koajs koa up to 2.16.3/3.1.1 API ctx.hostname input validation

Summaryinfo

A vulnerability has been found in koajs koa up to 2.16.3/3.1.1 and classified as problematic. This issue affects the function ctx.hostname of the component API. The manipulation leads to input validation. This vulnerability is referenced as CVE-2026-27959. Remote exploitation of the attack is possible. No exploit is available. The affected component should be upgraded.

Detailsinfo

A vulnerability was found in koajs koa up to 2.16.3/3.1.1 and classified as problematic. This issue affects the function ctx.hostname of the component API. The manipulation with an unknown input leads to a input validation vulnerability. Using CWE to declare the problem leads to CWE-20. The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. Impacted is integrity. The summary by CVE is:

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.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-27959 since 02/25/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details are known, but no exploit is available.

Upgrading to version 2.16.4 or 3.1.2 eliminates this vulnerability. Applying the patch 55ab9bab044ead4e82c70a30a4f9dc0fc9c1b6df is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the databases at EUVD (EUVD-2026-8816) and CERT Bund (WID-SEC-2026-1007). Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Affected

• IBM App Connect Enterprise

Productinfo

Vendor

• koajs

Name

• koa

Version

• 2.16.0
• 2.16.1
• 2.16.2
• 2.16.3
• 3.1.0
• 3.1.1

License

• open-source

Website

• Product: https://github.com/koajs/koa/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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