CVE-2012-6711 in Bash
Summary
by MITRE
A heap-based buffer overflow exists in GNU Bash before 4.3 when wide characters, not supported by the current locale set in the LC_CTYPE environment variable, are printed through the echo built-in function. A local attacker, who can provide data to print through the "echo -e" built-in function, may use this flaw to crash a script or execute code with the privileges of the bash process. This occurs because ansicstr() in lib/sh/strtrans.c mishandles u32cconv().
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Analysis
by VulDB Data Team • 10/06/2023
The vulnerability described in CVE-2012-6711 represents a critical heap-based buffer overflow within GNU Bash versions prior to 4.3, specifically when processing wide characters that are not supported by the current locale setting in the LC_CTYPE environment variable. This flaw demonstrates a fundamental issue in how bash handles character encoding conversions during output operations, particularly through the echo built-in function with the -e flag. The vulnerability arises from improper handling of Unicode character sequences when the system locale cannot properly interpret these wide characters, creating a scenario where malicious input can trigger memory corruption.
The technical root cause of this vulnerability lies in the ansicstr() function located in lib/sh/strtrans.c, which is responsible for converting string representations during bash's internal processing. When bash encounters wide characters that exceed the supported character set of the current locale, the u32cconv() function fails to properly validate or limit the memory allocation for these conversions. This improper handling creates a situation where the buffer allocated for character conversion can be exceeded, leading to heap corruption that allows for arbitrary code execution. The vulnerability specifically manifests when using echo -e with crafted input that contains wide character sequences, making it particularly dangerous in environments where bash scripts process untrusted input.
The operational impact of this vulnerability extends beyond simple script crashes to full system compromise, as a local attacker with the ability to inject data through bash scripts can leverage this flaw to execute arbitrary code with the privileges of the bash process. This presents a significant risk in environments where bash scripts are executed with elevated privileges or where users can influence script inputs. The vulnerability affects not only interactive shell sessions but also automated scripts and system services that rely on bash for execution, potentially allowing attackers to escalate privileges and gain unauthorized access to system resources. The heap overflow can be triggered through various attack vectors including command line arguments, environment variables, or input redirection, making it particularly challenging to defend against.
Mitigation strategies for CVE-2012-6711 primarily focus on updating bash to version 4.3 or later, which includes proper bounds checking and memory allocation handling for wide character processing. System administrators should also implement strict input validation and sanitization for all bash scripts that process external data, particularly when using echo -e functionality. Additionally, organizations can employ runtime protections such as stack canaries, address space layout randomization, and heap protection mechanisms to reduce the exploitability of such vulnerabilities. The flaw aligns with CWE-121, heap-based buffer overflow, and can be mapped to ATT&CK techniques involving privilege escalation and code injection through shell command execution. Regular security auditing of bash usage patterns and environment variable configurations can help identify systems potentially vulnerable to this class of attack, while monitoring for unusual echo command usage patterns may provide early detection of exploitation attempts.