CVE-2026-76172 in fast-uri
Summary
by MITRE • 08/24/2026
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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Analysis
by VulDB Data Team • 08/24/2026
The fast-uri library for Node.js contains a critical logic flaw in its URI parsing and serialization mechanisms that allows an attacker to bypass security controls relying on host validation or authority checks. The vulnerability stems from inconsistent handling of the scheme component during decoding and re-serialization. Specifically, the parser executes a legacy decoding pass over the scheme string but fails to re-escape the result before storing it internally. In contrast, other components like the host are properly escaped upon serialization. This asymmetry creates a discrepancy between how the URI is parsed for internal logic and how it is eventually serialized or normalized for output operations such as resolution against a base URL.
When an input contains percent-encoded slashes within the scheme component, the legacy decoder interprets these encoded characters during the initial parsing phase. Because the decoded value is not re-escaped, the parser may incorrectly determine that the URI lacks an authority section, resulting in undefined host and error properties internally. However, when this same malformed reference undergoes normalization or resolution against a base URL, the serialization process writes out the scheme verbatim without proper escaping of special characters like slashes. This causes the output to be interpreted as a network-path reference with an attacker-controlled authority rather than a relative path with no authority. Consequently, applications that whitelist specific hosts based on the parsed host value will inadvertently allow traffic destined for malicious domains because the internal check sees one thing while the actual resolution performs another.
This discrepancy enables severe security impacts including off-site redirects, server-side request forgery attacks, and address-policy bypasses. An attacker can craft a URI where the scheme contains encoded slashes that decode to form part of an authority section during serialization, effectively tricking downstream systems into treating a malicious external host as if it were safe or internal. Furthermore, the legacy decoder expands non-standard escape forms which widens the attack surface beyond what upstream filters might anticipate. Additionally, control characters such as carriage returns and line feeds present in the scheme can reach the output raw, potentially leading to HTTP response splitting or header injection vulnerabilities depending on how the resulting URI is processed by web servers or proxies.
The vulnerability affects fast-uri versions 2.3.1 through 2.4.4, 3.0.0 through 3.1.5, and 4.0.0 through 4.1.2. These versions fail to validate that the scheme remains valid after decoding before proceeding with further processing steps. The issue is resolved in patched releases including version 2.4.5, 3.1.6, and 4.1.3. In these fixed versions, the library rejects any URI where the decoded scheme does not conform to strict validity rules, thereby preventing the mismatch between parsing and serialization states that enables exploitation.
From a classification perspective, this vulnerability aligns with CWE-20 Improper Input Validation as it involves failing to properly validate input data before processing, leading to inconsistent state interpretation. It also relates to CWE-798 Use of Hard-coded Credentials if the bypass allows access to restricted resources, and more broadly to CWE-643 Improper Mitigation for XPATH Injection or similar injection flaws depending on context, though primarily it is a logic error in data handling. In terms of MITRE ATT&CK techniques, this flaw facilitates lateral movement via Server-Side Request Forgery (T1105) and potentially privilege escalation if the bypassed policy grants access to internal services. It also supports reconnaissance by allowing attackers to probe internal networks through SSRF vectors that evade host-based allowlists.
To mitigate this risk, organizations using fast-uri must upgrade immediately to version 2.4.5 or later for legacy branches, or versions 3.1.6 and 4.1.3+ for current releases. Developers should also implement defense-in-depth strategies by validating URIs at multiple layers of the application stack rather than relying solely on library-level parsing results. Implementing strict allowlists that verify both the parsed host and the final resolved URL can help detect discrepancies indicative of this type of bypass attack. Regular security audits focusing on URI handling logic are recommended to identify similar inconsistencies in other libraries or custom implementations.