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.

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: Data exfiltration - second variant

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

Figure 16: Data exfiltration - earliest variant

Features Comparison

Target Information	Latest Variant [June, 2022]	Second Variant [Dec, 2021]	Oldest Variant [March, 2021]
System Information	Yes*	Yes	Yes
Browser Information	Yes*	Yes*	Yes
Wallets Information	Yes	Yes	No
Telegram Information	Yes	Yes	No
FTP Information	Yes*	Yes	No
Files Collection	Yes	Yes	Yes
Steam Information	Yes	Yes	No
Discord Tokens	Yes	Yes	No
ScreenShot	Yes	Yes	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

ID	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
T1047	Windows Management Instrumentation
T1003	OS Credential Dumping
T1018	Remote System Discovery
T1552.002	Credentials in Registry
T1518.001	Security Software Discovery