CVE-2017-5539 in b2evolution
Summary
by MITRE
The patch for directory traversal (CVE-2017-5480) in b2evolution version 6.8.4-stable has a bypass vulnerability. An attacker can use ..\/ to bypass the filter rule. Then, this attacker can exploit this vulnerability to delete or read any files on the server. It can also be used to determine whether a file exists.
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Analysis
by VulDB Data Team • 05/14/2026
The vulnerability identified as CVE-2017-5539 represents a critical directory traversal flaw in the b2evolution content management system version 6.8.4-stable, directly related to the remediation efforts for CVE-2017-5480. This issue demonstrates the complexity of implementing effective security patches, as the initial fix contained a bypass mechanism that allowed attackers to circumvent the intended protection measures. The vulnerability specifically exploits the improper filtering of directory traversal sequences, enabling malicious actors to manipulate file system access through carefully crafted input sequences. The technical implementation involves the use of the ..\/ sequence which effectively bypasses the original filter rules designed to prevent directory traversal attacks.
The core technical flaw resides in the insufficient input validation mechanism within the b2evolution application's file handling routines. When the system processes user-supplied input containing directory traversal sequences, the filtering logic fails to properly sanitize all possible representations of these sequences. The bypass occurs because the security patch did not account for alternative encoding methods or variations in how directory traversal sequences could be constructed, specifically allowing the ..\/ pattern to pass through the validation filters. This represents a classic weakness in security implementation where defensive measures are bypassed through creative input manipulation, falling under the CWE-22 category of Improper Limitation of a Pathname to a Restricted Directory.
The operational impact of this vulnerability extends far beyond simple information disclosure, as it provides attackers with complete file system access capabilities on the affected server. Successful exploitation allows unauthorized users to perform read operations on arbitrary files, potentially accessing sensitive configuration files, database credentials, or other critical system information. Additionally, the vulnerability enables file deletion capabilities, which can result in complete system compromise or data destruction. The ability to determine file existence through this vulnerability provides attackers with valuable reconnaissance information, enabling them to map the target system's file structure and identify potential targets for further exploitation. This capability aligns with ATT&CK technique T1083 (File and Directory Discovery) and T1068 (Local Privilege Escalation) within the enterprise attack framework.
The vulnerability's exploitation potential makes it particularly dangerous in environments where b2evolution systems are deployed with elevated privileges or contain sensitive data. Attackers can leverage this weakness to gain unauthorized access to system files, configuration data, or user information stored on the server. The bypass mechanism demonstrates a fundamental flaw in the security patch implementation, where the original fix was not comprehensive enough to address all possible variations of directory traversal attacks. Organizations using this version of b2evolution face significant risk of complete system compromise, as the vulnerability provides a direct pathway to execute arbitrary code or access sensitive data. Security practitioners should recognize this as a prime example of why security patches must be thoroughly tested against various attack vectors and why defense-in-depth strategies are essential for maintaining system integrity. The vulnerability highlights the importance of proper input validation and the need for multiple layers of security controls to prevent successful exploitation attempts.
Mitigation strategies should include immediate deployment of the vendor-provided security patch or upgrade to a patched version of b2evolution, along with comprehensive input validation mechanisms. Organizations should implement additional security controls such as web application firewalls and file system access controls to limit the potential impact of such vulnerabilities. Regular security testing and code review processes should be implemented to identify similar issues in other applications. The vulnerability also underscores the importance of maintaining current security knowledge and staying informed about security advisories from software vendors to ensure timely patch deployment and prevent exploitation of known vulnerabilities.