CVE-2026-27959: Userinfo Host Header Injection in Koa

In January 2026, a security vulnerability was discovered in the Koa framework, which allowed for a userinfo host header injection attack due to its improper parsing of the HTTP Host header, especially when crafted to include a userinfo component. This flaw, tracked as CVE-2026-27959, could result in attacker-controlled hostnames being processed by applications using Koa's ctx.hostname feature. The vulnerability primarily stemmed from the framework's failure to account for the '@' character, which serves as a delimiter between userinfo and host in a URI authority component as per RFC 3986. The issue affected Koa versions 2.16.3 and below, as well as 3.0.0 through 3.1.1, and had a CVSS score of 7.5, indicating a high severity. Koa maintainers quickly addressed the problem, releasing patched versions 2.16.4 and 3.1.2 in February 2026, urging users to upgrade immediately to prevent exploitation, particularly in contexts such as password reset and email verification flows where the vulnerability could lead to significant security breaches. The update included enhanced validation to prevent unauthorized userinfo segments from masquerading as hostnames, and users are advised to implement additional security measures, such as validating host headers at the proxy layer and using environment-configured base URLs to mitigate further risks.

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