CVE-2025-62595 is a medium-severity (CVSS 4.3) Open Redirect vulnerability in koa. A fix is available for koa — see the affected versions and patch details below.
Koa Vulnerable to Open Redirect via Trailing Double-Slash (//) in back Redirect Logic
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2025-62595.
EPSS Exploitation Probability
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.
How urgent is this, really
CVE-2025-62595 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 379,145 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
koanpmDescription
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:
<img width="567" height="509" alt="3" src="https://github.com/user-attachments/assets/33de440a-8945-4e5f-9e0a-2011a3999458" />Request with a protocol‑relative Referer: curl -i -H "Referer: //haymiz.dev" http://127.0.0.1:3000/test
<img width="2072" height="1005" alt="1" src="https://github.com/user-attachments/assets/55c48c79-559d-46aa-8b76-c1d2d3536c8b" />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:
<img width="454" height="239" alt="2" src="https://github.com/user-attachments/assets/852ae81a-9f63-49c1-9ce5-72cd96bcea68" />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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | koa | ≥ 3.0.1&&< 3.0.3 | 3.0.3npm install koa@3.0.3 |
| 📦npm | koa | ≥ 2.16.2&&< 2.16.3 | 2.16.3npm install koa@2.16.3 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for koa, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update koa to 3.0.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-62595 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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2025-62595 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2025-62595. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2025-62595 in your dependencies?
O3 Security finds CVE-2025-62595 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.