GHSA-HRR3-GC8F-F4QJ
Vulnerability from github – Published: 2026-09-29 23:54 – Updated: 2026-09-29 23:54Impact
fast-uri folds the host to lowercase before it percent-decodes the host, so a percent-encoded uppercase unreserved octet such as %41 decodes to a literal A that is never folded. For a scheme-relative reference (//host) there is no scheme, so the host canonicalization that repairs this on a scheme-bearing URL does not run. As a result parse, normalize, and equal disagree on the same host: parse("//%41.com").host returns "A.com" while parse("//a.com").host and parse("//A.com").host return "a.com", and equal("//%41.com", "//a.com") is false even though equal("//A.com", "//a.com") is true. An application that makes a case-sensitive host decision on fast-uri output for a scheme-relative reference, for example a host allowlist or denylist that compares parse(url).host or uses fast-uri.equal, can be steered past the check with a percent-encoded uppercase octet. Because a hostname is case-insensitive in DNS and HTTP routing, the evading spelling reaches the same host the check meant to gate, so the effect is check evasion rather than reaching a different registrable host.
Patches
Upgrade to fast-uri 4.1.5, 3.1.8, or 2.4.7.
Workarounds
Compare hosts case-insensitively (lowercase the parsed host before any allowlist or denylist decision), or avoid making case-sensitive host decisions on scheme-relative input until upgrading.
{
"affected": [
{
"package": {
"ecosystem": "npm",
"name": "fast-uri"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "2.4.7"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "npm",
"name": "fast-uri"
},
"ranges": [
{
"events": [
{
"introduced": "3.0.0"
},
{
"fixed": "3.1.8"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "npm",
"name": "fast-uri"
},
"ranges": [
{
"events": [
{
"introduced": "4.0.0"
},
{
"fixed": "4.1.5"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-86472"
],
"database_specific": {
"cwe_ids": [
"CWE-178"
],
"github_reviewed": true,
"github_reviewed_at": "2026-09-29T23:54:25Z",
"nvd_published_at": "2026-09-15T11:17:12Z",
"severity": "MODERATE"
},
"details": "### Impact\n\n`fast-uri` folds the host to lowercase before it percent-decodes the host, so a percent-encoded uppercase unreserved octet such as `%41` decodes to a literal `A` that is never folded. For a scheme-relative reference (`//host`) there is no scheme, so the host canonicalization that repairs this on a scheme-bearing URL does not run. As a result `parse`, `normalize`, and `equal` disagree on the same host: `parse(\"//%41.com\").host` returns `\"A.com\"` while `parse(\"//a.com\").host` and `parse(\"//A.com\").host` return `\"a.com\"`, and `equal(\"//%41.com\", \"//a.com\")` is `false` even though `equal(\"//A.com\", \"//a.com\")` is `true`. An application that makes a case-sensitive host decision on `fast-uri` output for a scheme-relative reference, for example a host allowlist or denylist that compares `parse(url).host` or uses `fast-uri.equal`, can be steered past the check with a percent-encoded uppercase octet. Because a hostname is case-insensitive in DNS and HTTP routing, the evading spelling reaches the same host the check meant to gate, so the effect is check evasion rather than reaching a different registrable host.\n\n### Patches\n\nUpgrade to `fast-uri` 4.1.5, 3.1.8, or 2.4.7.\n\n### Workarounds\n\nCompare hosts case-insensitively (lowercase the parsed host before any allowlist or denylist decision), or avoid making case-sensitive host decisions on scheme-relative input until upgrading.",
"id": "GHSA-hrr3-gc8f-f4qj",
"modified": "2026-09-29T23:54:26Z",
"published": "2026-09-29T23:54:25Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/fastify/fast-uri/security/advisories/GHSA-hrr3-gc8f-f4qj"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-86472"
},
{
"type": "WEB",
"url": "https://github.com/fastify/fast-uri/commit/5dabb86732aa2a7655e258719970e61e12b6b4d1"
},
{
"type": "WEB",
"url": "https://cna.openjsf.org/security-advisories.html"
},
{
"type": "PACKAGE",
"url": "https://github.com/fastify/fast-uri"
},
{
"type": "WEB",
"url": "https://github.com/fastify/fast-uri/releases/tag/v2.4.7"
},
{
"type": "WEB",
"url": "https://github.com/fastify/fast-uri/releases/tag/v3.1.8"
},
{
"type": "WEB",
"url": "https://github.com/fastify/fast-uri/releases/tag/v4.1.5"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N",
"type": "CVSS_V3"
}
],
"summary": "fast-uri vulnerable to inconsistent host case normalization via percent-encoded octets"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
Related by attack behaviour
Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.