CVE-2019-16255 in Ruby
Summary
by MITRE
Ruby through 2.4.7, 2.5.x through 2.5.6, and 2.6.x through 2.6.4 allows code injection if the first argument (aka the "command" argument) to Shell#[] or Shell#test in lib/shell.rb is untrusted data. An attacker can exploit this to call an arbitrary Ruby method.
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Analysis
by VulDB Data Team • 05/10/2026
This vulnerability exists in ruby versions through 2.4.7, 2.5.x through 2.5.6, and 2.6.x through 2.6.4 within the shell.rb library where the Shell#[] and Shell#test methods fail to properly sanitize user input. The flaw occurs when the first argument to these methods is untrusted data, creating a code injection vector that allows attackers to execute arbitrary ruby methods. The vulnerability stems from insufficient input validation and sanitization within the shell command execution framework, specifically in how the shell library processes command arguments. This represents a classic command injection vulnerability that can be leveraged for remote code execution, as demonstrated by the ability to call arbitrary ruby methods through the vulnerable interface. The issue is particularly concerning because it affects core ruby functionality and can be exploited in applications that rely on shell command execution without proper input sanitization. The vulnerability is classified as CWE-77 and aligns with ATT&CK technique T1059.001 for command and scripting interpreter, specifically targeting ruby's shell execution capabilities. Attackers can exploit this by crafting malicious input that bypasses normal argument parsing and executes unintended ruby code within the application context, potentially leading to full system compromise. The vulnerability is particularly dangerous in web applications where user input flows directly into shell command execution without proper validation.
The technical implementation of this vulnerability involves the shell library's handling of command arguments in the Shell#[] and Shell#test methods, where user-supplied data is directly incorporated into shell commands without proper sanitization. When untrusted input is passed as the first argument, the ruby shell library fails to properly escape or validate the input before incorporating it into system commands, creating opportunities for attackers to inject malicious payloads. This flaw enables attackers to manipulate the shell command execution flow and invoke arbitrary ruby methods, effectively bypassing normal application security controls. The vulnerability is particularly dangerous because ruby's dynamic nature allows for method invocation through string manipulation, making the injection attack more potent than typical shell injection scenarios. The exploitation requires minimal privileges and can be performed remotely, making it an attractive target for attackers seeking to compromise ruby applications. The vulnerability affects the core shell execution functionality and can be leveraged to execute arbitrary commands on the underlying system, potentially leading to complete system compromise.
The operational impact of CVE-2019-16255 extends beyond simple code injection to potentially enable full system compromise through remote code execution. Applications that utilize shell command execution with user-supplied input are at risk, particularly those that do not properly validate or sanitize input before passing it to shell methods. The vulnerability can be exploited to execute arbitrary commands on the target system, potentially allowing attackers to escalate privileges, install backdoors, or exfiltrate sensitive data. Organizations running ruby applications on affected versions must consider this vulnerability as a critical threat to their infrastructure security. The impact is particularly severe in environments where ruby applications interact with external users or process untrusted input from network sources, as these scenarios provide attackers with direct exploitation opportunities. The vulnerability affects ruby's standard library functionality, meaning that any application using the shell.rb library is potentially at risk, regardless of the application's specific security measures. Security teams should prioritize patching affected ruby installations and implementing proper input validation controls to prevent exploitation attempts.
Mitigation strategies for CVE-2019-16255 involve immediate patching of ruby installations to versions that address the vulnerability, specifically ruby 2.4.8, 2.5.7, and 2.6.5 or later. Organizations should also implement proper input validation and sanitization practices when processing user data that may be used in shell command execution. The use of parameterized shell commands or alternative approaches that avoid direct shell execution should be considered as long-term security measures. Security controls should include input validation at multiple layers, including application-level filtering and proper escaping of shell metacharacters. Organizations should also implement network segmentation and monitoring to detect potential exploitation attempts. The ruby community recommends avoiding direct shell command execution with user-supplied data and instead using safer alternatives that do not expose the system to command injection risks. Regular security audits should verify that applications properly handle shell command arguments and that no vulnerable code paths exist. Additionally, implementing web application firewalls and runtime application self-protection mechanisms can provide additional defense-in-depth layers against exploitation attempts. The vulnerability serves as a reminder of the importance of secure coding practices and proper input handling in preventing command injection attacks.