Actor(s): APT31
According to PTSecurity, this stealer harvests system information which is then RC4 encrypted and Base64 encoded before sending it to the C2 server.
rule win_stealer_0x3401_auto { meta: author = "Felix Bilstein - yara-signator at cocacoding dot com" date = "2023-01-25" version = "1" description = "Detects win.stealer_0x3401." info = "autogenerated rule brought to you by yara-signator" tool = "yara-signator v0.6.0" signator_config = "callsandjumps;datarefs;binvalue" malpedia_reference = "https://malpedia.caad.fkie.fraunhofer.de/details/win.stealer_0x3401" malpedia_rule_date = "20230124" malpedia_hash = "2ee0eebba83dce3d019a90519f2f972c0fcf9686" malpedia_version = "20230125" malpedia_license = "CC BY-SA 4.0" malpedia_sharing = "TLP:WHITE" /* DISCLAIMER * The strings used in this rule have been automatically selected from the * disassembly of memory dumps and unpacked files, using YARA-Signator. * The code and documentation is published here: * https://github.com/fxb-cocacoding/yara-signator * As Malpedia is used as data source, please note that for a given * number of families, only single samples are documented. * This likely impacts the degree of generalization these rules will offer. * Take the described generation method also into consideration when you * apply the rules in your use cases and assign them confidence levels. */ strings: $sequence_0 = { eb69 68???????? 8d4dc0 e8???????? eb5a 68???????? 8d4dc0 } // n = 7, score = 100 // eb69 | jmp 0x6b // 68???????? | // 8d4dc0 | lea ecx, [ebp - 0x40] // e8???????? | // eb5a | jmp 0x5c // 68???????? | // 8d4dc0 | lea ecx, [ebp - 0x40] $sequence_1 = { 8b8da07dffff 40 3d00100000 722a f6c11f } // n = 5, score = 100 // 8b8da07dffff | mov ecx, dword ptr [ebp - 0x8260] // 40 | inc eax // 3d00100000 | cmp eax, 0x1000 // 722a | jb 0x2c // f6c11f | test cl, 0x1f $sequence_2 = { 8b1485c8710210 8a4c1a2d f6c104 7419 8a441a2e 80e1fb } // n = 6, score = 100 // 8b1485c8710210 | mov edx, dword ptr [eax*4 + 0x100271c8] // 8a4c1a2d | mov cl, byte ptr [edx + ebx + 0x2d] // f6c104 | test cl, 4 // 7419 | je 0x1b // 8a441a2e | mov al, byte ptr [edx + ebx + 0x2e] // 80e1fb | and cl, 0xfb $sequence_3 = { 0f8472010000 6a00 6a00 6a03 6a00 6a00 ffb5a4f5ffff } // n = 7, score = 100 // 0f8472010000 | je 0x178 // 6a00 | push 0 // 6a00 | push 0 // 6a03 | push 3 // 6a00 | push 0 // 6a00 | push 0 // ffb5a4f5ffff | push dword ptr [ebp - 0xa5c] $sequence_4 = { 894310 8b4614 894314 c7461407000000 837e1408 c7461000000000 } // n = 6, score = 100 // 894310 | mov dword ptr [ebx + 0x10], eax // 8b4614 | mov eax, dword ptr [esi + 0x14] // 894314 | mov dword ptr [ebx + 0x14], eax // c7461407000000 | mov dword ptr [esi + 0x14], 7 // 837e1408 | cmp dword ptr [esi + 0x14], 8 // c7461000000000 | mov dword ptr [esi + 0x10], 0 $sequence_5 = { 8b55ac f20f5ac0 51 8b4da8 f30f59c1 f30f59c1 f30f59c1 } // n = 7, score = 100 // 8b55ac | mov edx, dword ptr [ebp - 0x54] // f20f5ac0 | cvtsd2ss xmm0, xmm0 // 51 | push ecx // 8b4da8 | mov ecx, dword ptr [ebp - 0x58] // f30f59c1 | mulss xmm0, xmm1 // f30f59c1 | mulss xmm0, xmm1 // f30f59c1 | mulss xmm0, xmm1 $sequence_6 = { 83c6ff 7849 53 57 53 57 e8???????? } // n = 7, score = 100 // 83c6ff | add esi, -1 // 7849 | js 0x4b // 53 | push ebx // 57 | push edi // 53 | push ebx // 57 | push edi // e8???????? | $sequence_7 = { 74cb 8b1d???????? 0f1f4000 6666660f1f840000000000 33c0 c745ec07000000 c745e800000000 } // n = 7, score = 100 // 74cb | je 0xffffffcd // 8b1d???????? | // 0f1f4000 | nop dword ptr [eax] // 6666660f1f840000000000 | nop word ptr [eax + eax] // 33c0 | xor eax, eax // c745ec07000000 | mov dword ptr [ebp - 0x14], 7 // c745e800000000 | mov dword ptr [ebp - 0x18], 0 $sequence_8 = { 75f9 2bca 51 53 8d4c241c e8???????? 53 } // n = 7, score = 100 // 75f9 | jne 0xfffffffb // 2bca | sub ecx, edx // 51 | push ecx // 53 | push ebx // 8d4c241c | lea ecx, [esp + 0x1c] // e8???????? | // 53 | push ebx $sequence_9 = { 8b5d08 56 57 8bf9 897dfc 8b7704 3bde } // n = 7, score = 100 // 8b5d08 | mov ebx, dword ptr [ebp + 8] // 56 | push esi // 57 | push edi // 8bf9 | mov edi, ecx // 897dfc | mov dword ptr [ebp - 4], edi // 8b7704 | mov esi, dword ptr [edi + 4] // 3bde | cmp ebx, esi condition: 7 of them and filesize < 357376 }
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