Solarwinds供应链攻击武器SUNBURST和TEARDROP分析

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