CVE-2021-47755: Oliver Library Server Path Traversal Flaw

CVE-2021-47755 Overview

CVE-2021-47755 is a path traversal vulnerability affecting Oliver Library Server v5. The vulnerability exists within the FileServlet endpoint, which fails to properly sanitize the fileName parameter before processing file download requests. This allows unauthenticated attackers to access arbitrary files on the server's filesystem by manipulating the input with directory traversal sequences such as ../.

Critical Impact

Unauthenticated attackers can download sensitive system files including configuration files, credentials, and other confidential data from servers running Oliver Library Server v5.

Affected Products

• Oliver Library Server v5

Discovery Timeline

• 2026-01-15 - CVE CVE-2021-47755 published to NVD
• 2026-01-16 - Last updated in NVD database

Technical Details for CVE-2021-47755

Vulnerability Analysis

This vulnerability is classified as CWE-22 (Improper Limitation of a Pathname to a Restricted Directory), commonly known as path traversal or directory traversal. The FileServlet endpoint in Oliver Library Server v5 accepts a fileName parameter that is used to construct a file path for download operations. Due to insufficient input validation, the application does not properly sanitize or restrict directory traversal sequences in the user-supplied filename.

The network-accessible nature of this vulnerability combined with no authentication requirement significantly increases the risk exposure. An attacker can exploit this vulnerability remotely without any prior access to the system or valid credentials.

Root Cause

The root cause of this vulnerability lies in the improper validation of the fileName parameter within the FileServlet component. When a file download request is processed, the application directly uses the user-supplied filename to construct the target file path without filtering out directory traversal characters such as ../ or ..\. This allows an attacker to escape the intended download directory and access files anywhere on the server's filesystem where the web application has read permissions.

Attack Vector

The attack vector for CVE-2021-47755 is network-based, allowing remote exploitation without authentication. An attacker can craft malicious HTTP requests to the FileServlet endpoint, manipulating the fileName parameter to include path traversal sequences. By traversing up the directory structure and then specifying the target file path, attackers can download sensitive files such as /etc/passwd, application configuration files containing database credentials, or other sensitive data stored on the server.

The vulnerability can be exploited by sending crafted GET or POST requests to the vulnerable endpoint. Technical details and proof-of-concept information are available at Exploit-DB #50599.

Detection Methods for CVE-2021-47755

Indicators of Compromise

• HTTP requests to FileServlet containing path traversal sequences such as ../, ..%2f, or ..%5c in the fileName parameter
• Unusual file access patterns in web server logs showing requests for system files like /etc/passwd or Windows system files
• Multiple rapid file download requests from a single source targeting various system paths
• Web application logs showing errors related to file access outside the expected download directory

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block requests containing directory traversal patterns
• Configure intrusion detection systems to alert on HTTP requests with encoded path traversal sequences (%2e%2e%2f, %252e%252e%252f)
• Monitor web server access logs for unusual patterns in FileServlet requests
• Deploy file integrity monitoring on sensitive system files to detect unauthorized access attempts

Monitoring Recommendations

• Enable detailed logging for the FileServlet endpoint to capture all file download requests with full parameter details
• Set up real-time alerting for any web requests containing path traversal indicators
• Monitor for abnormal outbound data transfer volumes that may indicate mass file exfiltration
• Implement network traffic analysis to detect large file transfers from the server to external destinations

How to Mitigate CVE-2021-47755

Immediate Actions Required

• Restrict network access to the Oliver Library Server to trusted networks or IP addresses only
• Implement web application firewall rules to block path traversal patterns in HTTP requests
• Review and remove any unnecessary publicly accessible file download endpoints
• Audit server logs for evidence of exploitation attempts targeting the FileServlet endpoint

Patch Information

Organizations should consult Softlink Product Overview for official patch and upgrade information for Oliver Library Server. Contact the vendor directly to obtain the latest security updates that address this vulnerability.

Workarounds

• Deploy a reverse proxy or web application firewall in front of the Oliver Library Server to filter malicious requests
• Implement network segmentation to limit access to the vulnerable server from untrusted networks
• Configure file system permissions to restrict the web application's read access to only necessary directories
• Consider taking the FileServlet endpoint offline if file download functionality is not business-critical

# Example WAF rule to block path traversal attempts
# Add to ModSecurity configuration
SecRule REQUEST_URI|ARGS "@rx (\.\./|\.\.\\)" \
    "id:1001,phase:1,deny,status:403,msg:'Path Traversal Attempt Detected'"