CVE-2026-84394

fast-uri is a dependency-free RFC 3986 URI parser for Node.js, used by Fastify and ajv, that added a mailto scheme parser in version 4.1.3. In versions 4.1.3 and 4.1.4, the mailto parser compares each query field name to the reserved names to, subject, and body while the name is still percent-encoded, and decodes it only when storing it as a generic header, so a percent-encoded spelling of a reserved field name is not recognized as that field at parse time but is re-emitted as the literal field name when the parsed URI is serialized. An application that validates, logs, or displays the recipient list from the first parse and then serializes the URI and sends it can silently gain an attacker-chosen recipient, and the subject and body fields can be smuggled across the same roundtrip. The issue is fixed in fast-uri 4.1.5, and users should upgrade to 4.1.5 or later. As a workaround, do not act on a mailto URI that fast-uri has re-serialized without first decoding and re-validating its recipient, subject, and body fields.

fast-uri is a dependency-free RFC 3986 URI parser for Node.js, used by Fastify and ajv. In versions before 2.4.7, from 3.0.0 through 3.1.7, and from 4.0.0 through 4.1.4, fast-uri folds the host to lowercase before it percent-decodes the host, so a percent-encoded uppercase octet such as %41 decodes to a literal A that is never folded. For a scheme-relative reference such as //host there is no scheme, so the host canonicalization that would normally repair this does not run, and parse, normalize, and equal then disagree on the same host. An application that makes a case-sensitive host decision on fast-uri output, for example a host allowlist or denylist that compares the parsed host or uses equal, can be steered past the check with a percent-encoded uppercase octet, and because hostnames are case-insensitive in DNS and HTTP the evading spelling still reaches the host the check meant to gate. The issue is fixed in fast-uri 2.4.7, 3.1.8, and 4.1.5, and users should upgrade to one of those versions or later. As a workaround, compare hosts case-insensitively by lowercasing the parsed host before any allowlist or denylist decision.

fast-uri serializes the port component of a URI without validation. When recomposing the authority, the port is concatenated verbatim, so a non-digit port can inject authority delimiters, pointing the authority at an attacker-controlled host. This affects versions before 2.4.6, from 3.0.0 before 3.1.7, and from 4.0.0 before 4.1.4.

The fast-uri URI parser for Node.js mishandles schemes with percent-encoded slashes, allowing an attacker to manipulate the host and bypass allowlists. Affected versions are 2.3.1 up to 2.4.4, 3.0.0 up to 3.1.5, and 4.0.0 up to 4.1.2.

The fast-uri URI parser for Node.js incorrectly validates bracketed IPv6 literals, allowing a malicious host to be transformed into a different IPv6 destination, such as loopback or private address. Affected versions are 2.3.1 up to 2.4.4, 3.0.0 up to 3.1.5, and 4.0.0 up to 4.1.2.

The fast-uri URI parser for Node.js does not canonicalize the host in scheme-relative references, leading to inconsistencies between parse, resolve, normalize, and equal functions. Affected versions are 2.4.2 up to 2.4.4, 3.1.3 up to 3.1.5, and 4.0.1 up to 4.1.2.

The fast-uri URI parser for Node.js decodes percent escapes in the hostname twice, potentially changing the destination to loopback or another internal address. Affected versions are 2.4.1 up to 2.4.4, 3.1.2 up to 3.1.5, and 4.0.0 up to 4.1.2.

The fast-uri library before versions 4.1.2, 3.1.5, and 2.4.4 incorrectly parses URIs when a backslash is used instead of a double forward slash. Node.js treats backslash as interchangeable with forward slash, leading to differences in host parsing. Applications using fast-uri for host-based policy enforcement can be steered to an unintended host.

Vulnerability in fast-uri library (versions 2.3.1-4.1.0, 3.x up to 3.1.3, 2.x up to 2.4.2). Backslash is not treated as an authority delimiter, while Node's parser normalizes it to slash. Parsing discrepancy can bypass host-based policies (allowlists, SSRF filtering).

fast-uri versions 2.3.1 through 3.1.2 and 4.0.0 fail to canonicalize Unicode (IDN) hostnames for HTTP-family URLs. This causes discrepancies between fast-uri and Node's native URL parser, potentially bypassing host-based policies.