Koa has Host Header Injection via `ctx.hostname`
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.
- Products
- Not specified
- Affected versions
- Not specified
- Fixed versions
- Not specified
- CVEs
- CVE-2026-27959
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 7 Nov 2026, 01:08 UTC · Revision 1
Source evidence
Koa has Host Header Injection via `ctx.hostname`
core-osv · Original sourcePublished 26 Feb 2026, 01:45 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.
osv:CVE-2026-27959
Complete source record
{
"id": "eb36c75cc30d2fd626d8127c745dbbbcd72c9c421399bac2d5db80733e04f3c7",
"url": "https://osv.dev/vulnerability/CVE-2026-27959",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "Koa has Host Header Injection via `ctx.hostname`",
"summary": "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.",
"evidence": "osv:CVE-2026-27959",
"products": [],
"severity": "high",
"structured": {
"id": "CVE-2026-27959",
"aliases": [
"CVE-2026-27959",
"GHSA-7gcc-r8m5-44qm"
],
"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N"
}
],
"affected": [
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"type": "GIT",
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"database_specific": {
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"range": [
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{
"fixed": "2.16.4"
}
],
"source": "CPE_RANGE"
},
{
"cpe": "cpe:2.3:a:koajs:koa:*:*:*:*:*:node.js:*:*",
"range": [
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"introduced": "3.0.0"
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{
"range": [
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"source": "REFERENCES_COMMIT"
},
{
"range": [
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"source": "REFERENCES_COMMIT"
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"versions": [
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],
"database_specific": {
"source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-27959.json"
}
}
],
"modified": "2026-10-28T02:30:48.890865249Z",
"provider": "osv",
"references": [
"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-27959.json",
"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://github.com/CVEProject/cvelistV5/tree/main/cves/2026/27xxx/CVE-2026-27959.json",
"https://github.com/koajs/koa/security/advisories/GHSA-7gcc-r8m5-44qm",
"https://nvd.nist.gov/vuln/detail/CVE-2026-27959",
"https://bugzilla.redhat.com/show_bug.cgi?id=2442928",
"https://github.com/koajs/koa/commit/55ab9bab044ead4e82c70a30a4f9dc0fc9c1b6df",
"https://github.com/koajs/koa/commit/b76ddc01fdb703e51652b0fd131d16394cadcfeb"
],
"provider_metadata": {
"cwe_ids": [
"CWE-20"
],
"cna_assigner": "GitHub_M",
"osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/27xxx/CVE-2026-27959.json"
}
},
"related_cves": [
{
"value": "CVE-2026-27959",
"evidence": "/id"
}
],
"security_relevant": true,
"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 516859,
"published_at": "2026-02-26T01:45:45.668+00:00"
}