Excel with misleading macrosheet name spreading Zloader

SonicWall Capture Labs Threats Research team has been observing modifications in the techniques being used to distribute ZLoader using MS-Excel file. It all began around Jan 2020, when the first campaign was seen using XLM macro instead of the commonly used VBA macro. Since then, we have observed significant improvements like addition of evasion and sandbox bypassing techniques through XLM macro as already described in our previous blog.

This variant uses OOXML format based MS-Excel file. In the OOXML format based MS-Excel file, usually the XLM macro sheets are stored inside “macrosheets”  folder. The sheets are named either “Sheet<digit>.xml” or “intlsheet<digit>.xml”.  This variant uses a completely different folder and file name to store macro sheet. The macro sheet and folder are named “foto.png” and “bioxr” respectively, as shown in the below image:

Fig-1: XLM MacroSheet

Engines looking for macro sheets specifically inside “macrosheets” folder might fail to identify these files as XLM based Macro files. After careful inspection of the “workbook.xml.rels” file, we found that both the folder and the file name for the macro sheet are misleading as shown below:

Fig-2: workbook.xml.rels

Sample Analysis:

Upon opening the file, the user is displayed instructions to enable macro as shown below:

Fig-3: Excel File

The sample contains two sheets, one is a hidden macro sheet. It has a defined name “Aut0_Open”, which enables macro execution as soon as the file is opened. Font size in the sheet is kept small ( “2”) to inhibit reading of the content.

Fig-4: hidden macro sheet

Upon execution of XLM macro, payload belonging to Zloader family is downloaded and saved as C:\<random>\<random>\ServApi.exe

SonicWall Capture ATP protects against this threat as shown below:

Fig-5: Capture Report

Indicators of Compromise:

SHA256 of malicious Excel Files:

• 12047db782ec585e6c577248607f504869d166077ee33a4d455a66370ea6f9b4
• 189735e1fde7511cd9cedfb317f544971411691192c25ca36147998e492753d7
• 18d1cc06d96c741e0c21c1ceea194f37ca5941264cc0a26d89cba8e09c132485
• 18e6f2976642ca37a4e81358ea8da608b5d34a50b1954d0c3041e902ae23e192
• 18f33627843309fdef93e7edc7c24c856912d19a9622c2647165247e1aa16386
• 1a03a110254fe594cb08e5db44b5dd7d00ebedf5bf6944e2aff7807195b7bff6
• 1b29453e458e36c8b8b17371d4cb254a7cea4f1b035dc2d308e75ca1829766f3
• 20af190130ad3ac40a01df57341929d968616ef717bc9e691308ccaf4f41a683
• 211eb2bbaf1e1dcadd3f10c6c77ff2243f8690b1cd9f9dd5218d48d1b4edd02e
• 224b3303d4f32bc71fa3322d9385d004293459ed74885179178d04c880dbf6f8
• 2335e54b766bf5dc2a9078b995a4878ff350aa39d83ef7eabe77433c5c26e998

Network Connection:

• safedot[.]digital
  • Domain registred on 25-Feb-2021

Files:

• C:\<random>\<random>\ServApi.exe

Security News

The SonicWall Capture Labs Threat Research Team gathers, analyzes and vets cross-vector threat information from the SonicWall Capture Threat network, consisting of global devices and resources, including more than 1 million security sensors in nearly 200 countries and territories. The research team identifies, analyzes, and mitigates critical vulnerabilities and malware daily through in-depth research, which drives protection for all SonicWall customers. In addition to safeguarding networks globally, the research team supports the larger threat intelligence community by releasing weekly deep technical analyses of the most critical threats to small businesses, providing critical knowledge that defenders need to protect their networks.