Bug 2442928 (CVE-2026-27959)

Summary: CVE-2026-27959 koa: Koa: Host header injection vulnerability due to malformed HTTP Host header parsing
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: high Docs Contact:
Priority: high    
Version: unspecifiedCC: abarbaro, alizardo, caswilli, dschmidt, erezende, jchui, jhe, jkoehler, jlanda, kaycoth, kshier, ktsao, lphiri, mattdavi, nboldt, psrna, sdawley, simaishi, smcdonal, stcannon, teagle, yguenane
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in Koa’s ctx.hostname API used in Node.js applications. The function incorrectly parses specially crafted HTTP Host headers containing an @ character, which can cause the extracted hostname value to differ from the intended origin. An attacker can exploit this behavior by sending a malicious Host header to influence the hostname value returned by ctx.hostname. Applications that rely on this value for generating absolute URLs, password reset links, or email verification links without additional validation may be susceptible to Host header injection attacks.
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Description OSIDB Bzimport 2026-02-26 03:01:38 UTC 
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.