CVE-2021-4097 in phpservermon
Summary
by MITRE • 12/12/2021
phpservermon is vulnerable to Improper Neutralization of CRLF Sequences
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Analysis
by VulDB Data Team • 12/15/2021
The vulnerability identified as CVE-2021-4097 affects phpservermon, a popular open-source server monitoring application that provides real-time status updates for various server components. This particular weakness stems from improper handling of CRLF (Carriage Return Line Feed) sequences within the application's input validation mechanisms, creating a potential vector for malicious exploitation. The vulnerability resides in how the application processes user-supplied data that may contain CRLF characters, which are typically used to separate lines in text files and network protocols. When these sequences are not properly sanitized or neutralized, they can be manipulated to inject malicious content into the application's output streams or response headers.
The technical flaw manifests when phpservermon fails to adequately sanitize user input before processing it for display or transmission, particularly in contexts where CRLF characters could be interpreted as protocol delimiters or command terminators. This improper neutralization creates opportunities for attackers to inject malicious content into HTTP headers, manipulate response content, or potentially execute cross-site scripting attacks depending on how the application handles the processed data. The vulnerability is classified under CWE-117, which specifically addresses Improper Output Neutralization for Logs, as the CRLF sequences can be exploited to manipulate log entries or inject malicious payloads into the application's output streams. The flaw becomes particularly dangerous when the application uses user input in contexts where CRLF characters could be interpreted as protocol commands or header terminators, allowing attackers to manipulate HTTP responses or inject malicious data into the application's communication channels.
The operational impact of this vulnerability extends beyond simple data manipulation, as it can enable attackers to perform header injection attacks that may lead to session hijacking, cross-site scripting, or even cache poisoning attacks within the monitored environment. When exploited successfully, the vulnerability allows malicious actors to inject arbitrary CRLF sequences into HTTP responses, potentially causing the application to send malformed headers that could be interpreted by web browsers or intermediate proxies in unintended ways. This could result in users being redirected to malicious websites, session cookies being manipulated, or sensitive information being exposed through improperly sanitized log entries. The attack surface is particularly concerning in monitoring environments where phpservermon might be used to track critical infrastructure components, as the injection of malicious content could compromise the integrity of the monitoring data or provide attackers with additional attack vectors against the monitored systems. According to ATT&CK framework, this vulnerability maps to T1190 - Exploit Public-Facing Application, as it represents an entry point through which attackers can potentially compromise the application's integrity and confidentiality.
Mitigation strategies for CVE-2021-4097 should focus on implementing comprehensive input validation and sanitization mechanisms throughout the phpservermon application, particularly in areas where user input is processed and displayed. The primary recommendation involves ensuring that all user-supplied data undergoes proper neutralization of CRLF sequences before being processed or outputted, with specific attention to HTTP header generation and log entry creation. Organizations should implement strict input validation routines that strip or encode CRLF characters from user input, particularly in fields that are later used in HTTP headers or response generation. Additionally, regular security updates and patches should be applied to phpservermon installations, as the vulnerability was addressed in subsequent releases through improved input sanitization mechanisms. The implementation of web application firewalls and security monitoring solutions can provide additional layers of protection by detecting and blocking attempts to inject malicious CRLF sequences into the application's processing pipeline. System administrators should also conduct regular security assessments of their monitoring infrastructure to identify potential injection points and ensure that all user-facing components properly handle special characters and protocol delimiters according to established security best practices.