ClamAV Signature Creator (CASC) - IDA Pro plug-in to generate signatures

Disclaimer

THE SOFTWARE TOOL AND RELAED DATA (THE “TOOL”) AND ANY ALTERATIONS, MODIFICATIONS, ENHANCEMENTS AND IMPROVEMENTS THERETO AND TECHNICAL SUPPORT (IF ANY) ARE BEING PROVIDED TO YOU ON AN “AS-IS” BASIS, WITHOUT WARRANTY, EXPRESS OR IMPLIED, OF ANY KIND INCLUDING, BUT NOT LIMITED TO, ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK ARISING OUT OF YOUR USE OF THE TOOL REMAINS SOLELY WITH YOU. YOU ACKNOWLEDGE AND AGREE THAT USE OF THE TOOL IS SOLELY AT YOUR OWN RISK. IN NO EVENT SHALL CISCO OR ITS LICENSORS BE LIABLE FOR ANY DIRECT OR INDIRECT DAMAGES WHATSOEVER AS A RESULT OF YOUR USE OF THE TOOL, INCLUDING, WITHOUT LIMITATION, LOSS OF BUSINESS PROFITS, BUSINESS INTERRUPTION, LOSS OF DATA, OR OTHER LOSS ARISING OUT OF THE USE OF OR INABILITY TO USE THE TOOL, EVEN IF CISCO HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES, OR FOR ANY CLAIM BY YOU OR BASED ON A THIRD PARTY CLAIM.

CASC

The ClamAV Signature Creator (CASC), is an IDA Pro plug-in to aid reverse engineers in creating ClamAV NDB and LDB signatures from IDA Pro's Disassembly or Strings view.

CASC should run on any platform that supports IDA Pro 6.7 and higher. Limited functionality is available for IDA Pro 6.6. The signature analyzer part of CASC has only been tested on IDA 6.9.

README with pictures can be found on our wiki: https://github.com/vrtadmin/casc/wiki

Building

You need a working build system to build the yara-python library. On Ubuntu, you can get that by installing sudo apt-get install -y build-essential libpython2.7-dev libpython2.7-dev:i386 gcc-multilib libssl-dev libssl-dev:i386 on a 64 bit system (The build process has been developed for a 64 bit machine, and will not work out of the box on a 32 bit machine).

Run python package.py --output <output-dir> to build the plugin zip archives. This will build all combinations of 32/64 versions for Windows and Linux.

Installation

If you are using your system's python for IDA Pro (probably the case if you're on a 32 bit Linux system, or on a 64 bit Linux system and you're using IDA 7.0 or higher), you can install the packages ply, yara-python and ida-netnode, and then unzip the casc-*-universal.zip archive into your IDA Pro directory.

Otherwise, if you use the python bundled with IDA Pro, you'll need to install the libraries in this python. Either you can do this by yourself (e.g., by following the instructions from hexblog), or you use the archives with bundled dependencies that we provide. Those archives are built as described in the [building][#Building] step above, by running package.py. To install an archive, simply pick the casc-*-fat.zip corresponding to your system, and unzip it in your IDA Pro directory.

In case you don't want to install additional libraries, the plugin will degrade gracefully and hide the "Analyze" functionality which requires the libraries to be installed.

Support Information

ClamAV Signature Creator (CASC) is meant for creating ClamAV signatures on the sample as it exists on disk. Sub signatures could be based off unpacked code during the sample’s execution, however, ClamAV would not be able to match those signatures (some exception exist for automatic unpackers built into ClamAV).

Tested on

File Types

Opening Plug-in

Once the Python script is copied to the IDA Pro plug-ins folder, open IDA Pro with a sample. There are two ways of opening the plug-in.

• IDA Pro’s Plug-in Menu (Edit -> Plugins -> ClamAV Signature Creator
• Press ` (backtick)

Once the plug-in is opened you will be able to view sub signatures created in the past and saved in the IDB, add new misc ClamAV sub signatures, and add sub signatures generated from disassembly selected in the IDB.

