CVE-2016-4991 in nodepdf
Summary
by MITRE • 07/28/2022
Input passed to the Pdf() function is shell escaped and passed to child_process.exec() during PDF rendering. However, the shell escape does not properly encode all special characters, namely, semicolon and curly braces. This can be abused to achieve command execution. This problem affects nodepdf 1.3.0.
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Analysis
by VulDB Data Team • 07/29/2022
The vulnerability described in CVE-2016-4991 represents a critical command injection flaw within the nodepdf library version 1.3.0, specifically within the Pdf() function implementation. This issue stems from improper input validation and sanitization mechanisms that fail to adequately handle special shell characters during PDF rendering operations. The vulnerability manifests when user-supplied input is processed through shell escaping functions that do not account for all potentially dangerous characters in the shell context, creating an avenue for malicious command execution.
The technical flaw resides in the insufficient shell escaping mechanism that processes input before passing it to child_process.exec() function. While basic shell escaping may handle common special characters, the implementation fails to properly encode semicolon and curly brace characters which serve as critical delimiters in shell command parsing. These characters can be used to separate commands or create command substitution contexts, allowing attackers to inject arbitrary shell commands that execute with the privileges of the application process. The semicolon character acts as a command separator in shell environments, enabling attackers to chain multiple commands together, while curly braces provide command substitution capabilities that can be exploited to execute additional malicious payloads.
This vulnerability directly impacts the security posture of applications utilizing nodepdf 1.3.0, as it allows remote attackers to execute arbitrary commands on the underlying system. The operational impact extends beyond simple command execution to potentially enable full system compromise, data exfiltration, and persistence mechanisms. Attackers can leverage this vulnerability to escalate privileges, install backdoors, or perform reconnaissance activities on the compromised system. The vulnerability is particularly concerning because it operates at the application level where user input is processed, making it accessible to unauthenticated attackers who can manipulate PDF generation requests to inject malicious commands.
The exploitation of this vulnerability aligns with attack patterns documented in the MITRE ATT&CK framework under the T1059.001 technique for Command and Scripting Interpreter, specifically focusing on the execution of system commands through shell injection. From a CWE perspective, this represents a classic command injection vulnerability classified as CWE-77, which occurs when a program constructs a command that is sent to a shell for execution without proper validation or sanitization of user input. The improper handling of shell metacharacters in this context creates a direct path for attackers to bypass normal application security controls and execute arbitrary code on the target system.
Mitigation strategies for CVE-2016-4991 should prioritize immediate remediation through version updates to nodepdf library 1.3.1 or later, which contains proper shell escaping implementations. Organizations should implement input validation and sanitization measures that explicitly handle all shell metacharacters, including semicolons and curly braces, rather than relying on incomplete escaping functions. Additional defensive measures include implementing proper privilege separation where PDF generation processes run with minimal required permissions, employing input whitelisting techniques, and deploying web application firewalls that can detect and block suspicious command injection patterns. The remediation process should also include comprehensive code review of all shell command execution points to identify similar vulnerabilities that may exist in other parts of the application stack, ensuring a holistic approach to command injection prevention.