Automatic file extraction and scanning

• Automatic file extraction and scanning
  • User interface

Malcolm can leverage Zeek’s knowledge of network protocols to automatically detect file transfers and extract those files from PCAPs as Zeek processes them. This behavior can be enabled globally by modifying the ZEEK_EXTRACTOR_MODE variable in zeek.env, or on a per-upload basis for PCAP files uploaded via the browser-based upload form when Analyze with Zeek is selected.

To specify which files should be extracted, the following values are acceptable in ZEEK_EXTRACTOR_MODE:

• none: no file extraction
• interesting: extraction of files with mime types of common attack vectors
• notcommtxt: extraction of all files except common plain text files
• mapped: extraction of files with recognized mime types
• known: extraction of files for which any mime type can be determined
• all: extract all files

Extracted files can be examined through any of the following methods:

• submitting file hashes to VirusTotal; to enable this method, specify the VTOT_API2_KEY environment variable in zeek-secret.env
• scanning files with ClamAV; to enable this method, set the EXTRACTED_FILE_ENABLE_CLAMAV environment variable in zeek.env to true
• scanning files with Yara; to enable this method, set the EXTRACTED_FILE_ENABLE_YARA environment variable in zeek.env to true
• scanning PE (portable executable) files with Capa; to enable this method, set the EXTRACTED_FILE_ENABLE_CAPA environment variable in zeek.env to true

Files flagged via any of these methods will be logged as Zeek signatures.log entries, and can be viewed in the Signatures dashboard in OpenSearch Dashboards.

The EXTRACTED_FILE_PRESERVATION environment variable in zeek.env determines the behavior for preservation of Zeek-extracted files:

• quarantined: preserve only flagged files in ./zeek-logs/extract_files/quarantine
• all: preserve flagged files in ./zeek-logs/extract_files/quarantine and all other extracted files in ./zeek-logs/extract_files/preserved
• none: preserve no extracted files

The EXTRACTED_FILE_HTTP_SERVER_… environment variables in zeek.env and zeek-secret.env configure access to the Zeek-extracted files path through the means of a simple HTTPS directory server accessible at https://localhost/extracted-files/ if connecting locally. Beware that Zeek-extracted files may contain malware. As such, these files may be optionally ZIP archived (without a password or password-protected according to the WinZip AES encryption specification) or encrypted (to be decrypted using openssl, e.g., openssl enc -aes-256-cbc -d -in example.exe.encrypted -out example.exe) upon download. In other words:

• to disable the extracted files server:
  • EXTRACTED_FILE_HTTP_SERVER_ENABLE=false
• to enable the extracted file server:
  • EXTRACTED_FILE_HTTP_SERVER_ZIP=true
  • downloaded files are zipped, without a password:
    • EXTRACTED_FILE_HTTP_SERVER_ZIP=true
    • EXTRACTED_FILE_HTTP_SERVER_KEY=
  • downloaded files are zipped, AES-encrypted with a password:
    • EXTRACTED_FILE_HTTP_SERVER_ZIP=true
    • EXTRACTED_FILE_HTTP_SERVER_KEY=xxxxxxxxxxxxx
  • downloaded files are OpenSSL AES-256-CBC-compatibly encrypted:
    • EXTRACTED_FILE_HTTP_SERVER_ZIP=false
    • EXTRACTED_FILE_HTTP_SERVER_KEY=xxxxxxxxxxxxx
  • downloaded files are downloaded as-is, without archival or compression:
    • EXTRACTED_FILE_HTTP_SERVER_ZIP=false
    • EXTRACTED_FILE_HTTP_SERVER_KEY=

User interface

The files extracted by Zeek and the data about those files can be accessed through several of Malcolm’s user interfaces.

• The Files dashboard summarizes the file transfers observed in network traffic. The Extracted File Downloads table provides download links for the extracted files matching the currently applied filters. Note that the presence of these links don’t necessarily imply that the files they represent are available: depending on factors such as file preservation settings (above) and retention policies, files that were extracted and scanned may no longer be available. When this is the case, clicking one of the file download links will result in a “file not found” error. If one of these links refers to a file that was extracted and scanned on a Hedgehog Linux network sensor, Malcolm must be able to communicate with that sensor in order to retrieve and download the file.

• Viewing logs from Zeek’s files.log (e.g., event.provider == zeek && event.dataset == files), the Arkime session detail’s Extracted Filename URL field can be clicked for a context menu item to download the extracted file, if it was preserved as described above.

• Malcolm provides an extracted files directory listing to browse and download Zeek-extracted files. This interface is available at https://localhost/extracted-files/ if connecting locally. The Zeek uid and fuid values associated with these files and the sessions from which they were extracted are listed in the IDs column as filter links back into Dashboards. Similarly, files extracted and preserved on a Hedgehog Linux network sensor can be accessed at https://localhost/hh-extracted-files/X.X.X.X/, where X.X.X.X represents the IP address or hostname of the sensor (e.g., https://localhost/hh-extracted-files/192.168.122.57/ if the sensor’s IP address were 192.168.122.57).