CVE-2025-46814 is an HTTP header injection vulnerability found in the FastAPI Guard security library, versions prior to 2.0.0. FastAPI Guard is middleware designed to enhance FastAPI applications by controlling IP addresses, logging requests, and detecting penetration attempts. The vulnerability arises from improper neutralization of special elements in the X-Forwarded-For HTTP header, which is commonly used to identify the originating IP address of a client connecting through a proxy or load balancer. An attacker can manipulate this header to inject arbitrary IP addresses into the request context. This injection can lead to bypassing IP-based access controls, misleading logging and monitoring systems, and impersonating trusted clients. The vulnerability is categorized under CWE-74, indicating improper neutralization of special elements in output used by downstream components, which in this case is the IP address processing logic. Exploitation requires the attacker to send a crafted HTTP request with a malicious X-Forwarded-For header. The CVSS v3.1 score is 3.4 (low severity), reflecting that exploitation requires user interaction and high attack complexity, with limited impact on confidentiality and no impact on integrity or availability. No known exploits are currently reported in the wild. The recommended remediation is to upgrade FastAPI Guard to version 2.0.0 or later, where the vulnerability has been fixed by properly sanitizing and validating the X-Forwarded-For header input before use.

For European organizations using FastAPI Guard in their web applications, this vulnerability could allow attackers to bypass IP-based restrictions, which are often used to protect sensitive endpoints or administrative interfaces. This could lead to unauthorized access or evasion of security controls. Additionally, misleading IP information in logs could hamper incident response and forensic investigations, potentially delaying detection of malicious activity. While the direct impact on confidentiality, integrity, and availability is limited, the ability to impersonate trusted clients or evade detection could facilitate further attacks or data breaches. Organizations relying heavily on IP-based authentication or geo-restrictions are particularly at risk. The impact is more pronounced in sectors with strict regulatory requirements for access control and logging, such as finance, healthcare, and government services within Europe.

European organizations should immediately upgrade FastAPI Guard to version 2.0.0 or later to ensure the vulnerability is patched. Beyond upgrading, organizations should implement additional validation and sanitization of the X-Forwarded-For header at the application level, including rejecting or logging suspicious header formats. Where possible, avoid relying solely on client-supplied headers for critical security decisions; instead, use trusted network infrastructure to determine client IPs. Implement strict logging policies that correlate multiple data points (e.g., connection IP, authentication tokens) to detect anomalies. Employ Web Application Firewalls (WAFs) configured to detect and block malformed or suspicious X-Forwarded-For headers. Regularly audit and review access control policies that depend on IP addresses to ensure they are robust against header manipulation. Finally, conduct security awareness training for developers and operations teams about the risks of trusting client-supplied headers.