CVE-2025-48865 in Fabio
Summary
by MITRE • 05/30/2025
Fabio is an HTTP(S) and TCP router for deploying applications managed by consul. Prior to version 1.6.6, Fabio allows clients to remove X-Forwarded headers (except X-Forwarded-For) due to a vulnerability in how it processes hop-by-hop headers. Fabio adds HTTP headers like X-Forwarded-Host and X-Forwarded-Port when routing requests to backend applications. Since the receiving application should trust these headers, allowing HTTP clients to remove or modify them creates potential security vulnerabilities. Some of these custom headers can be removed and, in certain cases, manipulated. The attack relies on the behavior that headers can be defined as hop-by-hop via the HTTP Connection header. This issue has been patched in version 1.6.6.
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Analysis
by VulDB Data Team • 05/30/2025
The vulnerability identified as CVE-2025-48865 affects Fabio, an HTTP(S) and TCP router designed for deployment management within Consul environments. This router serves as a critical component in modern microservices architectures, facilitating traffic routing between clients and backend services while maintaining various HTTP headers that applications rely upon for proper functionality and security. The flaw exists in Fabio's handling of hop-by-hop headers, specifically in how it processes the HTTP Connection header to determine which headers should be removed during request forwarding. Prior to version 1.6.6, Fabio's implementation contained a significant security gap that allowed malicious clients to manipulate or remove specific HTTP headers that should remain intact for proper application operation and security enforcement.
The technical flaw stems from Fabio's improper interpretation of the HTTP specification regarding hop-by-hop headers, which are defined as headers that should not be forwarded by proxies or intermediaries in the communication chain. According to RFC 2616 and subsequent HTTP specifications, the Connection header explicitly lists which headers should be considered hop-by-hop, but Fabio's implementation incorrectly allowed clients to remove or modify certain X-Forwarded headers that are critical for proper application behavior. The vulnerability specifically impacts headers such as X-Forwarded-Host and X-Forwarded-Port that Fabio automatically adds when routing requests to backend applications, while leaving other X-Forwarded headers vulnerable to manipulation. This misconfiguration creates a situation where attackers can strip or modify these headers, potentially bypassing security controls or causing applications to misinterpret client information.
The operational impact of this vulnerability extends beyond simple header manipulation, creating potential security risks that could compromise application integrity and user privacy. When backend applications trust the X-Forwarded headers that Fabio provides, malicious actors can exploit this vulnerability to alter the perceived origin of requests, potentially bypassing authentication mechanisms, access controls, or logging systems that depend on accurate client information. The vulnerability particularly affects applications that rely on the X-Forwarded-Host header for proper URL generation or the X-Forwarded-Port header for secure connection handling. According to CWE-284, this represents an improper access control vulnerability where the router fails to properly validate or maintain critical header information. The attack vector is facilitated through the HTTP Connection header mechanism, which is defined in the HTTP/1.1 specification and commonly used by proxies to indicate which headers should be removed during forwarding operations.
This vulnerability aligns with several ATT&CK framework techniques including T1071.004 for application layer protocol usage and T1566 for credential access through manipulation of network traffic. The security implications are particularly concerning in environments where Fabio serves as a gateway between public networks and internal services, as attackers could potentially exploit this to gain unauthorized access to backend applications or manipulate session information. The vulnerability also relates to CWE-345, which addresses insufficient validation of data integrity, as the modified headers could lead to incorrect processing of requests by backend applications. Organizations using Fabio in production environments should consider this vulnerability as a potential entry point for more sophisticated attacks, especially when combined with other weaknesses in their network infrastructure or application security controls. The patch in version 1.6.6 addresses this by implementing proper header validation and ensuring that critical X-Forwarded headers are maintained regardless of client modifications to the Connection header or other hop-by-hop header specifications.