CryptoPP a_exp_b_mod_c | SOC Hunt Feed

Hunt Hypothesis

The CryptoPP a_exp_b_mod_c rule detects potential cryptographic operations that may indicate the use of custom or obfuscated cryptographic algorithms, which could be used by adversaries to exfiltrate data or establish covert communication channels. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and mitigate potential advanced persistent threats leveraging cryptographic techniques to evade standard detection mechanisms.

YARA Rule

rule CryptoPP_a_exp_b_mod_c
{	meta:
		author = "Maxx"
		description = "CryptoPP a_exp_b_mod_c"
	strings:
		$c0 = { 6A FF 68 ?? ?? ?? ?? 64 A1 00 00 00 00 50 64 89 25 00 00 00 00 81 EC ?? 00 00 00 56 8B B4 24 B0 00 00 00 57 6A 00 8B CE C7 44 24 0C 00 00 00 00 E8 ?? ?? ?? ?? 84 C0 0F 85 16 01 00 00 8D 4C 24 24 E8 ?? ?? ?? ?? BF 01 00 00 00 56 8D 4C 24 34 89 BC 24 A4 00 00 00 E8 ?? ?? ?? ?? 8B 06 8D 4C 24 3C 50 6A 00 C6 84 24 A8 00 00 00 02 E8 ?? ?? ?? ?? 8D 4C 24 48 C6 84 24 A0 00 00 00 03 E8 ?? ?? ?? ?? C7 44 24 24 ?? ?? ?? ?? 8B 8C 24 AC 00 00 00 8D 54 24 0C 51 52 8D 4C 24 2C C7 84 24 A8 }
		$c1 = { 6A FF 68 ?? ?? ?? ?? 64 A1 00 00 00 00 50 64 89 25 00 00 00 00 83 EC 4C 56 57 33 FF 8D 44 24 0C 89 7C 24 08 C7 44 24 10 ?? ?? ?? ?? C7 44 24 0C ?? ?? ?? ?? 89 44 24 14 8B 74 24 70 8D 4C 24 18 56 89 7C 24 60 E8 ?? ?? ?? ?? 8B 76 08 8D 4C 24 2C 56 57 C6 44 24 64 01 E8 ?? ?? ?? ?? 8D 4C 24 40 C6 44 24 5C 02 E8 ?? ?? ?? ?? C7 44 24 0C ?? ?? ?? ?? 8B 4C 24 6C 8B 54 24 68 8B 74 24 64 51 52 56 8D 4C 24 18 C7 44 24 68 03 00 00 00 E8 ?? ?? ?? ?? 8B 7C 24 4C 8B 4C 24 48 8B D7 33 C0 F3 }
		$c2 = { 6A FF 68 ?? ?? ?? ?? 64 A1 00 00 00 00 50 64 89 25 00 00 00 00 83 EC 34 56 57 33 FF 8D 44 24 0C 89 7C 24 08 C7 44 24 10 ?? ?? ?? ?? C7 44 24 0C ?? ?? ?? ?? 89 44 24 14 8B 74 24 58 8D 4C 24 18 56 89 7C 24 48 E8 ?? ?? ?? ?? 8B 0E C6 44 24 44 01 51 57 8D 4C 24 2C E8 ?? ?? ?? ?? 8D 4C 24 30 C6 44 24 44 02 E8 ?? ?? ?? ?? C7 44 24 0C ?? ?? ?? ?? 8B 54 24 54 8B 44 24 50 8B 74 24 4C 52 50 56 8D 4C 24 18 C7 44 24 50 03 00 00 00 E8 ?? ?? ?? ?? 8B 4C 24 30 8B 7C 24 34 33 C0 F3 AB 8B 4C }
	condition:
		any of them
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

Microsoft Defender for Endpoint — Custom indicators / advanced hunting
Email Gateway — Attachment scanning
File Share Monitoring — Periodic scanning of shared drives
YARA CLI — Manual threat hunting on endpoints

This rule contains 3 string patterns in its detection logic.

Source Rule

False Positive Guidance

Scenario: Scheduled system backup using rsync or tar with cryptographic operations
Filter/Exclusion: process.name != "rsync" AND process.name != "tar"
Scenario: Administrative task using openssl for certificate generation or key management
Filter/Exclusion: process.name != "openssl"
Scenario: Regularly executed script using gpg for secure data transfer or encryption
Filter/Exclusion: process.name != "gpg"
Scenario: Log rotation or data archival using logrotate with encryption via gpg
Filter/Exclusion: process.name != "logrotate" OR process.name != "gpg"
Scenario: Enterprise-grade password management tool like keepassxc performing encryption operations
Filter/Exclusion: process.name != "keepassxc"