What’s Vulnerable › Vulnerabilities

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

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-02-26 · updated 2026-07-15
• Weakness: CWE-20

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 products

• koajs koa: before 2.16.4; from 3.0.0 before 3.1.2 — fixed in 2.16.4, 3.1.2

How to fix

Upgrade to a fixed version: koajs koa 2.16.4, 3.1.2. Check the vendor advisory for mitigations.

References

• https://github.com/koajs/koa/commit/55ab9bab044ead4e82c70a30a4f9dc0fc9c1b6df
• https://github.com/koajs/koa/commit/b76ddc01fdb703e51652b0fd131d16394cadcfeb
• https://github.com/koajs/koa/security/advisories/GHSA-7gcc-r8m5-44qm
• https://access.redhat.com/errata/RHSA-2026:10184
• https://access.redhat.com/errata/RHSA-2026:7249
• https://access.redhat.com/security/cve/CVE-2026-27959
• https://bugzilla.redhat.com/show_bug.cgi?id=2442928
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-27959.json

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