According to Trend Micro, this is a backdoor abusing the Dropbox API, used by threat actor Earth Yako.
rule win_transbox_auto { meta: author = "Felix Bilstein - yara-signator at cocacoding dot com" date = "2026-01-05" version = "1" description = "Detects win.transbox." 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.transbox" malpedia_rule_date = "20260105" malpedia_hash = "19b79e7cab4eaf532122e5b45a77dd8f6bb5cc79" malpedia_version = "20251219" 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 = { 8bf1 8bda 57 837e1408 } // n = 4, score = 100 // 8bf1 | mov esi, ecx // 8bda | mov ebx, edx // 57 | push edi // 837e1408 | cmp dword ptr [esi + 0x14], 8 $sequence_1 = { 33d2 8b5d08 8955fc 8945e4 8945d8 8955e8 395104 } // n = 7, score = 100 // 33d2 | xor edx, edx // 8b5d08 | mov ebx, dword ptr [ebp + 8] // 8955fc | mov dword ptr [ebp - 4], edx // 8945e4 | mov dword ptr [ebp - 0x1c], eax // 8945d8 | mov dword ptr [ebp - 0x28], eax // 8955e8 | mov dword ptr [ebp - 0x18], edx // 395104 | cmp dword ptr [ecx + 4], edx $sequence_2 = { 8d4db8 e8???????? 8b35???????? 3b35???????? } // n = 4, score = 100 // 8d4db8 | lea ecx, [ebp - 0x48] // e8???????? | // 8b35???????? | // 3b35???????? | $sequence_3 = { 74e0 6a04 8d85f8fbffff 50 6a49 6a00 ff964c010000 } // n = 7, score = 100 // 74e0 | je 0xffffffe2 // 6a04 | push 4 // 8d85f8fbffff | lea eax, [ebp - 0x408] // 50 | push eax // 6a49 | push 0x49 // 6a00 | push 0 // ff964c010000 | call dword ptr [esi + 0x14c] $sequence_4 = { f7fb 56 8bf0 bbe0077e00 8bc3 2bc6 83f801 } // n = 7, score = 100 // f7fb | idiv ebx // 56 | push esi // 8bf0 | mov esi, eax // bbe0077e00 | mov ebx, 0x7e07e0 // 8bc3 | mov eax, ebx // 2bc6 | sub eax, esi // 83f801 | cmp eax, 1 $sequence_5 = { 57 8bf1 33ff 6804010000 57 8d4610 c706???????? } // n = 7, score = 100 // 57 | push edi // 8bf1 | mov esi, ecx // 33ff | xor edi, edi // 6804010000 | push 0x104 // 57 | push edi // 8d4610 | lea eax, [esi + 0x10] // c706???????? | $sequence_6 = { 894608 85c0 74e0 6a04 8d85f8fbffff 50 6a49 } // n = 7, score = 100 // 894608 | mov dword ptr [esi + 8], eax // 85c0 | test eax, eax // 74e0 | je 0xffffffe2 // 6a04 | push 4 // 8d85f8fbffff | lea eax, [ebp - 0x408] // 50 | push eax // 6a49 | push 0x49 $sequence_7 = { 8985d8faffff 8b8544faffff 8985dcfaffff 397e14 7204 8b16 } // n = 6, score = 100 // 8985d8faffff | mov dword ptr [ebp - 0x528], eax // 8b8544faffff | mov eax, dword ptr [ebp - 0x5bc] // 8985dcfaffff | mov dword ptr [ebp - 0x524], eax // 397e14 | cmp dword ptr [esi + 0x14], edi // 7204 | jb 6 // 8b16 | mov edx, dword ptr [esi] $sequence_8 = { e8???????? 83c420 b812000000 33d2 8a900c530110 6683bc96760a000000 7506 } // n = 7, score = 100 // e8???????? | // 83c420 | add esp, 0x20 // b812000000 | mov eax, 0x12 // 33d2 | xor edx, edx // 8a900c530110 | mov dl, byte ptr [eax + 0x1001530c] // 6683bc96760a000000 | cmp word ptr [esi + edx*4 + 0xa76], 0 // 7506 | jne 8 $sequence_9 = { 8d8504e1ffff 89bd44e1ffff 50 33db 89b540e1ffff 53 } // n = 6, score = 100 // 8d8504e1ffff | lea eax, [ebp - 0x1efc] // 89bd44e1ffff | mov dword ptr [ebp - 0x1ebc], edi // 50 | push eax // 33db | xor ebx, ebx // 89b540e1ffff | mov dword ptr [ebp - 0x1ec0], esi // 53 | push ebx condition: 7 of them and filesize < 288768 }
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