Dofoil Downloader Update Adds XOR-, RC4-Based Encryption

This blog was written by Sanchit Karve.

The Dofoil downloader (found in the wild since 2011) occasionally updates itself with new features and encryption techniques to hide communications with its control servers. The latest iteration uses a variation of XOR and RC4 algorithms similar to previous variants to encrypt the list of control servers within the binary and encrypt all traffic with the server.

The Dofoil sample we analyzed (D8AB2694A8AAA0FA729AC0FCC93767A2) contained many antianalysis tricks common to previous versions:

• Code obfuscation
• Self-modifying code
• Sleep for an infinite time if sample is named sample.exe
• Sleep for an infinite time if volume serial number of C:\ is 0xCD1A40 (anti-ThreatExpert) or 0x70144646 (unknown)
• CPU-specific checks
• Virtual machine presence based on an entry in HKLM\System\CurrentControlSet\Services\Disk\Enum
• Presence of sandboxing, etc.
• BeingDebugged and NTGlobalFlags checks in the process environment block

As in previous versions, a GET request to msn.com is made to confirm an active Internet connection on an infected machine.

After the confirmation, the sample proceeds to decrypt the location of its control servers, which are encrypted and stored in a lookup table.

The encrypted strings for the control server domain names are visible in high-entropy areas:

To decrypt, the samples use an XOR-based encryption scheme. The encrypted data conforms to the following format:

One decrypted byte is represented with two encrypted bytes in this scheme. Two bytes are read from the encrypted data and individually XORed with the one-byte key. The difference between the two values yields the decrypted byte. The size_of_encrypted_data field is a bit misleading because it contains an intentionally large value that the sample corrects during its decryption. When decrypted, the control servers are visible:

The sample we examined contains three control servers: hxxp://zoneserveryu(788|789|790)[dot]com

All communications with a server take place over HTTP POST requests; the commands are encrypted with an RC4-based algorithm. Unlike previous variants, in which the MD5 of the infected computer along with the volume serial number of C:\ was passed as the login parameter, the new variant uses a 160-bit hash composed of five components. For example, for the following command string, the login field translates to five DWORDs:

It’s unclear why the malware authors introduced redundancy in the hash. The command is encrypted, prefixed with its size and four-byte encryption key, and sent to the server like so:

When the command is decrypted with the following algorithm, we can see the original command:

The initial request gets a 404/not found response with an encoded body from the control server.

The body consists of encoded commands from the server along with a plug-in file (executable DLL) encrypted with the same algorithm listed above except that it uses a 13-byte key. It decrypts to:

The plug-in file usually has an exported function “Work” and could contain functionality for additional commands and features.

When the sample wishes to download additional malware, it passes a file number using the file parameter:

The server responds with a 404 response but passes on new malware in the content of the response. It also passes its own command in the “Vary” header.

The sample is equipped to handle four commands: to write downloaded files to disk and execute them, remove itself, silently register DLLs, and inject content directly into memory.

The sample returns the result of its command to the server. For example, if the server responds with “0-AAAAAA,” the sample writes the downloaded sample to disk (in %APPDATA% or %TEMP%) and executes it. If it succeeds, it responds with the run=ok command. If the sample fails to execute, it sends run=fail.

Eventually, the sample downloads password stealers and spam bots, which send spam claiming to be from Amazon.com, and embeds an attachment containing the original sample to spread itself:

McAfee customers are protected from this threat by Downloader-FAFW and other signatures.

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