SEPTEMBER 17, 2026
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Case File

CVE-2026-86818

MEDIUM · CVSS 4.8 EPSS 0.16% Public Exploit

Source: NVD + CISA KEV + EPSS · Published 2026-09-15 · Last synced 2026-09-17

CyberRota Analysis

AI-Generated

The vulnerability affects the fast-uri library used in Node.js applications, specifically in versions 4.1.3 and 4.1.4, where the mailto scheme parser mishandles percent-encoded reserved field names, potentially allowing attackers to manipulate recipient, subject, and body fields during URI serialization. This could lead to unauthorized email recipients and content being sent without proper validation. Developers using fast-uri, particularly those implementing mailto functionality, should prioritize upgrading to version 4.1.5 or later to mitigate this risk.

Public Exploit Signal

A public exploit, PoC, GitHub repository or Metasploit reference was detected for this CVE.

GitHub PoC Links

Note: these links are listed for security research and verification purposes only.

CVE
CVE-2026-86818
Severity
MEDIUM
CVSS
4.8
EPSS
0.16%

Original NVD 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.