Monday, July 27, 2020

During a recent review of file downloads from our ClamAV CDN network, we've noticed hundreds of IPs that seem to be downloading the daily.cvd and the main.cvd thousands of times a day.

There are about a dozen IPs that are downloading those to files more than 40,000 times a day. This is causing us to transfer about 250TB of data a day. We would encourage any users still doing this to cease as soon as possible. Not only does it waste our bandwidth — as we have much more efficient ways of downloading the updates — but it also wastes your bandwidth, as well.

Freshclam has the ability to download partial files of updates (called cdiffs).  Which are smaller, more incremental updates to the database. This allows users, and us, to manage our downloads in a much more efficient manner. We often receive the complaint, "I have to download the daily.cvd and main.cvd with Python and move the updates to an off-internet system."  That's fine — it's a use case we support. However, you can do the same with freshclam and the small cdiffs.

Furthermore, we also only release updates once a day.  Reducing the number of updates you check for (and, subsequently, download we assume through a crontab or periodic job of some type) would also alleviate this issue.

We will be constantly monitoring this in hopes that people migrate to using freshclam.  Over-abusers (for instance, the top 10 IPs that are downloading main.cvd 40,000 times a day), will be immediately blocked.  Further abusers may also be blocked, without notice.

To mitigate, please complete the following tasks:

1. Use Freshclam instead of Python or whatever downloading script you have cron'd.
2. Reduce the checks to once or twice a day.

Thank you for helping keep the ClamAV network healthy.

Any questions, please see us over on the ClamAV-Users list.

Thursday, July 23, 2020

By Nikhil Hegde.

MassLogger is a .NET executable and an info-stealer having the ability to take screenshots, log keystrokes, etc. The binary features two obfuscated loaders. The first loader decrypts the second using the Rijndael algorithm. The second loader decodes the payload using the values in a Bitmap image. The presence of multiple loaders and obfuscation made it a good choice for a walkthrough of ClamAV signature creation. In this post, we’ll walk through the creation of Clam signatures for this malware. It is our hope that this gives you a closer look at the work we do and reverse-engineer malware.

I picked up the sample hash, 2b7455d2a9434cfe516d9d886248b45f1073c0cc9fef73b15e9a1ef187fe4677 from a tweet by Nocturnus. The sample is available on VirusTotal.

Thursday, July 16, 2020

ClamAV 0.102.4 is out now. Users can head over to to download the release materials.

ClamAV 0.102.4 is a bug patch release to address the following issues:


Fixed a vulnerability a malicious user could exploit to replace a scan target's directory with a symlink to another path to trick clamscan, clamdscan, or clamonacc into removing or moving a different file (such as a critical system file). The issue would affect users that use the --move or --remove options for clamscan, clamdscan and clamonacc.

For more information about AV quarantine attacks using links, see RACK911 Lab's report.


Fixed a vulnerability in the ARJ archive-parsing module in ClamAV 0.102.3 that could cause a denial-of-service (DoS) condition. Improper bounds checking resulted in an out-of-bounds read that could cause a crash. The previous fix for this CVE in version 0.102.3 was incomplete. This fix correctly resolves the issue.


Fixed a vulnerability in the EGG archive module in ClamAV 0.102.0 - 0.102.3 that could cause a denial-of-service (DoS) condition. Improper error handling could cause a crash due to a NULL pointer dereference. This vulnerability is mitigated for those using the official ClamAV signature databases because the file type signatures in daily.cvd will not enable the EGG archive parser in affected versions.

We will be publishing a release candidate for version 0.103.0 in the next couple of weeks. Stay tuned!