SYMBOLCOMMON_NAMEaka. SYNONYMS
win.crytox (Back to overview)

Crytox


Ransomware.

References
2022-09-21ZscalerRomain Dumont
@online{dumont:20220921:technical:3feb7d0, author = {Romain Dumont}, title = {{Technical Analysis of Crytox Ransomware}}, date = {2022-09-21}, organization = {Zscaler}, url = {https://www.zscaler.com/blogs/security-research/technical-analysis-crytox-ransomware}, language = {English}, urldate = {2022-09-30} } Technical Analysis of Crytox Ransomware
Crytox
Yara Rules
[TLP:WHITE] win_crytox_auto (20230125 | Detects win.crytox.)
rule win_crytox_auto {

    meta:
        author = "Felix Bilstein - yara-signator at cocacoding dot com"
        date = "2023-01-25"
        version = "1"
        description = "Detects win.crytox."
        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.crytox"
        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 = { f76c2418 01c1 8b442450 11d3 f7ac24a8000000 01c1 8b442428 }
            // n = 7, score = 100
            //   f76c2418             | add                 esi, dword ptr [esp + 0x78]
            //   01c1                 | mov                 edi, ecx
            //   8b442450             | mul                 dword ptr [esp + 8]
            //   11d3                 | mov                 edi, edx
            //   f7ac24a8000000       | mov                 esi, eax
            //   01c1                 | add                 edi, ebx
            //   8b442428             | mov                 ebx, dword ptr [esp + 0xb0]

        $sequence_1 = { ff15???????? 8b45dc 83ec1c 89742414 895c2410 897c2408 c744241c00000000 }
            // n = 7, score = 100
            //   ff15????????         |                     
            //   8b45dc               | mov                 dword ptr [esp + 8], 1
            //   83ec1c               | sub                 esp, 0x10
            //   89742414             | test                eax, eax
            //   895c2410             | mov                 edi, eax
            //   897c2408             | mov                 edx, dword ptr [ebp - 0x4c]
            //   c744241c00000000     | jle                 0xc4

        $sequence_2 = { dee9 d9ac24a4000000 db9c24a0000000 d9ac24a6000000 8b8424a0000000 39d0 0f8e3a080000 }
            // n = 7, score = 100
            //   dee9                 | fld                 dword ptr [ebp - 4]
            //   d9ac24a4000000       | fld1                
            //   db9c24a0000000       | fxch                st(1)
            //   d9ac24a6000000       | fucomip             st(1)
            //   8b8424a0000000       | fstp                st(0)
            //   39d0                 | fsubp               st(1)
            //   0f8e3a080000         | fstp                dword ptr [ebp - 4]

        $sequence_3 = { e8???????? c9 c3 55 89e5 83ec58 8b450c }
            // n = 7, score = 100
            //   e8????????           |                     
            //   c9                   | mov                 dword ptr [ebp + 0x14], 2
            //   c3                   | lea                 eax, [edx + edi]
            //   55                   | jmp                 0x1e
            //   89e5                 | cmp                 byte ptr [ebx + 3], 0
            //   83ec58               | je                  0x2a
            //   8b450c               | mov                 dword ptr [ebp + 0x14], 0x18

        $sequence_4 = { e9???????? 0fbf55fe 8b45f4 83c224 8b44900c 8945e0 8b45f4 }
            // n = 7, score = 100
            //   e9????????           |                     
            //   0fbf55fe             | mov                 dword ptr [esp], 0xffff
            //   8b45f4               | sub                 esp, 4
            //   83c224               | cmp                 eax, dword ptr [esp + 0x40]
            //   8b44900c             | movups              xmmword ptr [ecx + 0x10], xmm0
            //   8945e0               | add                 ecx, 0x100
            //   8b45f4               | movups              xmmword ptr [ecx - 0xd0], xmm5

