Suspicious Alternate Data Stream (ADS) File Creation

hello@craftedsignal.io — Fri, 26 Jan 2024 18:00:00 +0000

This detection focuses on identifying the creation of Alternate Data Streams (ADS) on Windows systems, a technique often employed by adversaries to conceal malicious code or data within seemingly benign files. Attackers leverage scripting engines and command interpreters to write ADS to various file types, including executables, documents, and media files. This activity is uncommon in legitimate workflows, making it a valuable indicator of potential compromise. The rule is designed to trigger on file creation events where the process creating the file is a known script or command interpreter (cmd.exe, powershell.exe, etc.) and the target file has a suspicious extension. The detection excludes common legitimate ADS usage patterns. This technique is used for defense evasion, allowing malware to persist without being easily detected by traditional security measures.

Attack Chain

An attacker gains initial access to a Windows system (e.g., through phishing or exploiting a vulnerability).
The attacker uses a command interpreter (cmd.exe, powershell.exe, etc.) or scripting engine (wscript.exe, cscript.exe) to execute malicious code.
The malicious code creates an Alternate Data Stream (ADS) on a targeted file (e.g., an executable, document, or image). The targeted file’s extension could be pdf, dll, exe, dat, etc.
The attacker hides malicious code or data within the ADS, making it less visible to standard file system scans and security tools. The ADS is written to a file path using the C:\\*:\* syntax.
The attacker may rename or clean up any staging files to further conceal their activity.
The attacker can then execute the hidden code within the ADS, or use the ADS to store configuration data for later use.
The attacker maintains persistence by using the ADS to store and execute malicious code, bypassing typical file-based security measures.
The ultimate goal is to maintain unauthorized access to the system, potentially leading to data exfiltration, lateral movement, or other malicious activities.

Impact

Successful exploitation allows attackers to hide malicious code within legitimate files, evading detection by traditional security measures. This can lead to prolonged persistence on compromised systems, enabling data theft, ransomware deployment, or other malicious activities. While the specific number of victims is unknown, this technique is broadly applicable across Windows environments, potentially affecting a wide range of organizations.

Recommendation

Deploy the Sigma rule Suspicious ADS File Creation via Cmd to detect ADS creation events initiated by cmd.exe.
Deploy the Sigma rule Suspicious ADS File Creation via PowerShell to detect ADS creation events initiated by powershell.exe.
Enable Sysmon Event ID 15 (FileCreateStreamHash) to provide detailed information about ADS creation events, as referenced in the rule’s setup instructions.
Investigate any alerts generated by these rules, focusing on the file paths, creating processes, and command-line arguments involved, as detailed in the rule’s triage and analysis notes.

Rundll32 Execution with DLL Stored in Alternate Data Stream (ADS)

hello@craftedsignal.io — Mon, 08 Jan 2024 15:30:00 +0000

Rundll32 is a legitimate Windows utility used to execute DLLs. However, adversaries can abuse this functionality to execute malicious code while evading detection. This technique involves storing a malicious DLL within an Alternate Data Stream (ADS) of a file. ADS allows hiding data within existing files, making it less likely to be discovered by standard file system scans. When rundll32.exe is then used to execute the DLL from the ADS, it can bypass application whitelisting and other security measures, as the execution appears to originate from the trusted rundll32.exe process. This technique has been observed across various threat actors seeking to establish persistence or execute arbitrary code.

Attack Chain

An attacker gains initial access to the system through methods like phishing or exploiting a vulnerability.
The attacker uploads a malicious DLL to the target system.
The attacker uses a command-line utility to write the DLL into an Alternate Data Stream (ADS) of an existing file, such as a text file or image. For example: echo "DLL content" > legitimate_file.txt:malicious.dll.
The attacker uses rundll32.exe to execute the DLL stored in the ADS. The command typically looks like: rundll32.exe "C:\ads\file.txt:ADSDLL.dll",DllMain.
Rundll32.exe loads and executes the malicious DLL from the ADS.
The malicious DLL performs its intended actions, such as establishing persistence, downloading additional payloads, or exfiltrating data.
The attacker may use additional techniques to further conceal their activity, such as obfuscating the command line or using process injection.

Impact

Successful exploitation allows arbitrary code execution on the targeted system. Attackers can use this technique to establish persistence, escalate privileges, bypass security controls, and deploy further malware. The use of ADS makes detection more challenging, as the malicious DLL is hidden within a seemingly benign file. This can lead to data breaches, system compromise, and potential financial losses.

Recommendation

Enable Sysmon process creation logging to capture the command-line arguments used with rundll32.exe (as used in the Sigma rules below).
Deploy the Sigma rules in this brief to your SIEM to detect suspicious rundll32.exe executions from ADS.
Monitor for unusual file modifications that involve writing data to alternate data streams.
Implement application whitelisting to restrict the execution of unauthorized executables.

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Suspicious Alternate Data Stream (ADS) File Creation

Attack Chain

Impact

Recommendation

Rundll32 Execution with DLL Stored in Alternate Data Stream (ADS)

Attack Chain

Impact

Recommendation