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CVE-2026-27959 is a Host header parsing flaw in the Koa web framework for Node.js. In Koa versions prior to 3.1.2 and 2.16.4, the ctx.hostname API naively parses the HTTP Host (and/or :authority) value by taking the substring before the first colon, without validating that the resulting value conforms to RFC 3986 hostname syntax. When a malformed Host header containing an @ symbol is supplied, ctx.hostname can return an attacker-controlled hostname (e.g., evil.com). Applications that trust ctx.hostname (or derived values such as ctx.origin) for security-sensitive URL generation (password reset links, email verification URLs), redirects, or routing decisions can be coerced into using attacker-controlled hostnames, enabling Host Header Injection and downstream attacks.
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ctx.hostname/ctx.origin, this can also enable OAuth/SSO callback manipulation, web cache poisoning, and potentially SSRF or routing/tenant confusion in deployments that make security decisions based on the effective host.If you can’t patch tonight, do this now.
ctx.hostname or ctx.origin for security-sensitive URL generation, redirects, or routing without strict allowlisting/validation of the effective host. Enforce a canonical host at the edge (reverse proxy/load balancer) by validating and/or overriding Host and, when app.proxy=true, X-Forwarded-Host. Reject requests with malformed host/authority values (including any containing @ or otherwise not conforming to RFC 3986). Ensure the application is not configured as a default/catch-all virtual host for arbitrary Host headers.Patch, then assume compromise.
No valid public exploits. Mallory filtered out 1 candidate as fakes, detection scripts, or README-only repos.
All candidate exploits were filtered out by Mallory's validation.
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3 sources tracked across advisories and community write-ups. News coverage will land here when it surfaces.
No news coverage yet. Advisories and community discussion only.
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Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.