        $sequence_5 = { e8???????? e9???????? 8b4508 05c4020000 c744240806000000 c7442404b4e86a00 890424 }
            // n = 7, score = 100
            //   e8????????           |                     
            //   e9????????           |                     
            //   8b4508               | xor                 edx, edx
            //   05c4020000           | mov                 word ptr [ebp - 0x1a], dx
            //   c744240806000000     | movzx               eax, word ptr [ebp - 0x3c]
            //   c7442404b4e86a00     | sub                 ax, word ptr [ebp - 0x1a]
            //   890424               | jne                 0xc0

        $sequence_6 = { e9???????? 891c24 c744240402a00000 e8???????? 895c2420 83c414 5b }
            // n = 7, score = 100
            //   e9????????           |                     
            //   891c24               | sub                 eax, 1
            //   c744240402a00000     | imul                eax, eax, 0x64
            //   e8????????           |                     
            //   895c2420             | add                 eax, edx
            //   83c414               | mov                 eax, dword ptr [ebp + 0x2126c]
            //   5b                   | mov                 edi, dword ptr [esp + 0x8c]

        $sequence_7 = { e8???????? e9???????? 8b8424e0000000 8da82abc0100 8b44242c 8d500c 83c04c }
            // n = 7, score = 100
            //   e8????????           |                     
            //   e9????????           |                     
            //   8b8424e0000000       | je                  0x13c
            //   8da82abc0100         | movzx               eax, byte ptr [eax + 0x80]
            //   8b44242c             | mov                 dword ptr [esp], eax
            //   8d500c               | cmp                 dword ptr [eax + 0x138], 0
            //   83c04c               | mov                 ebx, eax

        $sequence_8 = { e9???????? 890424 e8???????? 89c1 8b44246c 99 f77c247c }
            // n = 7, score = 100
            //   e9????????           |                     
            //   890424               | add                 esp, 0x7c
            //   e8????????           |                     
            //   89c1                 | pop                 ebx
            //   8b44246c             | pop                 esi
            //   99                   | pop                 edi
            //   f77c247c             | pop                 ebp

        $sequence_9 = { e8???????? eb01 90 8b45f4 8b400c 890424 e8???????? }
            // n = 7, score = 100
            //   e8????????           |                     
            //   eb01                 | mov                 dword ptr [esp + 0xc], eax
            //   90                   | lea                 eax, [esp + 0xc8]
            //   8b45f4               | mov                 dword ptr [esp + 0x68], eax
            //   8b400c               | mov                 dword ptr [esp + 0x64], 0
            //   890424               | xor                 edi, edi
            //   e8????????           |                     

    condition:
        7 of them and filesize < 6156288
}
[TLP:WHITE] win_crytox_w0   (20220930 | Detect variants of Crytox Ransomware)
import "pe"
rule win_crytox_w0 {
  meta:
    description = "Detect variants of Crytox Ransomware"
    author = "Jake Goldi"
    date = "2022-09-29"
    packed_hash1 = "32eef267a1192a9a739ccaaae0266bc66707bb64768a764541ecb039a50cba67"
    hash2 = "11ea0d7e0ebe15b8147d39e72773221d11c2cf84e2d8d6164102c65e797eef6d"
    hash3 = "68fae79a2eca125090bd2a8badc46ed4324c38f2ff24db702d09c3d7687e0047"
    hash4 = "a0a6c2937b6a8b2bc1214ace8255adc6992b553b9e740c3fe1543e089e8437aa"
    source = "https://raw.githubusercontent.com/taogoldi/YARA/main/ransomware/crytox_ransom.yara"
    
    version="1.0"
    phase = "experimental"
    url = "https://www.zscaler.com/blogs/security-research/technical-analysis-crytox-ransomware"
    malware = "Win64.Ransom.Crytox"
    
    malpedia_reference = "https://malpedia.caad.fkie.fraunhofer.de/details/win.crytox"
    malpedia_rule_date = "20220930"
    malpedia_hash = ""
    malpedia_version = "20220930"
    malpedia_license = "CC BY-SA 4.0"
    malpedia_sharing = "TLP:WHITE"
    
    
  strings:
    $s1 = "utox" wide ascii nocase
    /*
        hash 2: 11ea0d7e0ebe15b8147d39e72773221d11c2cf84e2d8d6164102c65e797eef6d

