CVE-2025-48865: Manipulation of X-Forwarded Headers in Fabio Prior to Version 1.6.6

Overview

The Common Vulnerabilities and Exposures (CVE) system has recently identified a high-severity security vulnerability, CVE-2025-48865, that can potentially compromise systems or lead to data leakage. This vulnerability affects applications deployed using Fabio, an HTTP(S) and TCP router managed by consul. The vulnerability, which arises from the way Fabio processes hop-by-hop headers, is especially concerning due to its high CVSS severity score of 9.1 and the wide usage of Fabio as a routing mechanism in various applications.

Vulnerability Summary

CVE ID: CVE-2025-48865
Severity: High (CVSS: 9.1)
Attack Vector: Network
Privileges Required: None
User Interaction: None
Impact: Potential system compromise or data leakage

Affected Products

Product | Affected Versions

Fabio | Prior to 1.6.6

How the Exploit Works

The exploit takes advantage of a weakness in how Fabio processes hop-by-hop headers. Fabio adds HTTP headers like X-Forwarded-Host and X-Forwarded-Port when routing requests to backend applications. These headers are trusted by the receiving application. However, due to the vulnerability, an attacker can remove or modify these headers, leading to potential security vulnerabilities. This attack leverages the behavior that headers can be defined as hop-by-hop via the HTTP Connection header.

Conceptual Example Code

Here’s a conceptual example of how the vulnerability might be exploited via an HTTP request:

POST /vulnerable/endpoint HTTP/1.1
Host: target.example.com
Connection: close, X-Forwarded-Host
X-Forwarded-Host: malicious.com
Content-Type: application/json
{ "data": "..." }

In this example, an attacker manipulates the ‘X-Forwarded-Host’ header to redirect the traffic to a malicious server, potentially compromising the system or leading to data leakage.

Mitigation

Users of Fabio are strongly recommended to update to version 1.6.6 or later, which contains a patch for this vulnerability. If updating is not immediately possible, a temporary mitigation can be to use a Web Application Firewall (WAF) or Intrusion Detection System (IDS) to monitor and block any suspicious activity related to the manipulation of X-Forwarded headers.