win.formbook (Back to overview)

Formbook

URLhaus        

FormBook contains a unique crypter RunPE that has unique behavioral patterns subject to detection. It was initially called "Babushka Crypter" by Insidemalware.

References
http://blog.inquest.net/blog/2018/06/22/a-look-at-formbook-stealer/
http://cambuz.blogspot.de/2016/06/form-grabber-2016-cromeffoperathunderbi.html
http://www.vkremez.com/2018/01/lets-learn-dissecting-formbook.html
https://blog.talosintelligence.com/2018/06/my-little-formbook.html
https://thisissecurity.stormshield.com/2018/03/29/in-depth-formbook-malware-analysis-obfuscation-and-process-injection/
https://usualsuspect.re/article/formbook-hiding-in-plain-sight
https://www.arbornetworks.com/blog/asert/formidable-formbook-form-grabber/
https://www.botconf.eu/wp-content/uploads/2018/12/2018-R-Jullian-In-depth-Formbook-Malware-Analysis.pdf
https://www.cyberbit.com/blog/endpoint-security/formbook-research-hints-large-data-theft-attack-brewing/
https://www.fireeye.com/blog/threat-research/2017/10/formbook-malware-distribution-campaigns.html
https://www.peerlyst.com/posts/how-to-analyse-formbook-a-new-malware-as-a-service-sudhendu?trk=explore_page_resources_recent
https://www.peerlyst.com/posts/how-to-understand-formbook-a-new-malware-as-a-service-sudhendu?
https://www.virusbulletin.com/virusbulletin/2019/01/vb2018-paper-inside-formbook-infostealer/
Yara Rules
[TLP:WHITE] win_formbook_auto (20190620 | autogenerated rule brought to you by yara-signator)
rule win_formbook_auto {

    meta:
        author = "Felix Bilstein - yara-signator at cocacoding dot com"
        date = "2019-07-05"
        version = "1"
        description = "autogenerated rule brought to you by yara-signator"
        tool = "yara-signator 0.2a"
        malpedia_reference = "https://malpedia.caad.fkie.fraunhofer.de/details/win.formbook"
        malpedia_version = "20190620"
        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 / approach 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 = { 8b7d08 6a00 6a01 56 57 e8???????? }
            // n = 6, score = 1500
            //   8b7d08               | mov                 edi, dword ptr [ebp + 8]
            //   6a00                 | push                0
            //   6a01                 | push                1
            //   56                   | push                esi
            //   57                   | push                edi
            //   e8????????           |                     

        $sequence_1 = { 33c0 85c9 74?? 8d5604 }
            // n = 4, score = 1500
            //   33c0                 | xor                 eax, eax
            //   85c9                 | test                ecx, ecx
            //   74??                 |                     
            //   8d5604               | lea                 edx, [esi + 4]

        $sequence_2 = { 52 56 e8???????? 83c410 eb?? 6a0d }
            // n = 6, score = 1500
            //   52                   | push                edx
            //   56                   | push                esi
            //   e8????????           |                     
            //   83c410               | add                 esp, 0x10
            //   eb??                 |                     
            //   6a0d                 | push                0xd

        $sequence_3 = { 51 8b08 6a00 8d550c }
            // n = 4, score = 1500
            //   51                   | push                ecx
            //   8b08                 | mov                 ecx, dword ptr [eax]
            //   6a00                 | push                0
            //   8d550c               | lea                 edx, [ebp + 0xc]

        $sequence_4 = { 8b55fc 50 03cf 51 52 }
            // n = 5, score = 1500
            //   8b55fc               | mov                 edx, dword ptr [ebp - 4]
            //   50                   | push                eax
            //   03cf                 | add                 ecx, edi
            //   51                   | push                ecx
            //   52                   | push                edx

        $sequence_5 = { 03c3 33ff 83c408 8945f4 894df8 c745e468000000 }
            // n = 6, score = 1500
            //   03c3                 | add                 eax, ebx
            //   33ff                 | xor                 edi, edi
            //   83c408               | add                 esp, 8
            //   8945f4               | mov                 dword ptr [ebp - 0xc], eax
            //   894df8               | mov                 dword ptr [ebp - 8], ecx
            //   c745e468000000       | mov                 dword ptr [ebp - 0x1c], 0x68

        $sequence_6 = { e8???????? 83c418 85c0 78?? 8b95f4f7ffff }
            // n = 5, score = 1500
            //   e8????????           |                     
            //   83c418               | add                 esp, 0x18
            //   85c0                 | test                eax, eax
            //   78??                 |                     
            //   8b95f4f7ffff         | mov                 edx, dword ptr [ebp - 0x80c]

        $sequence_7 = { 33c0 3bdf 0f8????????? 56 }
            // n = 4, score = 1500
            //   33c0                 | xor                 eax, eax
            //   3bdf                 | cmp                 ebx, edi
            //   0f8?????????         |                     
            //   56                   | push                esi

    condition:
        7 of them
}
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