CVE-2017-5932 in Bash
Summary
by MITRE
The path autocompletion feature in Bash 4.4 allows local users to gain privileges via a crafted filename starting with a " (double quote) character and a command substitution metacharacter.
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Analysis
by VulDB Data Team • 08/23/2020
The vulnerability identified as CVE-2017-5932 represents a critical privilege escalation flaw within the bash shell's path autocompletion functionality. This issue specifically affects bash version 4.4 and demonstrates how seemingly innocuous autocompletion features can be exploited to bypass security controls and execute arbitrary commands with elevated privileges. The vulnerability stems from the improper handling of filenames that begin with double quote characters combined with command substitution metacharacters, creating a dangerous interaction that can be leveraged by local attackers.
The technical root cause of this vulnerability lies in bash's autocompletion system which processes filenames and command arguments without adequate sanitization of special characters. When a user interacts with bash autocompletion and encounters a filename starting with a double quote character, the shell's parsing logic fails to properly escape or handle the subsequent command substitution metacharacters. This parsing failure creates an environment where attacker-controlled input can be interpreted as executable commands rather than simple filename components. The vulnerability is classified under CWE-20, which addresses improper input validation, and specifically relates to improper handling of special characters in command line parsing contexts.
The operational impact of CVE-2017-5932 is significant as it allows local users to escalate their privileges from standard user level to root access without requiring authentication. Attackers can craft malicious filenames that, when processed through bash's autocompletion system, trigger command substitution and execute arbitrary code with elevated privileges. This privilege escalation can be achieved through various attack vectors including interactive shell sessions, automated scripts, or even through compromised applications that rely on bash for command execution. The vulnerability is particularly dangerous because it operates within the legitimate bash autocompletion workflow, making it difficult to detect and distinguish from normal system behavior.
Mitigation strategies for this vulnerability primarily focus on patching the affected bash versions and implementing additional input validation measures. System administrators should immediately update to bash versions that have addressed this vulnerability, specifically bash 4.4 patch 12 or later. Additional defensive measures include implementing proper file naming conventions that avoid starting filenames with special characters, disabling unnecessary autocompletion features in restricted environments, and monitoring shell command execution logs for suspicious patterns. Organizations should also consider implementing privilege separation mechanisms and ensuring that shell autocompletion features are properly configured to prevent command injection attacks. This vulnerability aligns with ATT&CK technique T1059.002 which covers command and scripting interpreter usage, particularly focusing on bash shell exploitation methods that leverage parsing vulnerabilities to achieve privilege escalation.