        FF 15 C9 22 00 00                       call    cs:GlobalAlloc
        48 89 85 58 FF FF FF                    mov     [rbp+hMem], rax ; unk_5F2520
        4C 8B E0                                mov     r12, rax
        
        48 8B C8                                mov     rcx, rax

        E8 DD 13 00 00                          call    sub_1402E940E
        48 C7 C1 C0 01 00 00                    mov     rcx, 448
        49 8D 94 24 08 04 00 00                 lea     rdx, [r12+408h]

        4C 8D 05 29 20 00 00                    lea     r8, unk_1402EA070
 
        4C 8D 8D A4 FD FF FF                    lea     r9, [rbp+var_25C]
        E8 A2 11 00 00                          call    sub_1402E91F5
        48 8B 8D 58 FF FF FF                    mov     rcx, [rbp+hMem] ; hMem
        FF 15 90 22 00 00                       call    cs:GlobalFree 

        -------------   

        hash 3: 68fae79a2eca125090bd2a8badc46ed4324c38f2ff24db702d09c3d7687e0047

        FF 15 C9 22 00 00                       call    cs:GlobalAlloc
        48 89 85 58 FF FF FF                    mov     [rbp+hMem], rax
        4C 8B E0                                mov     r12, rax
        
        90                                      nop
        50                                      push    rax
        59                                      pop     rcx
        
        E8 E2 13 00 00                          call    sub_1402E9413
        48 C7 C1 C0 01 00 00                    mov     rcx, 1C0h
        49 8D 94 24 08 04 00 00                 lea     rdx, [r12+408h]

        4C 8D 05 29 20 00 00                    lea     r8, unk_1402EA070
        
        4C 8D 8D A4 FD FF FF                    lea     r9, [rbp+var_25C]
        E8 A7 11 00 00                          call    sub_1402E91FA
        48 8B 8D 58 FF FF FF                    mov     rcx, [rbp+hMem] ; hMem
        FF 15 90 22 00 00                       call    cs:GlobalFree

        ------------- 

        hash 4: a0a6c2937b6a8b2bc1214ace8255adc6992b553b9e740c3fe1543e089e8437aa

        FF 15 C9 22 00 00                       call    cs:GlobalAlloc
        48 89 85 58 FF FF FF                    mov     [rbp+hMem], rax
        4C 8B E0                                mov     r12, rax
        48 8B C8                                mov     rcx, rax
        E8 61 12 00 00                          call    sub_1402E9292
        48 C7 C1 C0 01 00 00                    mov     rcx, 1C0h
        49 8D 94 24 08 04 00 00                 lea     rdx, [r12+408h]
        4C 8D 05 29 20 00 00                    lea     r8, unk_1402EA070
        4C 8D 8D A4 FD FF FF                    lea     r9, [rbp+var_25C]
        E8 26 10 00 00                          call    sub_1402E9079
        48 8B 8D 58 FF FF FF                    mov     rcx, [rbp+hMem] ; hMem
        FF 15 90 22 00 00                       call    cs:GlobalFre



    */

    $op1 = { FF 15 C9 22 00 00 48 89 85 58 FF FF FF 4C 8B E0 } 
    $op2 = { E8 ?? ?? 00 00 48 C7 C1 C0 01 00 00 49 8D 94 24 08 04 00 00 }
    $op3 = { 4C 8D 05 29 20 00 00 }
    $op4 = { 4C 8D 8D A4 FD FF FF E8 ?? 1? 00 00 48 8B 8D 58 FF FF FF FF 15 90 22 00 00 }

condition:
    uint16(0) == 0x5a4d and filesize < 5000KB and ((all of ($s*)) and (all of ($op*)))

}
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