CVE-2019-9924 in Bash
Summary
by MITRE
rbash in Bash before 4.4-beta2 did not prevent the shell user from modifying BASH_CMDS, thus allowing the user to execute any command with the permissions of the shell.
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Analysis
by VulDB Data Team • 08/04/2023
The vulnerability identified as CVE-2019-9924 represents a critical security flaw in the bash shell's restricted shell implementation known as rbash. This vulnerability specifically affects bash versions prior to 4.4-beta2 and fundamentally undermines the security assumptions that rbash is designed to maintain. The issue resides in the improper handling of the BASH_CMDS environment variable within the restricted shell context, which creates an unauthorized escape mechanism for privilege escalation.
The technical root cause of this vulnerability stems from rbash's failure to properly sanitize or restrict access to the BASH_CMDS variable, which serves as a dynamic command definition array in bash. When a user operates within an rbash session, the shell should normally prevent execution of commands that could bypass restrictions, but due to this flaw, users can manipulate the BASH_CMDS variable to redefine or extend command functionality. This manipulation allows attackers to effectively circumvent rbash's intended limitations and execute arbitrary commands with the same privileges as the shell itself, essentially breaking the fundamental security boundary that rbash is meant to enforce.
The operational impact of this vulnerability is severe and far-reaching within systems that rely on rbash for restricted user access. Any user who gains access to an rbash session can immediately escalate their privileges to gain full system control, as demonstrated by the ability to execute commands with elevated permissions. This vulnerability directly violates the principle of least privilege and can lead to complete system compromise when users have access to rbash environments. The attack vector is particularly concerning because it requires no additional privileges beyond access to the rbash session itself, making it an attractive target for both internal and external attackers.
This vulnerability aligns with CWE-284 Access Control Issues, specifically addressing improper access control mechanisms within restricted shell environments. From an ATT&CK framework perspective, this represents a privilege escalation technique that maps to T1068, and potentially T1548.002 for bypassing system protections. The flaw demonstrates how restricted shell implementations can be fundamentally undermined through improper variable handling, creating a direct path to command execution regardless of shell restrictions. Organizations using rbash for user access control should immediately consider this vulnerability as a critical risk requiring immediate remediation.
The recommended mitigation strategy involves upgrading to bash version 4.4-beta2 or later, where the vulnerability has been addressed through proper enforcement of access controls on the BASH_CMDS variable. System administrators should also conduct comprehensive audits of all rbash environments to identify and disable any existing rbash sessions until proper upgrades are implemented. Additional protective measures include implementing strict monitoring of environment variable modifications and ensuring that proper access controls are maintained at the operating system level, as the vulnerability essentially allows for complete bypass of shell-level restrictions through a single variable manipulation technique.