CVE-2026-76172
Received Received - Intake

URI Parsing Flaw in fast-uri Leads to SSRF

Publication date: 2026-08-24

Last updated on: 2026-08-24

Assigner: openjs

Description
fast-uri is a URI parser for Node.js. During parsing it runs a legacy decoding pass over the scheme component and never re-escapes the result, and serialization writes the scheme back out verbatim, unlike the host component which is re-escaped. As a result an input whose scheme carries percent-encoded slashes parses as a scheme with no authority, so the parsed host and error are both undefined, yet resolving or normalizing that same input emits a network-path reference whose authority is attacker-chosen and re-parses to that host. An application that allowlists on the parsed host, or treats a reference with no authority as safe to resolve against its base, gets the opposite of what it checked, giving an off-site redirect, server-side request forgery, or address-policy bypass. The legacy decoder also expands non-standard escape forms, widening the issue past upstream filters, and control characters in the scheme can reach the output as raw carriage return and line feed. The affected versions are 2.3.1 up to but not including 2.4.5, 3.0.0 up to but not including 3.1.6, and 4.0.0 up to but not including 4.1.3. The issue is fixed in 2.4.5, 3.1.6, and 4.1.3, which reject a scheme that is not valid after decoding. Users should upgrade to a patched version.

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Meta Information

Published
2026-08-24
Last Modified
2026-08-24
Generated
2026-08-24
AI Q&A
2026-08-24
EPSS Evaluated
N/A
NVD
CVE-2026-76172
EUVD
EUVD-2026-64773

Affected Vendors & Products

Showing 3 associated CPEs
Vendor Product Version / Range
fast-uri fast-uri to 2.4.5 (exc)
fast-uri fast-uri to 3.1.6 (exc)
fast-uri fast-uri to 4.1.3 (exc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-177 The product does not properly handle when all or part of an input has been URL encoded.

Attack-Flow Graph

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Executive Summary

This vulnerability affects the fast-uri library in Node.js. It involves improper handling of percent-encoded characters in the URL scheme component. The library decodes these characters but fails to re-escape or validate them properly, allowing attackers to manipulate URLs. For example, inputs like %2f%2fevil.example:/pwn can bypass security checks, leading to off-site redirects or server-side request forgery.

Detection Guidance

Check if your system uses vulnerable versions of fast-uri (2.3.1 to 2.4.4, 3.0.0 to 3.1.5, or 4.0.0 to 4.1.2) by inspecting package.json or dependency files. Use commands like 'npm ls fast-uri' or 'yarn list fast-uri' to verify installed versions.

Impact Analysis

This vulnerability can allow attackers to bypass security controls such as host allowlists or redirect validations. Applications that normalize or resolve untrusted URLs before checks could be tricked into interacting with attacker-controlled hosts, leading to off-site redirects, server-side request forgery, or address-policy bypass.

Compliance Impact

This vulnerability could lead to unauthorized data exposure or server-side request forgery, potentially violating GDPR's data protection requirements or HIPAA's safeguards for protected health information. Applications relying on host allowlisting or safe URL resolution may inadvertently redirect to malicious hosts, compromising compliance.

Mitigation Strategies

Upgrade fast-uri to patched versions (2.4.5+, 3.1.6+, or 4.1.3+). If upgrading is not possible, implement strict URL parsing and validation in your application to reject URLs with percent-encoded characters in the scheme component.

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