420).

Details:

The patched code attempts to treat values that startWith('/') as safe relative paths and only perform origin checks for absolute URLs. However, protocol‑relative URLs (those beginning with //host) also start with '/' and therefore match the startsWith('/') branch. A protocol‑relative referrer such as //evil.com with trailing double-slash is treated by the implementation as a safe relative path, but browsers interpret Location: //evil.com as a redirect to https://evil.com (or http:// based on context). This discrepancy allows an attacker to supply Referer: //evil.com and trigger an external redirect - bypassing the intended same‑origin protection.

Proof of concept (PoC):

Affected line of code: https://github.com/koajs/koa/blob/master/lib/response.js#L326 The problematic logic looks like:

Request with a protocol‑relative Referer: curl -i -H "Referer: //haymiz.dev" http://127.0.0.1:3000/test

Vulnerable response will contain: HTTP/1.1 302 Found Location: //haymiz.dev

A browser receiving that Location header navigates to https://haymiz.dev (or http:// depending on context), resulting in an open redirect to an attacker‑controlled host:

Recommendation / Patch:

Do not treat //host as a safe relative path. Explicitly exclude protocol‑relative values from any relative‑path branch.
Normalize the Referer by resolving it with a base (e.g., new URL(rawRef, ctx.href)), then compare resolved.origin (scheme+host+port) to ctx.origin (or ctx.host plus scheme/port) before allowing the redirect.

Impact:

An attacker who can cause a victim to visit a specially crafted link (or inject a request with a controlled Referer) can cause the victim to be redirected to an attacker‑controlled domain. This can be used for phishing, social engineering, or to bypass some protection rules that rely on same‑origin navigation.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`koa`	≥ 3.0.1&&< 3.0.3	3.0.3
📦npm	`koa`	≥ 2.16.2&&< 2.16.3	2.16.3

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for koa. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update koa to 3.0.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-g8mr-fgfg-5qpc is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-g8mr-fgfg-5qpc is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to GHSA-g8mr-fgfg-5qpc. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

### Summary: A bypass was discovered in the `Koa.js` framework affecting its back redirect functionality. In certain circumstances, an attacker can manipulate the Referer header to force a user’s browser to navigate to an external, potentially malicious website. This occurs because the implementation incorrectly treats some specially crafted URLs as safe relative paths. Exploiting this vulnerability could allow attackers to perform phishing, social engineering, or other redirect-based attacks on users of affected applications. This vulnerability affects the code referenced in GitHub Advisory

O3 Security · Impact-Aware SCA

Is GHSA-g8mr-fgfg-5qpc in your dependencies?

O3 detects GHSA-g8mr-fgfg-5qpc across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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