CVE-2025-50578: Docker-heimdall Header Injection Vulnerability

CVE-2025-50578 Overview

LinuxServer.io Heimdall 2.6.3-ls307 contains a critical vulnerability in how it handles user-supplied HTTP headers, specifically X-Forwarded-Host and Referer. An unauthenticated remote attacker can manipulate these headers to perform Host Header Injection and Open Redirect attacks. This allows the loading of external resources from attacker-controlled domains and unintended redirection of users, potentially enabling phishing, UI redress, and session theft. The vulnerability exists due to insufficient validation and trust of untrusted input, affecting the integrity and trustworthiness of the application.

Critical Impact

Unauthenticated attackers can exploit this vulnerability remotely to hijack user sessions, redirect users to malicious sites for phishing attacks, and load external resources from attacker-controlled domains, compromising the integrity of the application and user trust.

Affected Products

• LinuxServer docker-heimdall version 2.6.3-ls307

Discovery Timeline

• 2025-07-30 - CVE-2025-50578 published to NVD
• 2025-08-25 - Last updated in NVD database

Technical Details for CVE-2025-50578

Vulnerability Analysis

This vulnerability is classified as CWE-20 (Improper Input Validation), where the Heimdall application fails to properly validate and sanitize user-controlled HTTP headers before using them in security-sensitive operations. The X-Forwarded-Host and Referer headers are commonly used in web applications for routing, redirection logic, and resource loading decisions. When these headers are trusted without validation, attackers can inject malicious values that alter the application's behavior.

The attack requires no authentication and can be executed remotely over the network. An attacker can craft malicious HTTP requests with spoofed header values to manipulate how the application constructs URLs, loads resources, or redirects users. This can lead to multiple attack scenarios including phishing campaigns where users are redirected to attacker-controlled lookalike sites, session theft through manipulated authentication flows, and UI redress attacks where external malicious content is loaded within the legitimate application context.

Root Cause

The root cause of this vulnerability lies in the application's improper handling of the X-Forwarded-Host and Referer HTTP headers. These headers are user-controllable and should never be trusted for security-critical operations without proper validation. The Heimdall application uses these header values directly in URL construction and redirection logic, assuming they contain legitimate values from trusted proxies or the original request. This design flaw allows attackers to inject arbitrary hostnames and URLs that the application then uses to redirect users or load external resources.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker sends HTTP requests to the vulnerable Heimdall instance with manipulated X-Forwarded-Host or Referer headers containing attacker-controlled domain values. The application processes these headers and incorporates the malicious values into URL generation, causing two primary attack scenarios:

Host Header Injection: By manipulating the X-Forwarded-Host header, attackers can cause the application to generate URLs pointing to malicious domains. This can result in external resource loading from attacker-controlled servers, potentially injecting malicious scripts or content.

Open Redirect: By crafting specific header values, attackers can redirect users to arbitrary external URLs. This is particularly dangerous in authentication flows where users may be redirected to phishing sites that mimic legitimate login pages, enabling credential theft.

For detailed technical analysis and exploitation examples, refer to the Medium CVE-2025-50578 Exploit Analysis and GitHub Issue #1451.

Detection Methods for CVE-2025-50578

Indicators of Compromise

• Unusual or suspicious values in X-Forwarded-Host headers that don't match expected proxy configurations
• Unexpected Referer header values containing external domains in request logs
• User reports of being redirected to unfamiliar or suspicious websites after accessing Heimdall
• Network traffic showing connections to unexpected external domains originating from the Heimdall server

Detection Strategies

• Monitor web server access logs for requests containing suspicious X-Forwarded-Host header values that differ from legitimate proxy addresses
• Implement web application firewall (WAF) rules to detect and block requests with anomalous or unexpected host header values
• Deploy network monitoring to identify outbound connections to unknown domains from the Heimdall application server
• Configure SIEM alerts for patterns consistent with open redirect exploitation attempts

Monitoring Recommendations

• Enable detailed HTTP header logging on reverse proxies and web servers fronting Heimdall instances
• Implement alerting on sudden increases in redirect responses or unusual redirect destination patterns
• Monitor for phishing reports that reference your Heimdall deployment as a redirect source
• Review web application logs regularly for header injection attempt patterns

How to Mitigate CVE-2025-50578

Immediate Actions Required

• Update LinuxServer Heimdall to the latest available version that addresses this vulnerability
• Implement strict allowlist validation for X-Forwarded-Host header values at the reverse proxy or load balancer level
• Configure your web server or reverse proxy to override or strip untrusted X-Forwarded-Host headers from incoming requests
• Consider restricting network access to Heimdall deployments to trusted internal networks only

Patch Information

Organizations should monitor the GitHub Heimdall Project for official security updates and patched releases. Track the issue at GitHub Issue #1451 for the latest remediation guidance from the maintainers.

Workarounds

• Deploy a reverse proxy (such as nginx or Apache) in front of Heimdall and configure it to set explicit Host and X-Forwarded-Host headers, ignoring client-supplied values
• Implement network-level access controls to limit exposure of the Heimdall instance to trusted networks only
• Use a Web Application Firewall (WAF) with rules to detect and block header injection attempts
• Educate users about the risks of clicking links that may lead through the Heimdall application to external sites

# Example nginx configuration to mitigate host header injection
# Place this in your nginx server block for Heimdall

# Override X-Forwarded-Host with the actual host
proxy_set_header X-Forwarded-Host $host;
proxy_set_header Host $host;

# Optionally restrict to specific trusted hosts
# if ($host !~* ^(heimdall\.yourdomain\.com)$ ) {
#     return 444;
# }