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Vulnerability · Published Feb 26, 2026 · Assigned by GitHub_M

CVE-2026-27959Koa has Host Header Injection via `ctx.hostname`

High 7.5

In plain English

A high-severity improper input validation in koa from koajs. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 33% of all known vulnerabilities). Ready-made attack tooling exists: public proof-of-concept code. Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.5High
Exploit odds (30 days)0.4%top 66.6% of all CVEs
Exploited in the wildNo reportsCISA SSVC: poc
Attack tooling1Public PoC

Official description

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 packages & fixed versions

EcosystemPackageFixed inAdvisory
npmkoa3.1.2GHSA-7gcc-r8m5-44qm
npmkoa2.16.4GHSA-7gcc-r8m5-44qm

Affected products (as reported)

VendorProductVersions
koajs koa >= 3.0.0, < 3.1.2< 2.16.4

Weakness: CWE-20 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

References