0x01 事件描述
2020年12月，FireEye发现一起针对许多组织（主要是针对美国科技公司）的大规模网络攻击已经持续了几个月。这次网络攻击具有一定程度的复杂性，导致外国政府迅速介入，这次攻击中的战术和技术细节非常出色。攻击者没有进行常规的网络钓鱼或者漏洞利用攻击，而是进行了精心设计的供应链攻击。在本文中，我们重点分析了Solarwinds供应链中使用的后门（SUNBURST）及其攻击载荷之一（TEARDROP）的一些功能，包括对SUNBURST的哈希编码字符串进行彻底的模糊处理以及对TEARDROP的控制流和解密方法的分析。
0x02 技术细节分析
SUNBURST后门的技术细节已经被很多公司和组织分析透彻，这使我们可以专注于我们感兴趣的一个功能上进行深度分析，我们将分析后门精心设计的免杀方法。
SUNBURST所采用的规避技术在概念上类似于沙箱规避。沙盒规避的目的是确保恶意软件不会在检测恶意软件的虚拟机上运行。
后门文件的黑名单不是以可读字符串数组的形式给出的。而是将可读字符串替换为FNV-1a哈希值。通常的恶意软件都会采用此功能，，除了最近的一些恶意软件会使用FNV-1a，而不是SHA256和CRC32校验和，但这个后门厉害的地方在于它保持了代码合法性。
后门作者对仅将进程和服务列入黑名单并不满意，他们还将某些设备驱动程序和IP地址的整个范围列入了黑名单，通过将受感染机器的IP转换为域名，该功能用于将所有内部Solarwinds域都列入了黑名单。这告诉我们，不仅攻击者决定使用Solarwinds攻击Uber到达目标，而且还详细了解了Solarwinds内部网络的拓扑结构。总体而言，嵌入在SUNBURST中的哈希编码字符串列表是SUNBURST加入黑名单的200多个域。结果表明，即使是最先进，最顽强的攻击者也不认为自己是无敌的，他们相信自己足够立于不败之地。
可在Addeneum I中找到SUNBURST中包含的FNV-1a混淆字符串的完整列表。
毫无疑问，这种攻击是一项令人难以置信的大规模黑客技术成就。250多个组织被SolarWinds后门感染，其中一半在美国，在推送恶意代码时，他们确保遵循Solarwinds的编码约定；他们在其初始有效载荷中包含了“逻辑漏洞”，以将恶意活动从初始感染推迟整整两周，并进行动态分析。他们将横向移动限制为使用被盗但有效的用户凭据进行的合法操作。出于所有这些原因，值得注意的是，这种Übermensch层的攻击被用来部署TEARDROP，这仅仅是人为的恶意软件删除程序。
在发现之时，TEARDROP是一种新的攻击载荷，从未见过，甚至可能是针对这种攻击量身定制的。它仅针对少数几个目标部署，TEARDROP在内存中运行，但是它确实注册了Windows服务，还会编辑注册表。
TEARDROP的控制流程非常简单，在服务执行期间调用DLL导出函数Tk_CreateImageType，该函数将JPEG图像写入当前目录，其名称各不相同；赛门铁克披露过upbeat_anxiety.jpg和festive_computer.jpg，FireEye披露过gracious_truth.jpg。在未经训练的人看来，这些似乎是人为命名的。但是更有可能是通过将两个硬编码单词连接在一起而在任意计算机上随机生成的，而该机器是用来编译该恶意软件的。
然后TEARDROP使用自制密码和硬编码密0x96钥执行解密。使用以下方法来实现该过程：
从较高的角度看，这就像某种自制PRNG，它决定每次使用哪个密钥字节。当动态运行时，生成的键索引可以简单地映射到0, 1, 2, ..., 149, 0, 1, ...等等。事实证明，这不是PRNG，而是对模运算的编译器优化实现。
一旦了解了优化，解密代码就等同于以下内容：
CTXT_START_OFFSET = 0x30 KEY_LENGTH = 0x96 PREV_CTXT_BYTE_INITIAL_DEFAULT = 0xcc prev_ctxt_byte = PREV_CTXT_BYTE_INITIAL_DEFAULT for i, ctxt_byte in enumerate(ciphertext[CTXT_START_OFFSET:]): ptxt_byte = ctxt_byte ^ (prev_ctxt_byte ^ key[i % KEY_LENGTH]) plaintext[i] = ptxt_byte prev_ctxt_byte = ctxt_byte
因此，原始加密是一个简单的XOR，然后还将每个密文字节与前一个密文字节进行XOR，没有人会随机针对内存中二进制Blob的Kasiski攻击，以期遇到XOR密文。
解密后的有效负载具有以下自定义header格式，其读取方式类似于正确的PE header：
第一幅图显示了在TEARDROP上找到的解密BEACON有效载荷的代码，第二幅图显示了一个已知BEACON样本的代码，PE的基本地址一样。
TEARDROP的BEACON有效载荷
Cobalt Strike的BEACON（sha256：3cfbf519913d703a802423e6e3fb734abf8297971caccc7ae45df172196b6e84）
TEARDROP的构建方式摈弃了BEACON，它是Cobalt Strike 附带的有效载荷。根据Cobalt Strike网站的说法，BEACON的目的是为高级攻击者建模。它支持跨各种协议的网络横向移动，用于C2签入的“被动”和“主动”模式，以及可用于模仿其他恶意软件或与目标网络的合法流量融合的可配置C2通信方案。
0x03 分析总结
如果我们在这次攻击事件发生后不得不做个总结，那就是“纵深防御”。似乎我们一直以来都是这样的陈词滥调，但它源于美国国家安全局（NSA）在2012年发表的一篇论文，其背后的原理是合理且相关的：不要将所有精力都花在建一堵墙上。没有完美的围墙，总有一天，有人会穿过这面墙。
通过分析SUNBURST和TEARDROP的二进制文件，我们了解到，即使这个非常成功的攻击也存在问题。为了确保这一点，必须创建大量的域和进程黑名单。
0x04 附录I：SUNBURST中包含的FNV-1a混淆字符串列表
Processes: 2597124982561782591 = apimonitor-x64 2600364143812063535 = apimonitor-x86 13464308873961738403 = autopsy64 4821863173800309721 = autopsy 12969190449276002545 = autoruns64 3320026265773918739 = autoruns 12094027092655598256 = autorunsc64 10657751674541025650 = autorunsc 11913842725949116895 = binaryninja 5449730069165757263 = blacklight 292198192373389586 = cff explorer 12790084614253405985 = cutter 5219431737322569038 = de4dot 15535773470978271326 = debugview 7810436520414958497 = diskmon 13316211011159594063 = dnsd 13825071784440082496 = dnspy 14480775929210717493 = dotpeek32 14482658293117931546 = dotpeek64 8473756179280619170 = dumpcap 3778500091710709090 = evidence center 8799118153397725683 = exeinfope 12027963942392743532 = fakedns 576626207276463000 = fakenet 7412338704062093516 = ffdec 682250828679635420 = fiddler 13014156621614176974 = fileinsight 18150909006539876521 = floss 10336842116636872171 = gdb 12785322942775634499 = hiew32demo 13260224381505715848 = hiew32 17956969551821596225 = hollows_hunter 8709004393777297355 = idaq64 14256853800858727521 = idaq 8129411991672431889 = idr 15997665423159927228 = ildasm 10829648878147112121 = ilspy 9149947745824492274 = jd-gui 3656637464651387014 = lordpe 3575761800716667678 = officemalscanner 4501656691368064027 = ollydbg 10296494671777307979 = pdfstreamdumper 14630721578341374856 = pe-bear 4088976323439621041 = pebrowse64 9531326785919727076 = peid 6461429591783621719 = pe-sieve32 6508141243778577344 = pe-sieve64 10235971842993272939 = pestudio 2478231962306073784 = peview 9903758755917170407 = peview 14710585101020280896 = ppee 13611814135072561278 = procdump64 2810460305047003196 = procdump 2032008861530788751 = processhacker 27407921587843457 = procexp64 6491986958834001955 = procexp 2128122064571842954 = procmon 10484659978517092504 = prodiscoverbasic 8478833628889826985 = py2exedecompiler 10463926208560207521 = r2agent 7080175711202577138 = rabin2 8697424601205169055 = radare2 7775177810774851294 = ramcapture64 16130138450758310172 = ramcapture 506634811745884560 = reflector 18294908219222222902 = regmon 3588624367609827560 = resourcehacker 9555688264681862794 = retdec-ar-extractor 5415426428750045503 = retdec-bin2llvmir 3642525650883269872 = retdec-bin2pat 13135068273077306806 = retdec-config 3769837838875367802 = retdec-fileinfo 191060519014405309 = retdec-getsig 1682585410644922036 = retdec-idr2pat 7878537243757499832 = retdec-llvmir2hll 13799353263187722717 = retdec-macho-extractor 1367627386496056834 = retdec-pat2yara 12574535824074203265 = retdec-stacofin 16990567851129491937 = retdec-unpacker 8994091295115840290 = retdec-yarac 13876356431472225791 = rundotnetdll 14968320160131875803 = sbiesvc 14868920869169964081 = scdbg 106672141413120087 = scylla_x64 79089792725215063 = scylla_x86 5614586596107908838 = shellcode_launcher 3869935012404164040 = solarwindsdiagnostics 3538022140597504361 = sysmon64 14111374107076822891 = sysmon64 7982848972385914508 = task explorer 8760312338504300643 = task explorer-x64 17351543633914244545 = tcpdump 7516148236133302073 = tcpvcon 15114163911481793350 = tcpview 15457732070353984570 = vboxservice 16292685861617888592 = win32_remote 10374841591685794123 = win64_remotex64 3045986759481489935 = windbg 17109238199226571972 = windump 6827032273910657891 = winhex64 5945487981219695001 = winhex 8052533790968282297 = winobj 17574002783607647274 = wireshark 3341747963119755850 = x32dbg 14193859431895170587 = x64dbg 17439059603042731363 = xwforensics64 17683972236092287897 = xwforensics 700598796416086955 = redcloak 3660705254426876796 = avgsvc 12709986806548166638 = avgui 3890794756780010537 = avgsvca 2797129108883749491 = avgidsagent 3890769468012566366 = avgsvcx 14095938998438966337 = avgwdsvcx 11109294216876344399 = avgadminclientservice 1368907909245890092 = afwserv 11818825521849580123 = avastui 8146185202538899243 = avastsvc 2934149816356927366 = aswidsagent 13029357933491444455 = aswidsagenta 6195833633417633900 = aswengsrv 2760663353550280147 = avastavwrapper 16423314183614230717 = bccavsvc 2532538262737333146 = psanhost 4454255944391929578 = psuaservice 6088115528707848728 = psuamain 13611051401579634621 = avp 18147627057830191163 = avpui 17633734304611248415 = ksde 13581776705111912829 = ksdeui 7175363135479931834 = tanium 3178468437029279937 = taniumclient 13599785766252827703 = taniumdetectengine 6180361713414290679 = taniumendpointindex 8612208440357175863 = taniumtracecli 8408095252303317471 = taniumtracewebsocketclient64 