Exploit:Linux/CVE-2021-4034.B!MTB

The security flaw originates from improper command-line argument validation within the pkexec utility's source code. The main() function contains logic that begins processing command-line arguments at index position 1. A critical edge case occurs when the program is launched with an empty argument list, creating a scenario where the argument count (argc) equals zero. Under these specific conditions, the program's internal indexing variable incorrectly remains set to 1 despite the absence of valid arguments in the array. 

This programming error triggers a sequence of memory access violations. The initial fault manifests as an out-of-bounds read operation when the code attempts to access the memory location for argv[1]. Due to the standard memory organization on the program stack, where the environment pointer array (envp) immediately follows the argument pointer array (argv), this invalid memory read retrieves the content stored in the first environment variable slot, envp[0]. 

The exploitation process continues with a more severe out-of-bounds write operation. The pkexec program passes this incorrectly obtained value from envp[0] to the g_find_program_in_path() function, which searches the system's PATH environment variable for a matching binary. When this search successfully locates a corresponding executable file, the program writes the complete filesystem path of this binary back to the original invalid memory address of argv[1], which overwrites and corrupts the envp[0] environment variable entry. 

This memory corruption sequence establishes a fundamental exploit primitive that allows threat actors to reintroduce dangerous environment variables into pkexec's launch context. These specific environment variables are normally filtered and removed through security measures when any setuid program runs. The exploitation methodology focuses on manipulating the GCONV_PATHenvironment variable, which controls the filesystem location for gconv module libraries. By poisoning this environment variable through the memory corruption primitive, threat actors can deceive pkexec into loading and launching a maliciously crafted shared library with full root-level system privileges. 

A functional proof-of-concept implementation involves creating multiple coordinated files. The core component is an exploit loader program (conventionally named exploit.c) specifically designed to execute pkexec with precisely manipulated environment conditions that trigger the vulnerability. This is accompanied by malicious library source code (evil-so.c) containing payload instructions that spawn a privileged root shell when launched.  

This source code is compiled into a shared object file (evil.so) placed within a controlled directory. Finally, a configuration file (gconv-modules) provides explicit instructions to the system's internationalization framework, directing it to load the malicious shared library during the character set conversion process. This comprehensive file set collaborates to exploit the memory corruption vulnerability and achieve complete privilege escalation from unprivileged user to root-level device access.