Executive Summary

This vulnerability exists in the fast-uri library versions up to 3.1.0. The issue is that the library decodes percent-encoded path separators and dot segments before applying dot-segment removal in its normalize() and equal() functions. Because encoded path data is treated like actual slashes and parent-directory references, different URIs that should be distinct can end up collapsing into the same normalized path.

As a result, applications that rely on normalizing or comparing attacker-controlled URLs to enforce path-based policies can be bypassed. An attacker can craft a path that appears to be confined under an allowed prefix but actually normalizes to a different location, potentially circumventing security controls.

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Impact Analysis

The vulnerability can allow attackers to bypass path-based security policies by exploiting the way URLs are normalized. This means that an attacker could access resources or perform actions that should be restricted by the application’s URL path controls.

Since the CVSS score indicates a high impact on integrity (I:H) with no impact on confidentiality or availability, the main risk is unauthorized modification or manipulation of data or application behavior through crafted URLs.

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Mitigation Strategies

To mitigate this vulnerability, update fast-uri to version 3.1.1 or later, as versions up to 3.1.0 are affected.

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