Services: windows Defender: 5183687599225757871 = msmpeng 917638920165491138 = windefend Windows Sense: 10063651499895178962 = mssense 16335643316870329598 = sense Windows Sensor: 10501212300031893463 = microsoft.tri.sensor 155978580751494388 = microsoft.tri.sensor.updater NIST: 17204844226884380288 = cavp Carbon Black: 5984963105389676759 = cb 11385275378891906608 = carbonblack 13693525876560827283 = carbonblackk 17849680105131524334 = cbcomms 18246404330670877335 = cbstream CrowdStrike: 8698326794961817906 = csfalconservice 9061219083560670602 = csfalconcontainer 11771945869106552231 = csagent 9234894663364701749 = csdevicecontrol 8698326794961817906 = csfalconservice FireEye: 15695338751700748390 = xagt 640589622539783622 = xagtnotif 9384605490088500348 = fe_avk 6274014997237900919 = fekern 15092207615430402812 = feelam 3320767229281015341 = fewscservice ESET: 3200333496547938354 = ekrn 14513577387099045298 = eguiproxy 607197993339007484 = egui 15587050164583443069 = eamonm 9559632696372799208 = eelam 4931721628717906635 = ehdrv 2589926981877829912 = ekrnepfw 17997967489723066537 = epfwwfp 14079676299181301772 = ekbdflt 17939405613729073960 = epfw F-SECURE: 521157249538507889 = fsgk32st 14971809093655817917 = fswebuid 10545868833523019926 = fsgk32 15039834196857999838 = fsma32 14055243717250701608 = fssm32 5587557070429522647 = fnrb32 12445177985737237804 = fsaua 17978774977754553159 = fsorsp 17017923349298346219 = fsav32 17624147599670377042 = f-secure gatekeeper handler starter 16066651430762394116 = f-secure network request broker 13655261125244647696 = f-secure webui daemon 3421213182954201407 = fsma 14243671177281069512 = fsorspclient 16112751343173365533 = f-secure gatekeeper 3425260965299690882 = f-secure hips 9333057603143916814 = fsbts 3413886037471417852 = fsni 7315838824213522000 = fsvista 13783346438774742614 = f-secure filter 2380224015317016190 = f-secure recognizer 3413052607651207697 = fses 3407972863931386250 = fsfw 10393903804869831898 = fsdfw 3421197789791424393 = fsms 541172992193764396 = fsdevcon Drivers: 17097380490166623672 = cybkerneltracker.sys 15194901817027173566 = atrsdfw.sys 12718416789200275332 = eaw.sys 18392881921099771407 = rvsavd.sys 3626142665768487764 = dgdmk.sys 12343334044036541897 = sentinelmonitor.sys 397780960855462669 = hexisfsmonitor.sys 6943102301517884811 = groundling32.sys 13544031715334011032 = groundling64.sys 11801746708619571308 = safe-agent.sys 18159703063075866524 = crexecprev.sys 835151375515278827 = psepfilter.sys 16570804352575357627 = cve.sys 1614465773938842903 = brfilter.sys 12679195163651834776 = brcow_x_x_x_x.sys 2717025511528702475 = lragentmf.sys 17984632978012874803 = libwamf.sys domain names: 1109067043404435916 = swdev.local 15267980678929160412 = swdev.dmz 8381292265993977266 = lab.local 3796405623695665524 = lab.na 8727477769544302060 = emea.sales 10734127004244879770 = cork.lab 11073283311104541690 = dev.local 4030236413975199654 = dmz.local 7701683279824397773 = pci.local 5132256620104998637 = saas.swi 5942282052525294911 = lab.rio 4578480846255629462 = lab.brno 16858955978146406642 = apac.lab HTTP: 8873858923435176895 = expect 6116246686670134098 = content-type 2734787258623754862 = accept 6116246686670134098 = content-type 7574774749059321801 = user-agent 1475579823244607677 = 100-continue 11266044540366291518 = connection 9007106680104765185 = referer 13852439084267373191 = keep-alive 14226582801651130532 = close 15514036435533858158 = if-modified-since 16066522799090129502 = date
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