Impact
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.
References
fg0x0 Reporter
mcollina Remediation developer
UlisesGascon Remediation reviewer
Impact
fast-urifolds the host to lowercase before it percent-decodes the host, so a percent-encoded uppercase unreserved octet such as%41decodes to a literalAthat 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 resultparse,normalize, andequaldisagree on the same host:parse("//%41.com").hostreturns"A.com"whileparse("//a.com").hostandparse("//A.com").hostreturn"a.com", andequal("//%41.com", "//a.com")isfalseeven thoughequal("//A.com", "//a.com")istrue. An application that makes a case-sensitive host decision onfast-urioutput for a scheme-relative reference, for example a host allowlist or denylist that comparesparse(url).hostor usesfast-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-uri4.1.5, 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.
References