Attack Chain

1. Attacker gains local access to the system.
2. Attacker crafts a malicious payload containing junk data, shellcode, and a return address.
3. Attacker invokes the yasr binary with the -p parameter, supplying the crafted payload as its argument.
4. The yasr application attempts to process the oversized argument without proper bounds checking.
5. The crafted payload overwrites the stack, including critical data such as return addresses.
6. The application attempts to return from a function, but instead jumps to the attacker-controlled shellcode.
7. The attacker's shellcode executes with the privileges of the yasr process.
8. Attacker achieves arbitrary code execution, potentially compromising the system.

Impact

Successful exploitation of CVE-2016-20041 can lead to a denial-of-service condition (application crash) or, more severely, arbitrary code execution with the privileges of the yasr process. The number of potential victims depends on the prevalence of yasr version 0.6.9-5 in local environments. If exploited, attackers could gain unauthorized access to sensitive data or further compromise the affected system.

Recommendation

• Monitor process creation events for invocations of yasr with unusually long arguments to the -p parameter, using the process_creation Sigma rule provided.
• Consider implementing application control or whitelisting to restrict the execution of yasr to authorized users and use cases.
• Although patching is not possible due to the age of this software, consider removing this software from production to mitigate against exploitation of CVE-2016-20041.