Excel 4 Macros: "Abnormal Sheet Visibility"
Excel 4 macros are composed of formulas (commands) and values stored inside a sheet.
Each sheet in a spreadsheet can be "visible", "hidden" or "very hidden". Malware authors will often make Excel 4 macro sheets hidden or very hidden.
In .xls files, spreadsheet data is stored in the Workbook stream as BIFF records. There is a BIFF record for sheets: the BOUNDSHEET record. The byte value at position 5 in a BOUNDSHEET record defines the visibility of a sheet: visible (0x00), hidden (0x01) or very hidden (0x02):
Encoding the visibility of a sheet is done with the 2 least significant bits. Per Microsoft's documentation, the 6 more significant bits are unused bits and must be ignored. In spreadsheets created with Excel, these bits are set to 0.
From time to time, I find malicious Excel 4 macro documents, where these bits are not zero:
oledump's plugin_biff will report this: "reserved bits not zero".
The "visibility" value is 0x0A, that's 0x08 + 0x02: thus the sheet is very hidden (0x02).
Excel has no problem at all opening a spreadsheet like this (the unused bits must be ignored). But if you use or develop detection rules like YARA, Suricata, ... ; be aware that these unused bits can be set to 1 in stead of 0.
You might wonder: 2 bits to encode visibility. Visible (0x00), hidden (0x01) or very hidden (0x02).
What about 0x03?
When a sheet's visibility is set to 0x03 (I do this by patching the .xls with a binary editor), my tests with Excel 2016 and 2019 show that an Excel 4 macro sheet will behave as "very hidden", and the macro code will be executed.
However, before a user is prompted to enable macros, that user will have to click through extra warnings:
Didier Stevens
Senior handler
Microsoft MVP
blog.DidierStevens.com DidierStevensLabs.com
Comments
Anonymous
Dec 3rd 2022
9 months ago
Anonymous
Dec 3rd 2022
9 months ago
<a hreaf="https://technolytical.com/">the social network</a> is described as follows because they respect your privacy and keep your data secure. The social networks are not interested in collecting data about you. They don't care about what you're doing, or what you like. They don't want to know who you talk to, or where you go.
<a hreaf="https://technolytical.com/">the social network</a> is not interested in collecting data about you. They don't care about what you're doing, or what you like. They don't want to know who you talk to, or where you go. The social networks only collect the minimum amount of information required for the service that they provide. Your personal information is kept private, and is never shared with other companies without your permission
Anonymous
Dec 26th 2022
9 months ago
Anonymous
Dec 26th 2022
9 months ago
<a hreaf="https://defineprogramming.com/the-public-bathroom-near-me-find-nearest-public-toilet/"> nearest public toilet to me</a>
<a hreaf="https://defineprogramming.com/the-public-bathroom-near-me-find-nearest-public-toilet/"> public bathroom near me</a>
Anonymous
Dec 26th 2022
9 months ago
<a hreaf="https://defineprogramming.com/the-public-bathroom-near-me-find-nearest-public-toilet/"> nearest public toilet to me</a>
<a hreaf="https://defineprogramming.com/the-public-bathroom-near-me-find-nearest-public-toilet/"> public bathroom near me</a>
Anonymous
Dec 26th 2022
9 months ago
Anonymous
Dec 26th 2022
9 months ago
https://defineprogramming.com/
Dec 26th 2022
9 months ago
distribute malware. Even if the URL listed on the ad shows a legitimate website, subsequent ad traffic can easily lead to a fake page. Different types of malware are distributed in this manner. I've seen IcedID (Bokbot), Gozi/ISFB, and various information stealers distributed through fake software websites that were provided through Google ad traffic. I submitted malicious files from this example to VirusTotal and found a low rate of detection, with some files not showing as malware at all. Additionally, domains associated with this infection frequently change. That might make it hard to detect.
https://clickercounter.org/
https://defineprogramming.com/
Dec 26th 2022
9 months ago
rthrth
Jan 2nd 2023
8 months ago