CVE-2004-2065 in DansGuardian
Summary
by MITRE
DansGuardian 2.8 and earlier allows remote attackers to bypass the extension filtering rule via a hex encoded extension or . in the filename.
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Analysis
by VulDB Data Team • 06/25/2018
The vulnerability identified as CVE-2004-2065 affects DansGuardian versions 2.8 and earlier, representing a significant security flaw in web content filtering systems that can be exploited by remote attackers to circumvent content restrictions. DansGuardian is widely deployed as a proxy server filtering solution that blocks access to inappropriate or unwanted web content based on various filtering criteria including file extensions, keywords, and categories. This particular vulnerability specifically targets the extension filtering mechanism that is fundamental to how the software determines which files to block or allow based on their file extensions. The flaw stems from insufficient validation of filename extensions within the filtering logic, allowing malicious actors to craft filenames that appear to contain blocked extensions while actually bypassing the filtering rules through hexadecimal encoding or strategic placement of periods within filenames.
The technical implementation of this vulnerability exploits a weakness in the regular expression matching or string parsing functions used by DansGuardian to identify file extensions. When a user attempts to access a resource, the filtering system examines the filename to determine if it matches any blocked extensions. However, the software fails to properly normalize or canonicalize filenames before performing these checks, creating a window where attackers can manipulate the filename structure to evade detection. The hex encoding technique involves representing the extension characters in hexadecimal format, while the period manipulation exploits how the software processes filename components. This type of vulnerability is classified as a bypass or evasion mechanism that directly impacts the integrity of the filtering system's rule enforcement capabilities.
The operational impact of this vulnerability extends beyond simple content bypassing, as it fundamentally undermines the security posture of organizations relying on DansGuardian for content control. Organizations using this software may experience unauthorized access to restricted content categories, including adult material, social media platforms, gaming sites, or other inappropriate resources that should be blocked according to policy. The remote nature of the attack means that threat actors do not require local system access or credentials to exploit the vulnerability, making it particularly dangerous for network environments where the filtering system serves as a primary security control. This vulnerability can be exploited by attackers from anywhere on the internet, potentially allowing them to access restricted resources while the organization's security policies appear to be functioning properly.
Organizations should immediately upgrade to DansGuardian versions 2.9 or later where this vulnerability has been addressed through improved filename parsing and validation mechanisms. The mitigation strategy involves not only applying the vendor-provided patch but also implementing additional monitoring to detect potential exploitation attempts. Security teams should review existing network traffic logs for patterns that might indicate attempts to exploit this vulnerability, particularly looking for unusual filename structures or hex encoded content in web requests. This vulnerability aligns with CWE-20, which describes improper input validation, and can be mapped to ATT&CK technique T1071.004 for application layer protocol manipulation. The remediation process should include comprehensive testing of the patched version to ensure that legitimate filtering functionality remains intact while the bypass mechanism is properly addressed, as the fix must maintain the software's core filtering capabilities while preventing the specific evasion techniques that exploit the vulnerability.