# CVE-2026-86818: fast-uri mailto header injection via percent-encoded field-name desynchronization

Source: https://junglewise.ai/threats/cve-2026-86818-fast-uri-mailto-header-injection-via-percent-encoded-field-name  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-15. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-86818 · Severity: medium · CVSS 4.8 · Published 2026-09-15

fast-uri is a URI parser library used by Node.js projects including Fastify and ajv. A flaw in its mailto scheme parser allows attackers to inject unintended email recipients, subjects, and body content by using percent-encoded field names that bypass validation checks but materialize after the URI is serialized. An application that validates an email's recipients before sending could silently add attacker-controlled recipients without detection.

Technologies: [fast-uri (npm)](https://junglewise.ai/threats/technologies/npm-fast-uri). Vendors: [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2026-86818 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-86818 being exploited in the wild.

**How severe is CVE-2026-86818?** Medium severity, with a CVSS score of 4.8, published 15 September 2026.

**Which products and versions are affected?** fast-uri (npm) >= 4.1.3, < 4.1.5, SEMVER:introduced=4.1.3,fixed=4.1.5.

**Is there a fix for CVE-2026-86818?** Yes. fast-uri (npm): upgrade to 4.1.5 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-86818 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-86818?** Check whether you run fast-uri (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- fast-uri (npm): upgrade to 4.1.5 or later

  ```sh
  npm install fast-uri@4.1.5
  ```


## Technical details

The vulnerability is an encoding/decoding desynchronization (CWE-172, CWE-436) in fast-uri versions 4.1.3 and 4.1.4's mailto parser. During parsing, the parser compares query field names (to, subject, body) while still percent-encoded, but only decodes them when storing generic headers. On serialization, decoded names are re-emitted as literals, so a percent-encoded reserved name like %74o (encoding "to") is not recognized as a recipient during parse() but becomes a literal "to=" field after serialize(). An attacker can inject multiple recipients or modify subject/body by percent-encoding these reserved names. The attack requires an application to parse an untrusted mailto URI, make a validation/allowlist decision on parse().to, then serialize and forward the result to a mail client or send path. The fix is available in fast-uri 4.1.5.

## Affected products

- OpenJS Foundation fast-uri 4.1.3 to 4.1.4

## Identifiers

- CVE-2026-86818
- GHSA-jvvf-x445-j334

## Timeline

- 2026-09-15: disclosed: CVE-2026-86818 and GHSA-jvvf-x445-j334 published
- 2026-09-15: patched: fast-uri 4.1.5 released with fix

## References

- [https://cna.openjsf.org/security-advisories.html](https://cna.openjsf.org/security-advisories.html)
- [https://github.com/fastify/fast-uri/security/advisories/GHSA-jvvf-x445-j334](https://github.com/fastify/fast-uri/security/advisories/GHSA-jvvf-x445-j334)

## Related threats

- [fast-uri vulnerable to scheme injection via an unescaped first-segment colon in suffix references](https://junglewise.ai/threats/cve-2026-110725-fast-uri-is-a-dependency-free-rfc-3986-uri-parser-and-serializer)
- [fast-uri URI scheme spoofing via percent-encoded scheme](https://junglewise.ai/threats/cve-2026-110306-fast-uri-is-a-dependency-free-rfc-3986-uri-parser-and-serializer)
- [fast-uri host confusion via unchecked WHATWG canonicalization](https://junglewise.ai/threats/cve-2026-110180-fast-uri-is-a-dependency-free-rfc-3986-uri-parser-and-serializer)
- [fast-uri vulnerable to improper path equivalence in relative reference normalization](https://junglewise.ai/threats/cve-2026-110095-fast-uri-is-a-dependency-free-rfc-3986-uri-parser-and-serializer)
- [fast-uri inconsistent host case normalization via percent-encoded octets](https://junglewise.ai/threats/cve-2026-86472-fast-uri-inconsistent-host-case-normalization-via-percent-encoded)