MetaDefender Core V4.x includes the option to block or allow files by creating a BLACKLIST or WHITELIST.
The user can select files to be blocked or allowed based on:
The documentation is here https://onlinehelp.opswat.com/corev4/3.6.2._Workflow_template_configuration.html
The conventional usage of this feature would be, to create a list of files to be blocked or allowed, by any of the three selectors mentioned above (filetype groups, mime-types, file names) or a combination of them.
Using filetype groups VS. MIME-types VS file extensions
When possible, It is better to use Filetype groups over MIME types, and MIME types over File names.
It is shorter to define, leaves less space for human error and can also leverage OPSWAT's file detection mechanism, so that even if an imposter file has the extension .doc but in reality, it is a .exe it will be treated as .exe
Using Regular Expressions
The rules we create can consist of literal strings but can also include wild cards in the form of Regular Expressions.
For example, if we use the string : ^.*\.docx$ in the "Blacklist by file names", it will test as True for all files whose name is terminating with docx.
Each file processed by MetaDefender Core will be tested against the rules defined in the blacklist.
As soon as any of the rules tests as True the file will be blocked.
Sometimes the business rule is something like "Block all files except...".
Such a scenario is accommodated in the system by the usage of Regular Expressions.
In Regular Expression, we can create an expression that will test as True when a certain string is NOT found (known as Negative Look Ahead)
For example, if we use the string: ^.*\.((?!docx$).)*$ in the "Blacklist by file names", it will test as True for files whose names do NOT terminate with docx.
* To make the above Regular Expression case insensitive we can use: ^.*\.((?![dD][oO][cC][xX]$).)*$
In many cases, we will need to allow more than one file type.
For example, if we use the string: ^.*\.((?!docx$)(?!xls$).)*$ in the "Blacklist by file names", it will test as True for files whose names do NOT terminate with either docx or xls.
The Negative Look Ahead block (?!XYZ$) can be repeated as many times as required.
In the example given above "Block all files except docx" there is a hidden problem.
.docx files are actually archive files, containing other files (such as xml, gif, jpeg etc...)
This means that if the business rule is block everything except docx it most likely means block everything except docx and all the files it contains.
Note: You can use a tool such as https://regex101.com/ to create and test your regular expressions.
This article applies to MetaDefender Core v4 Windows
This article was last updated on 2019-10-06