CVE-2019-3877 in mod_auth_mellon
Summary
by MITRE
A vulnerability was found in mod_auth_mellon before v0.14.2. An open redirect in the logout URL allows requests with backslashes to pass through by assuming that it is a relative URL, while the browsers silently convert backslash characters into forward slashes treating them as an absolute URL. This mismatch allows an attacker to bypass the redirect URL validation logic in apr_uri_parse function.
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Analysis
by VulDB Data Team • 05/19/2025
The vulnerability identified as CVE-2019-3877 resides within the mod_auth_mellon authentication module for Apache HTTP Server, specifically affecting versions prior to v0.14.2. This authentication module facilitates SAML-based single sign-on functionality and operates by handling authentication requests and responses between the web server and SAML identity providers. The flaw manifests in the logout URL processing mechanism where the application fails to properly validate redirect URLs, creating a pathway for malicious actors to manipulate the authentication flow. The issue stems from how the module handles URL parsing and validation, particularly when backslash characters are present in the redirect URI.
The technical implementation of this vulnerability exploits a fundamental inconsistency in URL parsing behavior between the mod_auth_mellon module and web browsers. When a logout request contains a URL with backslash characters, the module's apr_uri_parse function processes these backslashes as relative path indicators, treating them as legitimate relative URLs. However, browsers automatically normalize backslashes to forward slashes, effectively converting the malicious URL into an absolute path that bypasses the intended validation checks. This mismatch between the module's interpretation and browser behavior creates a window for open redirect exploitation. The vulnerability specifically targets the URL validation logic within the authentication module's logout functionality, where redirect URLs are supposed to be restricted to prevent unauthorized redirection.
The operational impact of this vulnerability extends beyond simple open redirect scenarios, potentially enabling attackers to construct malicious logout URLs that redirect users to phishing sites or malicious domains. An attacker could craft a logout request with a URL containing backslashes that, when processed by the vulnerable module, would appear as a legitimate relative URL to the module but as an absolute URL to the browser. This creates a scenario where users might be redirected to attacker-controlled domains during the logout process, potentially capturing session tokens or credentials. The vulnerability undermines the security of SAML-based authentication flows and could lead to session hijacking or credential theft in environments relying on mod_auth_mellon for authentication. The issue affects organizations using SAML authentication where logout functionality is critical for maintaining secure session management and preventing unauthorized access to protected resources.
Mitigation strategies for CVE-2019-3877 require immediate deployment of mod_auth_mellon version 0.14.2 or later, which includes proper URL validation logic that addresses the backslash handling inconsistency. System administrators should also implement additional security measures such as validating all redirect URLs against a strict whitelist of approved domains and implementing proper input sanitization for logout parameters. Organizations should consider configuring the module to enforce strict URL parsing rules that prevent backslash character interpretation as path separators in redirect contexts. The fix addresses the core issue by ensuring that URL validation logic properly handles special characters and maintains consistency between the module's interpretation and browser behavior, preventing the bypass of redirect validation mechanisms. This vulnerability aligns with CWE-601 Open Redirect and ATT&CK technique T1566 Phishing, as it enables attackers to redirect users to malicious sites during authentication flows, potentially compromising user credentials and session integrity.