CVE-2026-27959: Koajs Koa

High severity, CVSS 7.5. EPSS: 0.5% chance of exploitation in the next 30 days.

Koa is 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. When a malformed Host header containing a `@` symbol is received, `ctx.hostname` returns `evil[.]com` - an attacker-controlled value. Applications using `ctx.hostname` for URL generation, password reset links, email verification URLs, or routing decisions are vulnerable to Host header injection attacks. Versions 3.1.2 and 2.16.4 fix the issue.

Affected products

• Koajs Koa: before 2.16.4 (fixed in 2.16.4); from 3.0.0, before 3.1.2 (fixed in 3.1.2)

Published 2026-02-26. Last modified 2026-07-15.