Koa has Host Header Injection via ctx.hostname

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-27959: 0.39% estimated exploitation activity in the next 30 days; percentile 33.30 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary 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 (e.g., evil.com:fake@legitimate.com) 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. Details The vulnerability exists in Koa's hostname getter in lib/request.js: The host getter retrieves the raw header value with HTTP/2 and proxy support: The Problem The parsing logic simply splits on the first : and returns the first segment. There is no validation that the resulting string is a valid hostname per RFC 3986 Section 3.2.2. RFC 3986 Section 3.2.2 defines the host component as: The @ character is explicitly NOT permitted in the host component - it is the…

Published by the source

26 Feb 2026, 22:42 UTC

Last modified by the source

26 Feb 2026, 23:41 UTC

Source last checked

26 Oct 2026, 12:42 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-7gcc-r8m5-44qm

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-7gcc-r8m5-44qm

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# Patch Next advisory brief: CVE-2026-27959

Advisory: Koa has Host Header Injection via ctx.hostname
IDs: CVE-2026-27959, GHSA-7gcc-r8m5-44qm
Record: https://patchnext.com/advisories/cve-2026-27959 (revision 2, source checked 26 Oct 2026, 12:42 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: koa 3.1.2, 2.16.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm koa: affected >= 3.0.0, < 3.1.2; < 2.16.4; fixed in 2.16.4, 3.1.2

## Suggested inspection (read-only)
- `npm ls koa --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on koa.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-7gcc-r8m5-44qm (OSV record GHSA-7gcc-r8m5-44qm lists affected ranges: npm koa >=3.0.0 =0 <2.16.4 (fixed))
- Verified: https://github.com/advisories/GHSA-7gcc-r8m5-44qm (GitHub-reviewed advisory GHSA-7gcc-r8m5-44qm lists affected ranges: npm koa >=3.0.0 =0 <2.16.4)

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