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X-FILES Stealer Evolution – An Analysis and Comparison Study

by Stuti Chaturvedi on August 4, 2022

Introduction

Zscaler’s ThreatLabz threat research team recently has spotted a new variant of the emerging X-FILES infostealer attack with enhanced features to exfiltrate sensitive information. X-FILES is a stealer that aims to steal sensitive information, including logins and financial data.

This blog will walk through the differences between the variants of X-FILES that we have observed until now, including differences in features, attack chains, and command-and-control (C2) patterns. Following our in-depth analysis, we’ll include a tabular feature comparison.

Interesting Facts

X-FILES stealer was first observed in March 2021 by 3xp0rt. A second variant was observed in the month of December, 2021 again by 3xp0rt.
In June 2022, ThreatLabz discovered a revised version of the stealer.
We have observed that the malware is mostly coming from phishing domains hosted on Russian IPs. Even the C2 panel (xfilesreborn[.]ru), for the latest variant, is hosted on Russian IP (46[.]8[.]153[.]137).
Recently, it has been seen that the threat actors are now exploiting the Follina vulnerability to deliver X-FILES stealer.
Like other infostealers, X-FILES aims to steal and exfiltrate sensitive information such as saved browser credentials, Crypto wallets, FTP credentials, and credit card information.
All the variants that we have stumbled upon are written using C# programming language, with new features added over time by the threat actors.
With the latest variant, the threat actors have switched to hiding interesting strings in base64 format rather than keeping it in plain text format. Changes in C2 patterns are also observed.

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Website Analysis

Our investigation has revealed a number of phishing websites that have been created and used by threat actors to distribute X-FILES stealer, with some still active.

In Scenario 1, the threat actors have distributed malware by pretending to be legitimate VPN software and Nitro Generator software, respectively. The downloaded files from the phishing websites are the X-FILES stealer.

Figure 1: Phishing websites 1 and 2

In Scenario 2, the main payload was downloaded by another malicious file hosted on a phishing website, which is a Russian domain associated with multiple malwares. As the domain is currently down, the following screenshot is taken from VirusTotal to show the relationship graph of the malicious domain.

Figure 2: Graphical representation of the malicious domain

Attack Chain

From the above scenarios, we have deduced the layout of the attack chain, illustrated in Figure 3.

Figure 3 : X-FILES attack chain

Technical Analysis

In this section, we will lay out the differences and additional features that we have seen amongst different variants of the stealer, obfuscation of interesting strings, and the C2 pattern of the latest variant.

Note:- For the purpose of studying differences in features, the following md5s were analyzed:

Latest Variant :123fd0237ca90f8a606009461fe2bb76 (June, 2022)
Second Variant : 1ed070e0d33db9f159a576e6430c273c (Dec, 2021)
Oldest Variant : 1b85d1786c4dde6ca1ee03a95e19531e(March, 2021)

System Information

Along with the information of IP, Country, Region, City, Operating System and Screen resolution (all of which were data collected by previous variants), the latest variant collects additional information about Windows Activation key, graphic cards, memory, processor, and antiviruses installed on the victim’s machine.

Figure 4: Code comparison

The PC info is collected in the following manner by the latest variant:

Figure 5: System Information collected by the latest variant

Wallet Information

As in the second variant (but not the first), the latest variant collects information about wallets and crypto wallet extensions. The uniqueness of this variant is that, unlike the second variant in which file paths were embedded in code, in this variant a list of targeted files gets downloaded from the C2 panel first and then the information is collected.

#Latest Variant

Figure 6: Paths of Wallets and crypto-wallets extensions from C2 server

#Second Variant

Figure 7: Paths of wallets and crypto-wallet extensions embedded in the code

Browser Information

The latest variant is, like earlier variants, capable of stealing saved browser information. However, the interesting thing is that in the latest variant, the targeted files are searched using a directory crawling technique at targeted folders. After getting a list of the matched patterns and file paths, the same are used for further stealing activities. It is worth noting that the paths are hard-coded in the second and the oldest variant.

# Latest variant

Figure 8: Latest variant code

#Second & Oldest variant

Figure 9: Older variants code

FTP Information

Both the latest and the second variant are capable of collecting FTP-related information, which wasn’t present in the oldest version. It is noteworthy that the second variant steals only Filezilla-related information, whereas the latest variant is also capable of stealing WinScp information, as shown in the below snapshot. Moreover, the latest variant is making use of XmlReader to get values, whereas in the second variant Regex is used to get the targeted information.

#Filezilla [Latest variant]

Figure 10: Filezilla Information stealing code in latest variant

#WinScp [Latest variant]

Figure 11: WinScp Information stealing code in latest variant

# Second variant

Figure 12: Filezilla Information stealing code in older variant

Strings Before and After Decryption

In order to hide the stuff at static level, the latest variant is now making use of base64 encoded strings (refer to the below snapshot), whereas in earlier versions the strings were in plain text format.

Figure 13: Base64 encoded and decoded strings.

C2 Communications

After performing stealing activities, the malware then exfiltrates data in JSON format to its embedded C2 server.

Note:- The attackers nowadays prefer using JSON as a data exchange mechanism as it can be used with any programming language and is easy to handle. Also, as it is a lightweight and structured notation, it is relatively easy to serialize and deserialize the data.

Figure 14: JSON data exfiltration – latest variant

The description of the C2 pattern of the latest variant is as follows:

Parameters

Description

cookies_x

Number of cookies information collected

country_x

Country Code

credit_x

Number of Credit cards information retrieved

ice_o_lator_hash

MD5 hash value of zip file

ip_x

IP information

passwords_x

Number of password retrieved

postal_x

Postal code

tag_x

Attacker’s hardcoded predefined value

user_id

Attacker’s hardcoded predefined value

wallets_x

Names of wallets for which information is collected

x_type

Type of coverage i.e full or partial

zipx

Base64 encrypted ZIP file consisted of files created by the stealer

In the second variant, the POST request is also made and sent with similar parameters, but not in JSON format.

Figure 15: JSON data exfiltration – second variant

In the oldest variant, the C2 pattern was simple and in readable format as shown below:

Figure 16: JSON data exfiltration – earliest variant

Features Comparison

Target Information

Latest Variant

[June, 2022]

Second Variant

[Dec, 2021]

Oldest Variant

[March, 2021]

System Information

Yes*

Yes

Browser Information

Yes*

Yes

Wallets Information

Yes

Telegram Information

Yes

FTP Information

Yes*

Yes

Files Collection

Yes

Steam Information

Yes

Discord Tokens

Yes

ScreenShot

Yes

Note: ”*” implies additional features have been added

Conclusion

It seems that the threat actors behind the X-FILES stealer campaign are continuously making changes or enhancement in the code and delivery mechanisms to steal a wider variety of sensitive user and system information. In the future, we anticipate additional variants that continue in this trend. Zscaler’s ThreatLabz team is continuously monitoring the campaign and will publish any new findings.

MITRE ATT&CK AND TTP Mapping

Tactic

T1189

Drive-by Compromise

T1140

Deobfuscate/Decode Files or Information

T1082

System Information Discovery

T1083

File and Directory Discovery

T1005

Data from Local System