CVE-2026-86818: fast-uri vulnerable to mailto header injection via percent-encoded field-name desynchronization
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.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
fast-urito a version that resolves this vulnerability.Fixed in 4.1.5 - Compensating control
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.
Event History
Frequently Asked Questions
Who is exposed to this issue?
Applications using fast-uri 4.1.3 or 4.1.4 are exposed when they process attacker-controlled mailto URIs and later act on a URI after fast-uri has serialized it. Risk is especially relevant if the application validates, logs, or displays recipients from an initial parse before sending the re-serialized URI.
What does an attacker need to exploit this?
An attacker needs to supply a crafted mailto URI containing percent-encoded spellings of reserved query field names. This can cause recipient, subject, or body values to be interpreted differently after a parse-and-serialize roundtrip.
What should be done if upgrading is not immediately possible?
Do not act on a mailto URI re-serialized by fast-uri unless its recipient, subject, and body fields have first been decoded and re-validated. This prevents relying on the initial parse result when making sending decisions.
How can I determine whether my application is affected?
Check whether the deployed fast-uri version is 4.1.3 or 4.1.4 and whether the application accepts or handles mailto URIs. Then identify flows that parse a mailto URI, inspect or validate fields, serialize it, and use the serialized result to send email.