Adversaries may use XOR-like encryption in C2 communications to obfuscate command and control traffic, making it harder to detect through traditional signature-based methods. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify potential stealthy malware activity that evades standard detection mechanisms.
YARA Rule
rule elknot_xor : malware
{
meta:
author = "[email protected]"
date = "2016-04-25"
description = "elknot/Billgates variants with XOR like C2 encryption scheme"
reference = "http://liuya0904.blogspot.tw/2016/04/new-elknotbillgates-variant-with-xor.html"
sample = "474429d9da170e733213940acc9a2b1c, 2579aa65a28c32778790ec1c673abc49"
strings:
//md5=474429d9da170e733213940acc9a2b1c
/*
seg000:08130801 68 00 09 13 08 push offset dword_8130900
seg000:08130806 83 3D 30 17 13 08 02 cmp ds:dword_8131730, 2
seg000:0813080D 75 07 jnz short loc_8130816
seg000:0813080F 81 04 24 00 01 00 00 add dword ptr [esp], 100h
seg000:08130816 loc_8130816:
seg000:08130816 50 push eax
seg000:08130817 E8 15 00 00 00 call sub_8130831
seg000:0813081C E9 C8 F6 F5 FF jmp near ptr 808FEE9h
*/
$decrypt_c2_func_1 = {08 83 [5] 02 75 07 81 04 24 00 01 00 00 50 e8 [4] e9}
// md5=2579aa65a28c32778790ec1c673abc49
/*
.rodata:08104D20 E8 00 00 00 00 call $+5
.rodata:08104D25 87 1C 24 xchg ebx, [esp+4+var_4] ;
.rodata:08104D28 83 EB 05 sub ebx, 5
.rodata:08104D2B 8D 83 00 FD FF FF lea eax, [ebx-300h]
.rodata:08104D31 83 BB 10 CA 02 00 02 cmp dword ptr [ebx+2CA10h], 2
.rodata:08104D38 75 05 jnz short loc_8104D3F
.rodata:08104D3A 05 00 01 00 00 add eax, 100h
.rodata:08104D3F loc_8104D3F:
.rodata:08104D3F 50 push eax
.rodata:08104D40 FF 74 24 10 push [esp+8+strsVector]
*/
$decrypt_c2_func_2 = {e8 00 00 00 00 87 [2] 83 eb 05 8d 83 [4] 83 bb [4] 02 75 05}
condition:
1 of ($decrypt_c2_func_*)
}This YARA rule can be deployed in the following contexts:
This rule contains 2 string patterns in its detection logic.
Scenario: A system administrator is using PowerShell to perform routine system diagnostics, which includes XOR-based string manipulation for obfuscation purposes.
Filter/Exclusion: Check for ProcessName containing powershell.exe and CommandLine containing -Command or -File with known admin scripts.
Scenario: A scheduled job runs a script that uses XOR encryption to securely store credentials in memory, as part of a secure credential management process.
Filter/Exclusion: Filter by ProcessName matching schtasks.exe or Task Scheduler and check for known internal credential management scripts.
Scenario: A network monitoring tool like Wireshark or tcpdump is used to capture and analyze encrypted traffic, which may include XOR-like encryption for data obfuscation during analysis.
Filter/Exclusion: Filter by ProcessName containing tcpdump.exe or Wireshark.exe and check for known network analysis tools in the process tree.
Scenario: A devops pipeline running on Jenkins or GitLab CI uses XOR encryption to obfuscate API keys or secrets in the pipeline configuration files.
Filter/Exclusion: Filter by ProcessName containing jenkins.exe or gitlab-runner.exe and check for known CI/CD tooling processes.
Scenario: A Windows service running a custom application uses XOR encryption to encrypt logs or communication with a backend server for security purposes.
Filter/Exclusion: Filter by ServiceName matching the known internal service name and check for ProcessName matching the internal application.