CVE-2018-18426 in S-Cms
Summary
by MITRE
s-cms 3.0 allows remote attackers to execute arbitrary PHP code by placing this code in a crafted User-agent Disallow value in the robots.php txt parameter.
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Analysis
by VulDB Data Team • 04/04/2020
The vulnerability identified as CVE-2018-18426 affects s-cms version 3.0 and represents a critical remote code execution flaw that stems from improper input validation within the robots.php script. This vulnerability specifically targets the User-agent header processing mechanism where the application fails to properly sanitize user-supplied input before incorporating it into the robots.txt file generation process. The flaw allows attackers to inject malicious PHP code directly through the Disallow value of the User-agent header, which is then executed by the web server when the robots.txt file is accessed.
The technical implementation of this vulnerability resides in the application's failure to properly validate and escape user input within the robots.php script. When a request is made to the robots.php endpoint with a crafted User-agent header containing a Disallow directive, the system processes this input without adequate sanitization measures. This creates a classic command injection scenario where attacker-controlled code becomes part of the generated robots.txt file and subsequently executed as PHP code by the web server. The vulnerability operates at the application layer and requires no authentication or privileged access to exploit, making it particularly dangerous for publicly accessible web applications.
The operational impact of CVE-2018-18426 extends beyond simple code execution to potentially compromise entire web servers and underlying infrastructure. Successful exploitation allows attackers to execute arbitrary PHP commands with the privileges of the web server process, which typically includes access to database credentials, file system resources, and potentially the ability to escalate privileges within the hosting environment. This vulnerability aligns with CWE-94, which describes improper control of generation of code, and represents a direct violation of secure coding practices for input validation and output encoding. The attack surface is particularly concerning as it leverages standard HTTP headers that are commonly sent by web crawlers and security tools, making detection more challenging.
Mitigation strategies for this vulnerability must address both immediate remediation and long-term architectural improvements. The primary fix involves implementing proper input validation and sanitization within the robots.php script to prevent any user-supplied data from being directly embedded into executable code. This includes escaping special characters, implementing strict input filtering, and ensuring that all user-controllable parameters are properly validated before processing. Organizations should also consider implementing web application firewalls to detect and block suspicious User-agent headers containing potentially malicious code patterns. The vulnerability demonstrates the importance of following ATT&CK framework principles for defensive measures, particularly in preventing initial access and command execution phases. Regular security audits and input validation testing should be implemented to prevent similar issues in other application components, as this flaw represents a fundamental breakdown in application security controls that could affect other input handling mechanisms within the same codebase.