CVE-2020-5217 in secure_headers
Summary
by MITRE
In Secure Headers (RubyGem secure_headers), a directive injection vulnerability is present in versions before 3.8.0, 5.1.0, and 6.2.0. If user-supplied input was passed into append/override_content_security_policy_directives, a semicolon could be injected leading to directive injection. This could be used to e.g. override a script-src directive. Duplicate directives are ignored and the first one wins. The directives in secure_headers are sorted alphabetically so they pretty much all come before script-src. A previously undefined directive would receive a value even if SecureHeaders::OPT_OUT was supplied. The fixed versions will silently convert the semicolons to spaces and emit a deprecation warning when this happens. This will result in innocuous browser console messages if being exploited/accidentally used. In future releases, we will raise application errors resulting in 500s. Depending on what major version you are using, the fixed versions are 6.2.0, 5.1.0, 3.8.0.
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Analysis
by VulDB Data Team • 03/25/2024
The vulnerability identified as CVE-2020-5217 affects the Secure Headers RubyGem, a widely used security library designed to implement HTTP security headers in web applications. This directive injection flaw represents a critical weakness in the Content Security Policy (CSP) implementation that could allow attackers to bypass security controls. The vulnerability specifically manifests when user-supplied input is processed through the append/override_content_security_policy_directives methods, creating opportunities for malicious input injection that can fundamentally alter security policies.
The technical exploitation of this vulnerability relies on the improper handling of semicolon characters within CSP directive values, which are normally used to separate multiple directives within a single header. When user input containing semicolons is passed through the vulnerable methods, these characters can be interpreted as directive separators rather than literal characters within directive values. This creates a condition where attackers can inject additional directives that would normally be ignored due to the alphabetical sorting mechanism that prioritizes the first occurrence of each directive. Since most security directives appear alphabetically before script-src, an attacker could effectively override or modify the script-src directive, potentially allowing malicious JavaScript execution.
The operational impact of this vulnerability extends beyond simple policy bypassing, as it can be leveraged to circumvent fundamental security protections that organizations rely upon for preventing cross-site scripting attacks and other client-side exploits. The vulnerability's behavior is particularly concerning because it operates silently in affected versions, converting semicolons to spaces and issuing deprecation warnings rather than causing immediate application failures. This design choice means that security breaches could occur without immediate detection, allowing attackers to manipulate CSP policies without raising obvious alarms. The fact that previously undefined directives can receive values even when SecureHeaders::OPT_OUT is specified demonstrates a fundamental flaw in the input sanitization and policy enforcement mechanisms.
Organizations using affected versions of Secure Headers should immediately implement mitigations including upgrading to the fixed versions 3.8.0, 5.1.0, or 6.2.0 depending on their major version, as these releases introduce automatic semicolon-to-space conversion with deprecation warnings. The mitigation strategy should also include comprehensive input validation and sanitization of all user-supplied data that might be processed through CSP directive methods. Security teams should conduct thorough audits of their CSP implementations to identify any potential exploitation that may have occurred, while also monitoring application logs for deprecation warnings that might indicate attempted exploitation. The vulnerability aligns with CWE-94, which addresses the injection of code or commands, and maps to ATT&CK technique T1059.007 for script injection, highlighting the need for robust input validation and secure coding practices in security-critical libraries. The transition to error-raising behavior in future releases ensures that such injection attempts will result in application failures rather than silent security bypasses, providing better visibility into potential attacks.