Zegost - analysis of the Chinese backdoor

1 decade ago 120

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Interesting features:
- Rootkit on board;
- Dropped driver has ~100MB size on disk;
- Contains AVKill code;
- Injected DLL as a payload.

https://twitter.com/artem_i_baranov/status/283497092427694080

Original dropper fingerprints:

SHA256: 030340a429180da10df3dee1092701aa3b9e38dac45445badb457de44c198061
SHA1: ecb9626b9a2cd0c75a078f1c17cbead251380ba6
MD5: 48c093b0e24d65838e1ee0f5b7b4337e
File size: 98304 bytes

Dropper is detected by almost all vendors:

Resource section is interesting, because it stores the rootkit driver in packed state (APLib).

Point of driver loading by dropper is trivial - using of ntdll!ZwLoadDriver.

For loading the driver the first time, it creates the same file and service name.

\Registry\Machine\System\CurrentControlSet\Services\HideKey
C:\recycler\hidekey.txt

To mislead some static detectors and analyzing tools, the trojan uses the trick of dropping rootkit with total file size ~100MB. Actual size ~70KB.

A similar technique is used in Darkmegi rootkit, but the total size is smaller.

Resource with 104 number in dropper contains reg-file for setup driver.

Point of its loading.

To ensure the survival after reboot, the dropper creates AppSvcHlp.sys - a copy of hidekey.txt in standard drivers directory.

Driver/rootkit:

SHA256: 061e60a81dd01207b08f5243eb54fb9fe2e492d51e9e691f18ae9581607a625e
SHA1: 19889145b193926b8fa2827c5eff966b450b3a19
MD5: 2d613204d44fb0455ef0fa5384d5352c
File size: 75776 bytes

Driver body contains malicious dll [antivshlp32.dll] and has main purpose dll installation:

Driver targeted to disruption a lot of AV products: Qihoo 360, Kaspersky AV, ESET Nod32, Malwarebytes Anti-Malware.

DLL:

SHA256: bf876fef476ec8d7e712422d0411099834810747e447102818f0af93591b53eb
SHA1: e7356ab76d223ce18845942bf62cf55123b9b686
MD5: e72b0a5d85f1e9d3413745d2b696b714
File size: 40960 bytes

Autorun from:

DLL has on board another exe-file injected into IE. This exe being stored in resource section and packed with APLib.

Injection chronicles:

Injected module - final payload:

SHA256: 5577a888fa4477c47a3bf3159b5e46de16a75582ba4888d38b5f2a8b527a9c18
SHA1: dae0f132166d008878491baa65424e221792669f
MD5: 8e8d86259b9e94a8febc36407964cfe3
File size: 14336 bytes

Performs response to remote server from context of shadow IE:
Remote server - hxxp://iyy.conimes.com http://whois.domaintools.com/conimes.com

Communication:

With help of:

Request:
http://iyy.conimes.com/cgi/online.asp?hostname=ComputerName&httptype=[1][not httptunnel]

Other requests:

/cgi/binup.asp
/cgi/textup.asp
http://%s/cgi/%s.txt
http://%s/cgi/command.asp?hostname=%s&command=test&del=delfile
http://%s/cgi/update.exe

posted by https://twitter.com/artem_i_baranov

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