CVE-2026-86818 fast-uri vulnerable to mailto header injection via percent-encoded field-name desynchronization
Attackers can inject malicious recipients or headers into mailto URIs by exploiting percent-encoding desynchronization in the fast-uri parser.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-86818?
CVE-2026-86818 is a medium-severity vulnerability in fast-uri, published on 15 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Which versions are affected by CVE-2026-86818?
2 open source packages ship an affected version, including fast-uri.
| Package | Affected | Fixed in |
|---|---|---|
fast-uri npm | From 4.1.3 | 4.1.5 |
fast-uri npm | From 4.1.3 | 4.1.5 |
How do I fix CVE-2026-86818?
Upgrade fast-uri to 4.1.5 or later.
Is CVE-2026-86818 being exploited?
There is no known exploitation of CVE-2026-86818 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-86818, GHSA-jvvf-x445-j334, MINI-2pc9-58jr-mjmf, UBUNTU-CVE-2026-86818.