CVE-2026-84292 in fast-uri
Summary
by MITRE • 09/02/2026
fast-uri serializes the port component of a URI without validating it. When recomposing the authority, the userinfo and host components are escaped but the port is concatenated verbatim, so a port value that is not a sequence of digits can inject authority delimiters, demoting the intended host to userinfo and pointing the authority at an attacker-controlled host. Both fast-uri and Node's URL read the result back as the attacker's host with no error, so re-validating the built URI does not catch it. This affects applications that build URIs from parts and assign untrusted data to the port component through the serialize, normalize, or equal functions in their object forms. The issue affects fast-uri versions before 2.4.6, from 3.0.0 before 3.1.7, and from 4.0.0 before 4.1.4. It is fixed in 2.4.6, 3.1.7, and 4.1.4, where recomposeAuthority rejects any port that is not a digit sequence per RFC 3986.
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Analysis
by VulDB Data Team • 09/03/2026
The vulnerability identified as fast-uri CVE-2024-xxxx represents a critical logic flaw in the URI serialization process within the fast-uri library for Node.js environments. This issue stems from an inconsistency in how different components of a Uniform Resource Identifier are processed during reconstruction. Specifically, while the userinfo and host segments undergo proper escaping to prevent injection attacks, the port component is concatenated verbatim without any validation or sanitization. According to RFC 3986, which defines the syntax for URIs, the port must consist exclusively of digits. The failure to enforce this constraint allows an attacker who controls the input data passed to the serialize, normalize, or equal functions in their object forms to inject arbitrary characters into the authority section of the URI.
The operational impact of this flaw is severe because it enables a form of host header injection and URL parsing bypass that can lead to server-side request forgery or open redirect vulnerabilities depending on how the application consumes the generated URIs. By injecting specific non-digit sequences, an attacker can manipulate the structure of the authority component such that standard delimiters are introduced prematurely. This effectively demotes the intended legitimate host into the userinfo segment and redirects the actual authority portion to a domain controlled by the attacker. Because both fast-uri and Node.js built-in URL parser read back this malformed URI without raising errors, subsequent validation steps within an application may fail to detect the manipulation. The application perceives the resulting object as valid, thereby trusting maliciously crafted URIs that point to external or internal resources under the attacker's control.
This vulnerability specifically affects applications that dynamically construct URIs from untrusted inputs and assign these values directly to the port property of URI objects before serialization. It impacts fast-uri versions prior to 2.4.6, as well as version ranges starting from 3.0.0 up to but not including 3.1.7, and similarly for major version four where any release below 4.1.4 is susceptible. The core technical failure lies in the recomposeAuthority function which lacks strict type checking or regex validation against numeric-only patterns before appending the port value to the host string. This oversight creates a pathway for attackers to bypass security controls that rely on hostname verification, potentially leading to data exfiltration, session hijacking if cookies are domain-specific, or access to internal network services via SSRF vectors.
From a classification perspective, this issue aligns with CWE-20 Improper Input Validation and CWE-79 Cross-site Scripting in the context of URL parsing logic, though it is more accurately described as an Open Redirect vulnerability precursor due to its ability to redirect trust boundaries. In terms of MITRE ATT&CK mapping, this technique relates to T1566 Phishing where malicious links are crafted to appear legitimate or bypass filtering mechanisms, and potentially T1078 Valid Accounts if used in conjunction with credential theft via SSRF. The vulnerability exploits the assumption that standard library parsers will reject malformed URIs, which is not always true across different implementations like Node.js URL vs fast-uri internal parsing logic.
To mitigate this risk, organizations must immediately upgrade to patched versions of fast-uri: 2.4.6 for legacy systems using major version two, 3.1.7 for those on the three series, and 4.1.4 or later for modern deployments. These updates introduce strict validation in the recomposeAuthority function ensuring that only digit sequences are accepted as port numbers, thereby preventing delimiter injection. For applications unable to upgrade immediately due to dependency constraints, a temporary workaround involves manually validating any untrusted input assigned to URI components before passing them to fast-uri functions. Developers should implement regex checks to ensure port values match /^\d+$/ and reject or sanitize inputs that do not conform to this pattern. Additionally, implementing strict allowlists for allowed hosts in SSRF mitigation strategies can provide a secondary layer of defense against the resulting malicious redirects.