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JsJaws Service

This Assemblyline service integrates components from six open-source projects:

  • Malware Jail, which provides a sandbox for semi-automatic Javascript malware analysis, deobfuscation and payload extraction.
  • Box.js, which is a sandbox tool for studying JavaScript malware.
  • JS-X-Ray, which is a tool for static analysis via SAST scanning.
  • Synchrony, which is a tool for deobfuscating JavaScript that has been obfuscated with obfuscator.io (https://obfuscator.io).
  • WScript Emulator, which is a tool for emulating/tracing the Windows Script Host functionality. The libraries that this tool uses have been integrated into the MalwareJail environment.
  • GootLoaderAutoJsDecode, which contains GootLoaderAutoJsDecode.js - a tool used for automatically decoding Gootloader files using static analysis.

Both sandboxes use Node VM under the hood for malware sandboxing, although Box.js prefers a modified version of Node VM called vm2.

Signatures

JsJaws also uses signatures for the majority of its scoring. These can be run on both the file contents and the sandbox outputs.

In the signatures folder are a bunch of signatures that will affect the scoring of samples. If you have a sample that needs better detection from this service, and you see output from the MalwareJail or Box.js tool that could have a signature written for it, then please make a Pull Request or share the sample! Let's improve this!

Service Parameters

  • allow_download_from_internet - [default: false]: See "Features included with Internet connectivity" section.
  • max_payloads_extracted - [default: 50]: Maximum payload files extracted if deep scan is turned off.
  • raise_malware_jail_exc - [default: false]: Raise a noisy exception if the MalwareJail tool errors, rather than silently letting the other tools output.
  • total_stdout_limit - [default: 10000]: The limit to number of stdout lines analyzed that werre captured from tools.
  • send_tool_stderr_to_pipe - [default: false]: If you don't want a tool's STDERR clogging up your terminal, set to true
  • max_gauntlet_runs - [default: 30]: The maximum number of times that the gauntlet should be run. This usually gets exceeded when a script writes randomly generated content to the DOM.

Submission Parameters

Generic parameters:

  • tool_timeout: The length of time we will allow both Malware Jail and Box.js to individually run for.
  • add_supplementary: If you want supplementary files to be added to the result, select this.
  • static_signatures: If you want the signatures to be run against the file contents as well rather than just the dynamic excecution output.
  • display_sig_marks: If you want the lines of code that caused the signatures to be raised to be displayed in the ResultSections.
  • static_analysis_only: If you do not want the file to be executed via Box.js and MalwareJail, and only with static analysis tools such as JS-X-Ray and Synchony, set this to "true".
  • ignore_stdout_limit: The service-level config total_stdout_limit will be ignored if you set this flag to "true".

Box.js parameters:

  • no_shell_error: For Box.js, select this flag if you want to.

MalwareJail parameters:

  • browser: Browser type for detonation.
  • wscript_only: By default, detonation takes place within a sandboxed browser. This option allows for the sample to be run in WScript only.
  • throw_http_exc: By selecting the throw_http_exc flag, the sandbox will throw an error in every network call. This is useful for attempting different code execution paths.
  • download_payload: If the service should allow the sample to download any payload from the Internet.
  • extract_function_calls: Files that each represent a Function Call can be noisy and not particularly useful. This flag turns on this extraction.
  • extract_eval_calls: Files that each represent a Eval Call can be noisy and not particularly useful. This flag turns on this extraction.
  • log_errors: Setting this parameter to true will insert a log of the exception into exception-catching clauses within a script. This is useful for debugging.
  • override_eval: Setting this parameter to true will use indirect links to eval calls. This is key when scoped variables are used. See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#direct_and_indirect_eval for more information. Use wisely.
  • file_always_exists: This parameter when set to true will cause the Scripting.FileSystemObject.FileExists method to return true.

Synchrony parameters:

  • enable_synchrony: Synchrony will most likely extract a "cleaned" file given any JavaScript file, which adds load to Assemblyline. So only enable this option if you are sure you want this.

Features included with Internet connectivity

jQuery Fetching

There have been samples that embed malicious code within standard jQuery libraries. If the service Docker container has access to the Internet, then we can fetch the actual jQuery library and compare the two files, determining the difference between them and then extracting the difference (aka malicious code). If the service Docker container does not have Internet access, then please set the docker_config value of allow_internet_access to False in the service_manifest.yml.

Assemblyline System Safelist

JsJaws-specific safelisted items

The file at al_config/system_safelist.yaml contains suggested safelisted values that can be added to the Assemblyline system safelist either by copy-and-pasting directly to the text editor on the page https://<Assemblyline Instance>/admin/tag_safelist or through the Assemblyline Client.