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Vulnerability record · CVE-2026-27959 · published 26 February 2026

CVE-2026-27959: Koajs koa improper input validation vulnerability

KKoajs · Koa

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.

7.5 CVSS 3.1 High EPSS 0.52% · top 58.1% CWE-20 · Improper input validation
7.5CVSS 3.1 base score
0.52%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
8References, 1 tagged exploit
15 Jul 2026Last modified by NVD

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.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/koajs/koa/commit/55ab9bab044ead4e82c70a30a4f9dc0fc9c1b6df Patch
https://github.com/koajs/koa/commit/b76ddc01fdb703e51652b0fd131d16394cadcfeb Patch
https://github.com/koajs/koa/security/advisories/GHSA-7gcc-r8m5-44qm ExploitVendor Advisory
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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Source: NIST National Vulnerability Database (record CVE-2026-27959), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.