Creating Sub Signatures

Sub signatures can either be created from disassembly viewable from within IDA Pro or manually from entering/creating a valid ClamAV sub signature.

Insert Misc. Sub Signature

A custom ClamAV sub signature can be created in a couple of different ways:

• Within the CASC plug-in window, press the Ins key
• Within the CASC plug-in window, right click and select "Insert"
• Within the Strings window; select the string(s) of interest, right click, and select "Add string to ClamAV Signature Creator"

Insert Assembly Sub Signature

There are several ways to create a sub signature from disassembly within the IDB. All methods involve first selecting the code you are interested in creating a signature from. Either highlight the code or position and click your cursor in the basic block of interest, then:

• Within the CASC plug-in window, press Ctrl+Ins
• Within the CASC plug-in window, right click and select "Insert Assembly"
• Within the IDA View window by
  • Pressing Ctrl+`
  • Right click and select "Add Assembly to ClamAV Sig Creator…"

The Assembly to Signature window will allow you to insert notes for the sub signature, apply various masking options, and scroll through the opcodes/assembly associated with that sub signature.

Selecting a masking option will change the opcodes and assembly text if the masking option can be applied. Selecting "Customize" will allow you to edit the opcodes (note the assembly area will not update for any customizations made). If you uncheck “Customize” then all previously applied masking options will be applied and the customizations will be deleted.

Common Problems

If a masking option is selected but the opcodes and assembly don’t change:

ESP Offsets
    This will apply to [esp+offset] operands only
EBP Offsets
    This will apply to [ebp+offset] operands only

Absolute Calls

IDA might display the disassembly as

call memset

However, that instruction may be a call to a function within the sample that directly calls or jumps to the actual memset function. If that is the case, no changes will be made. Global Offsets Still in testing, report any issues to
https://github.com/vrtadmin/casc/issues

Editing Sub Signatures

To edit a signature, simply double click on the signature within the CASC plug-in window and the signature will open up for editing. Any changes made will be saved only if you press OK. Prior to saving a sub signature it will undergo a verification to ensure it correctly conforms to a ClamAV signature component. If any problems exist, clicking the OK but will result in an error message to its right. The error must be corrected before the sub signature will be saved.

Creating ClamAV Signature

Before creating a signature, make sure to give it a descriptive ClamAV signature name (the default is Win.Trojan.Agent). Once a sub signature(s) is created, you can select one or more sub signatures from the CASC plug-in window (use Ctrl or Shift keys to select multiple signatures) and click the "Create ClamAV Signature"

Once the “Create ClamAV Signature” button is click a dialog box with a formatted email will be displayed for the user to send to ClamAV’s community-sigs list. Selecting the [email protected] hyperlink is a mailto: link. It will attempt to copy the signature information displayed to the systems default mail client. Keep in mind if any special characters are used then the email’s contents may not be correct and will need to be manually copied over.

Analyzing ClamAV signatures

In the main plugin panel, use the tabbed pane to switch to the "Analyze" mode. Note that this tab is not available if you don't have the required libraries installed. You can now paste a ClamAV .ldb or .ndb signature in the text field above the "Add Signature" button. Once you then click Add signature, the signature will appear in the left top list (if it was well formatted and parsed by the plugin, otherwise check the IDA Pro output window for errors). Now you can double-click on the signature in the list. For .ndb signatures this will directly bring you to the matched part of the binary, and color the match in red. For .ldb signatures, the subsignatures will be displayed in the right list element. If you double-click any of the sub signature entries, it will bring you to the match for this subsignature.

Bugs and Support

There is no support provided with CASC.

If you think you've found a bug, please report it at: https://github.com/vrtadmin/casc/issues

In order to help us solve your issues as quickly as possible, please include the following information when filing a bug:

• The version of IDA Pro you're using
• The operating system used to run IDA Pro
• Any CASC related errors printed out in IDA Pro's output window
• Task trying to accomplish
• Any other relevant information

Other options for communication can be found at: https://github.com/vrtadmin/casc/wiki