Hostile Takeover: Dissecting Koa's Host Header Injection
Vulnerability ID: CVE-2026-27959
CVSS Score: 7.5
Published: 2026-02-26
A high-severity Host Header Injection vulnerability in the popular Node.js framework Koa allows attackers to manipulate context.hostname via malformed headers. By exploiting Koa's naive string-splitting logic, attackers can spoof the hostname used by the application for critical functions like URL generation and routing, leading to password reset poisoning and potential cache poisoning.
TL;DR
Koa versions < 2.16.4 and < 3.1.2 utilized a flawed string-splitting method to parse the HTTP Host header. Attackers can inject an '@' symbol to trick the parser into identifying a malicious domain as the hostname. This enables URL spoofing attacks, notably compromising password reset flows.
⚠️ Exploit Status: POC
Technical Details
- Attack Vector: Network (Remote)
- CVSS v3.1: 7.5 (High)
- Impact: Integrity (High)
- Exploit Status: PoC Available
- Weakness: Improper Input Validation
- EPSS: 0.05%
Affected Systems
- Koa Framework (Node.js)
-
koa: < 2.16.4 (Fixed in:
2.16.4) -
koa: >= 3.0.0 < 3.1.2 (Fixed in:
3.1.2)
Exploit Details
- GitHub Advisory: Advisory detailing the PoC logic for Host Header Injection
Mitigation Strategies
- Upgrade Koa framework to patched versions
- Validate Host headers at the reverse proxy level (Nginx/Apache)
- Avoid using ctx.hostname for critical URL generation
- Use server-side configuration variables for domain generation
Remediation Steps:
- Run
npm auditto identify the vulnerable Koa dependency. - Execute
npm update koato fetch version 2.16.4+ or 3.1.2+. - Verify
package-lock.jsonensures the resolved version is correct. - Audit code for usages of
ctx.hostname,ctx.host, andctx.origin.
References
Read the full report for CVE-2026-27959 on our website for more details including interactive diagrams and full exploit analysis.
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