fast-uri up to 4.1.4 Mailto Scheme Parser input validation

Summaryinfo

A vulnerability has been found in fast-uri up to 4.1.4 and classified as critical. Affected by this issue is some unknown functionality of the component Mailto Scheme Parser. The manipulation leads to input validation. This vulnerability is documented as CVE-2026-86818. The attack can be initiated remotely. There is not any exploit available. The affected component should be upgraded.

Detailsinfo

A vulnerability was found in fast-uri up to 4.1.4 and classified as critical. This issue affects an unknown part of the component Mailto Scheme Parser. The manipulation with an unknown input leads to a input validation vulnerability. Using CWE to declare the problem leads to CWE-20. The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-86818 since 09/08/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Neither technical details nor an exploit are publicly available.

Upgrading to version 4.1.5 eliminates this vulnerability.

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Productinfo

Name

• fast-uri

Version

• 4.1.0
• 4.1.1
• 4.1.2
• 4.1.3
• 4.1.4

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