Encoding Error in fast-uri - CVE-2026-86818
Published: September 16, 2026
Vulnerability identifier: #VU150186
CSH Severity: Medium
CVSS v4: 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]
CVE-ID: CVE-2026-86818
CWE-ID: CWE-172
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
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.
Affected software
fast-uri
How to mitigate CVE-2026-86818
Install security update from vendor's website.
fast-uri - update to 4.1.5