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CVE-2026-27959 Koajs CVE debrief

CVE-2026-27959 is a HIGH severity vulnerability in Koa, a middleware for Node.js using ES2017 async functions. Prior to versions 3.1.2 and 2.16.4, Koa's `ctx.hostname` API performs naive parsing of the HTTP Host header, extracting everything before the first colon without validating the input conforms to RFC 3986 hostname syntax. This allows for Host header injection attacks when a malformed Host header containing a `@` symbol is received. Applications using `ctx.hostname` for URL generation, password reset links, email verification URLs, or routing decisions are vulnerable.

Vendor
Koajs
Product
Koa
CVSS
HIGH 7.5
CISA KEV
Not listed in stored evidence
Original CVE published
2026-02-26
Original CVE updated
2026-06-16
Advisory published
2026-02-26
Advisory updated
2026-06-16

Who should care

Developers and administrators using Koa versions prior to 3.1.2 or 2.16.4 should be aware of this vulnerability and take steps to mitigate it.

Technical summary

The vulnerability arises from Koa's `ctx.hostname` API not properly validating the HTTP Host header according to RFC 3986 hostname syntax. This allows an attacker to inject malicious values by including a `@` symbol in the Host header, potentially leading to security issues such as unauthorized URL generation or routing decisions.

Defensive priority

HIGH

Recommended defensive actions

  • Upgrade to Koa version 3.1.2 or 2.16.4 or later.
  • Review and validate the HTTP Host header in your application to prevent Host header injection attacks.

Evidence notes

Evidence from the NVD and GitHub security advisories confirms the vulnerability and provides patches for affected versions.

Official resources

CVE-2026-27959 was published on 2026-02-26T02:16:23.317Z and modified on 2026-06-16T19:38:55.347Z.