CVE-2025-62595

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.

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.

Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites. Typical impact: phishing and credential harvesting via a trusted domain.

CVE-2025-62595 has a CVSS score of 4.7 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.0.3, 2.16.3); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2025-62595? CVE-2025-62595 is a medium-severity open redirect vulnerability in koa (npm), affecting versions >= 3.0.1, < 3.0.3. It is fixed in 3.0.3, 2.16.3. Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites.
2. How severe is CVE-2025-62595? CVE-2025-62595 has a CVSS score of 4.7 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of koa are affected by CVE-2025-62595? koa (npm) versions >= 3.0.1, < 3.0.3 is affected.
4. Is there a fix for CVE-2025-62595? Yes. CVE-2025-62595 is fixed in 3.0.3, 2.16.3. Upgrade to this version or later.
5. Is CVE-2025-62595 exploitable, and should I be worried? Whether CVE-2025-62595 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-62595 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-62595?
   • Upgrade koa to 3.0.3 or later
   • Upgrade koa to 2.16.3 or later