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rule win_mailto_auto { meta: author = "Felix Bilstein - yara-signator at cocacoding dot com" date = "2026-09-17" version = "1" description = "Detects win.mailto." 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.mailto" malpedia_rule_date = "20260916" malpedia_hash = "53a643781c18e08f4a50949af60172d837a0b6f6" malpedia_version = "20260917" 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 = { 89742418 6689442416 e8???????? 6a01 8d4c2418 51 8b4058 } // n = 7, score = 400 // 89742418 | mov dword ptr [esp + 0x18], esi // 6689442416 | mov word ptr [esp + 0x16], ax // e8???????? | // 6a01 | push 1 // 8d4c2418 | lea ecx, [esp + 0x18] // 51 | push ecx // 8b4058 | mov eax, dword ptr [eax + 0x58] $sequence_1 = { 51 ff742434 8d4c2434 51 } // n = 4, score = 400 // 51 | push ecx // ff742434 | push dword ptr [esp + 0x34] // 8d4c2434 | lea ecx, [esp + 0x34] // 51 | push ecx $sequence_2 = { 885c2412 85ed 0f847d000000 8d4e32 0f1f440000 } // n = 5, score = 400 // 885c2412 | mov byte ptr [esp + 0x12], bl // 85ed | test ebp, ebp // 0f847d000000 | je 0x83 // 8d4e32 | lea ecx, [esi + 0x32] // 0f1f440000 | nop dword ptr [eax + eax] $sequence_3 = { e8???????? 83c404 5d 53 e8???????? 83c404 5b } // n = 7, score = 400 // e8???????? | // 83c404 | add esp, 4 // 5d | pop ebp // 53 | push ebx // e8???????? | // 83c404 | add esp, 4 // 5b | pop ebx $sequence_4 = { 33c0 5b 83c410 c3 8b442410 8b4c241c 8b5078 } // n = 7, score = 400 // 33c0 | xor eax, eax // 5b | pop ebx // 83c410 | add esp, 0x10 // c3 | ret // 8b442410 | mov eax, dword ptr [esp + 0x10] // 8b4c241c | mov ecx, dword ptr [esp + 0x1c] // 8b5078 | mov edx, dword ptr [eax + 0x78] $sequence_5 = { 33c0 5e c3 8d46ff 5e c3 8d44240c } // n = 7, score = 400 // 33c0 | xor eax, eax // 5e | pop esi // c3 | ret // 8d46ff | lea eax, [esi - 1] // 5e | pop esi // c3 | ret // 8d44240c | lea eax, [esp + 0xc] $sequence_6 = { 55 56 57 e8???????? 83c40c 85c0 741a } // n = 7, score = 400 // 55 | push ebp // 56 | push esi // 57 | push edi // e8???????? | // 83c40c | add esp, 0xc // 85c0 | test eax, eax // 741a | je 0x1c $sequence_7 = { 83c40c c7400c01000000 8d4704 5f 5e 5b c3 } // n = 7, score = 400 // 83c40c | add esp, 0xc // c7400c01000000 | mov dword ptr [eax + 0xc], 1 // 8d4704 | lea eax, [edi + 4] // 5f | pop edi // 5e | pop esi // 5b | pop ebx // c3 | ret $sequence_8 = { 75d1 8b6c2414 55 e8???????? 8b6c242c 83c404 } // n = 6, score = 400 // 75d1 | jne 0xffffffd3 // 8b6c2414 | mov ebp, dword ptr [esp + 0x14] // 55 | push ebp // e8???????? | // 8b6c242c | mov ebp, dword ptr [esp + 0x2c] // 83c404 | add esp, 4 $sequence_9 = { 89842484000000 8944246c 8b442420 03c5 c1c707 33d0 897c2410 } // n = 7, score = 400 // 89842484000000 | mov dword ptr [esp + 0x84], eax // 8944246c | mov dword ptr [esp + 0x6c], eax // 8b442420 | mov eax, dword ptr [esp + 0x20] // 03c5 | add eax, ebp // c1c707 | rol edi, 7 // 33d0 | xor edx, eax // 897c2410 | mov dword ptr [esp + 0x10], edi condition: 7 of them and filesize < 180224 }
rule win_mailto_w0 { meta: copyright = "(c) 2020 Crowdstrike Inc." author = "Crowdstrike" description = "Detects the Netwalker ransomware" reports = "CSIT-20081" source = "https://go.crowdstrike.com/rs/281-OBQ-266/images/ReportCSIT-20081e.pdf" version = "202004281747" last_modified = "2020-04-28" malware_family = "Netwalker" malpedia_reference = "https://malpedia.caad.fkie.fraunhofer.de/details/win.mailto" malpedia_rule_date = "20210224" malpedia_hash = "" malpedia_version = "20210224" malpedia_sharing = "TLP:WHITE" strings: $salsaconst = "expand 32-byte kexpand 16-byte k" $ins_getapi = {55 8b ec a1 ?? ?? ?? ?? 5d c3} $ins_crc32 = {25 20 83 b8 ed 33 d0} $ins_push1137 = {68 39 05 00 00 68 69 7a 00 00} $ins_rc4 = {8b 45 ( e? | f? ) 83 c0 01 33 d2 b9 00 01 00 00 f7 f1 89 55} $in_c25519 = {6a 00 68 41 db 01 00} condition: 3 of them }
rule win_mailto_w1 { meta: copyright = "(c) 2020 Crowdstrike Inc." author = "Crowdstrike" description = "Detects the Netwalker ransomware" reports = "CSIT-20081" source = "https://go.crowdstrike.com/rs/281-OBQ-266/images/ReportCSIT-20081e.pdf" version = "202004281748" last_modified = "2020-04-28" malware_family = "Netwalker" malpedia_reference = "https://malpedia.caad.fkie.fraunhofer.de/details/win.mailto" malpedia_rule_date = "20210224" malpedia_hash = "" malpedia_version = "20210224" malpedia_sharing = "TLP:WHITE" strings: $ = "namesz" fullword $ = "crmask" fullword $ = "idsz" fullword $ = "lend" fullword $ = "lfile" fullword $ = "mpk" fullword $ = "namesz" fullword $ = "pspath" fullword $ = "rwsz" fullword $ = "spsz" fullword $ = "svcwait" fullword $ = "unlocker" fullword $ = "onion1" fullword condition: 10 of them }
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