CVE-2019-12524 in Web Proxy
Summary
by MITRE
An issue was discovered in Squid through 4.7. When handling requests from users, Squid checks its rules to see if the request should be denied. Squid by default comes with rules to block access to the Cache Manager, which serves detailed server information meant for the maintainer. This rule is implemented via url_regex. The handler for url_regex rules URL decodes an incoming request. This allows an attacker to encode their URL to bypass the url_regex check, and gain access to the blocked resource.
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Analysis
by VulDB Data Team • 05/26/2024
The vulnerability identified as CVE-2019-12524 represents a significant security flaw in Squid proxy server versions through 4.7, specifically addressing a bypass mechanism within the url_regex rule implementation. This issue stems from the improper handling of URL decoding during request processing, creating an avenue for unauthorized access to sensitive administrative resources. The vulnerability exploits the fundamental architecture of Squid's access control mechanisms, where default configurations include rules designed to prevent access to the Cache Manager interface, which contains detailed server information intended exclusively for system administrators. The Cache Manager interface serves as a critical administrative component providing insights into server operations, cache statistics, and configuration details that could be exploited by malicious actors to gain unauthorized visibility into the proxy server's internal workings.
The technical flaw manifests in the url_regex rule handler's processing of incoming requests, where the system performs URL decoding on the requested URLs before applying the access control rules. This decoding process creates a discrepancy between the original encoded URL presented to the rule engine and the decoded URL that is actually evaluated against the access control policies. When attackers craft maliciously encoded URLs, they can manipulate the encoded representation to bypass the url_regex pattern matching that should normally block access to the Cache Manager interface. The vulnerability is classified under CWE-284 Access Control Bypass, which specifically addresses situations where access control mechanisms can be circumvented through improper implementation or processing of security checks. This particular implementation flaw allows attackers to exploit the URL decoding behavior as an attack vector to gain unauthorized access to administrative resources that should remain protected.
The operational impact of this vulnerability extends beyond simple unauthorized access, as it enables attackers to potentially gather sensitive information about the proxy server infrastructure, including cache performance metrics, server configuration details, and operational statistics. This information could be leveraged to plan more sophisticated attacks against the network infrastructure or identify additional vulnerabilities within the proxy server environment. The attack surface is particularly concerning given that Squid is widely deployed as a caching proxy server in enterprise environments, making this vulnerability potentially exploitable across numerous organizational networks. From an attacker's perspective, this vulnerability aligns with ATT&CK technique T1083, which involves discovering system information, as it allows adversaries to gather detailed insights into the proxy server's operational state and configuration. The ability to bypass default security controls also represents a critical failure in the principle of least privilege, as it allows unauthorized users to access administrative resources that should only be available to system maintainers.
Organizations utilizing Squid proxy servers should implement immediate mitigations to address this vulnerability, including updating to versions that have patched the URL decoding behavior in url_regex rules. The recommended approach involves ensuring that all instances of Squid are upgraded to versions beyond 4.7, where the underlying URL decoding mechanism has been corrected to prevent bypass attempts. Additionally, administrators should review and validate their custom access control rules to ensure that no additional bypass mechanisms exist within their specific configurations. Security teams should also implement monitoring for unusual access patterns to the Cache Manager interface, as this vulnerability could be exploited to gain unauthorized administrative access. Network segmentation and additional access controls should be considered as compensating measures while awaiting official patches, and organizations should conduct thorough security assessments to identify any potential exploitation attempts that may have occurred prior to implementing the fix. The vulnerability demonstrates the critical importance of proper input validation and consistent security controls throughout the application processing pipeline, as even seemingly minor implementation details can create significant security risks.