SB2026091645 - Multiple vulnerabilities in fast-uri

Description

This security bulletin contains information about 2 vulnerabilities.

1) Encoding Error (CVE-ID: CVE-2026-86818)

CWE-ID: CWE-172 - Encoding Error

CVSSv4: 6.3 [CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N]

The vulnerability allows a remote attacker to inject attacker-controlled mail recipients, subjects, or message bodies.

The vulnerability exists due to an encoding error in the fast-uri mailto scheme parser when parsing percent-encoded mailto query field names. A remote attacker can supply a crafted mailto URI containing percent-encoded reserved field names to inject attacker-controlled mail recipients, subjects, or message bodies.

Exploitation requires an application to make a display, allowlist, or logging decision based on the initial parsed recipient list and then serialize the URI before passing it to a mail client or outbound send path.

2) Improper Handling of Case Sensitivity (CVE-ID: CVE-2026-86472)

CWE-ID: CWE-178 - Improper Handling of Case Sensitivity

CVSSv4: 6.3 [CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N]

The vulnerability allows a remote attacker to bypass host allowlist or denylist checks.

The vulnerability exists due to improper handling of case sensitivity in the fast-uri host parsing, normalization, and equality functions when processing scheme-relative references containing percent-encoded uppercase unreserved host octets. A remote attacker can supply a crafted scheme-relative URL to bypass host allowlist or denylist checks.

The evading hostname resolves and routes to the same host intended to be restricted.

Remediation

Install update from vendor's website.

References

• https://github.com/fastify/fast-uri/security/advisories/GHSA-jvvf-x445-j334
• https://github.com/fastify/fast-uri/security/advisories/GHSA-hrr3-gc8f-f4qj