CVE-2016-9401 in Bash
Summary
by MITRE
popd in bash might allow local users to bypass the restricted shell and cause a use-after-free via a crafted address.
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Analysis
by VulDB Data Team • 05/14/2026
The vulnerability identified as CVE-2016-9401 resides within the bash shell's popd command implementation, representing a critical security flaw that can be exploited by local attackers to circumvent restricted shell environments. This issue specifically targets the way bash handles directory stack operations, where the popd command is responsible for removing directories from the stack and returning to the previous directory. The vulnerability manifests through a use-after-free condition that occurs when bash processes a crafted address parameter, creating an exploitable memory management error that can be leveraged for privilege escalation and arbitrary code execution.
The technical root cause of this vulnerability stems from improper memory management within bash's directory stack handling mechanisms. When the popd command processes a specially crafted address, it fails to properly validate or sanitize the input before performing memory operations that lead to a use-after-free scenario. This memory corruption vulnerability allows attackers to manipulate the heap memory layout and potentially execute malicious code with elevated privileges. The flaw specifically affects bash versions prior to 4.4 and represents a classic memory safety issue that has been documented under CWE-416, which covers use-after-free conditions in software implementations. The vulnerability's exploitation requires local access and can be particularly dangerous in restricted shell environments where users are intended to be limited in their system access.
The operational impact of CVE-2016-9401 extends beyond simple privilege escalation, as it can be leveraged to bypass security controls that rely on restricted shell environments for access control. Attackers can exploit this vulnerability to gain shell access and execute arbitrary commands, potentially leading to full system compromise. The vulnerability is particularly concerning in multi-user environments where restricted shells are employed to limit user capabilities, as it can be used to escape these confinement mechanisms and gain broader system access. From an attack perspective, this vulnerability aligns with ATT&CK technique T1059.002 for command and scripting interpreter and T1068 for exploit for privilege escalation, making it a significant threat vector for attackers seeking to elevate their privileges on affected systems.
Mitigation strategies for CVE-2016-9401 primarily focus on updating bash to version 4.4 or later, which contains the necessary patches to address the use-after-free condition in the popd command. System administrators should prioritize patching affected systems, particularly those running older bash versions or those in environments where restricted shells are utilized. Additional protective measures include implementing proper access controls, monitoring for suspicious shell behavior, and ensuring that systems are regularly updated with security patches. Organizations should also consider implementing runtime protections and monitoring for memory corruption patterns that could indicate exploitation attempts. The vulnerability serves as a reminder of the importance of proper memory management in shell implementations and highlights the critical need for regular security updates to address memory safety issues that can be exploited for privilege escalation.