CVE-2019-5477 in Nokogiri
Summary
by MITRE
A command injection vulnerability in Nokogiri v1.10.3 and earlier allows commands to be executed in a subprocess via Ruby's `Kernel.open` method. Processes are vulnerable only if the undocumented method `Nokogiri::CSS::Tokenizer#load_file` is being called with unsafe user input as the filename. This vulnerability appears in code generated by the Rexical gem versions v1.0.6 and earlier. Rexical is used by Nokogiri to generate lexical scanner code for parsing CSS queries. The underlying vulnerability was addressed in Rexical v1.0.7 and Nokogiri upgraded to this version of Rexical in Nokogiri v1.10.4.
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Analysis
by VulDB Data Team • 11/26/2023
The vulnerability described in CVE-2019-5477 represents a critical command injection flaw within the Nokogiri library that affects versions 1.10.3 and earlier. This security issue stems from improper handling of user input within the CSS parsing functionality of Nokogiri, specifically when the undocumented method Nokogiri::CSS::Tokenizer#load_file is invoked with unsafe file paths. The flaw creates a dangerous execution path where attacker-controlled input can be interpreted as command arguments rather than simple file references, potentially enabling arbitrary code execution on the affected system. This vulnerability operates at the intersection of parsing libraries and system command execution, making it particularly insidious as it leverages legitimate parsing functionality to achieve malicious code execution.
The technical root cause of this vulnerability can be traced to the Rexical gem version 1.0.6 and earlier, which Nokogiri uses to generate lexical scanner code for CSS query parsing. When Rexical processes user input during the generation of scanner code, it fails to properly sanitize or escape special characters that could be interpreted as shell metacharacters. This flaw manifests specifically when the load_file method receives user-controlled input that contains shell command injection sequences, allowing attackers to execute arbitrary commands with the privileges of the running process. The vulnerability is classified as a command injection issue under CWE-77 and aligns with ATT&CK technique T1059.001 for command and scripting interpreter, demonstrating how parsing libraries can become attack vectors for remote code execution. The flaw exists because the generated scanner code does not properly handle special characters in file paths that could be interpreted by the underlying Ruby Kernel.open method.
The operational impact of CVE-2019-5477 extends beyond simple privilege escalation to encompass full system compromise when exploited successfully. Applications using vulnerable versions of Nokogiri that process user-supplied CSS queries or file paths become potential attack vectors for remote command execution, allowing attackers to execute arbitrary system commands, access sensitive data, or establish persistence mechanisms. This vulnerability is particularly concerning because it can be exploited in web applications that parse CSS content from untrusted sources, making it applicable to content management systems, web scrapers, and any application that processes external CSS data. The attack surface is broad since many applications rely on Nokogiri for HTML and XML processing, and CSS parsing functionality is commonly used in web applications. Security teams must consider this vulnerability as part of their application security testing, particularly when reviewing applications that use external CSS libraries or perform dynamic parsing of user-supplied content.
Mitigation strategies for CVE-2019-5477 focus on upgrading to patched versions of both Rexical and Nokogiri, with the recommended approach being to upgrade to Nokogiri v1.10.4 or later, which includes the patched Rexical v1.0.7 version. Organizations should conduct comprehensive vulnerability assessments to identify all applications using vulnerable versions of Nokogiri and prioritize their remediation based on risk exposure. Additional protective measures include implementing proper input validation and sanitization for all user-supplied file paths, using whitelisting approaches for file operations, and employing principle of least privilege for processes that utilize Nokogiri. Security monitoring should include detection of unusual command execution patterns and file access from web applications. The vulnerability serves as a reminder of the importance of dependency management and regular security updates, as issues in seemingly innocuous parsing libraries can have severe consequences. Organizations should also consider implementing automated dependency scanning tools to identify and remediate similar vulnerabilities before they can be exploited in production environments.