Advisory: [TZO-09-2020] - Bitdefender Malformed Archive bypass (RAR Uncompressed Size)

Release mode    : Forced Disclosure
Ref             : [TZO-09-2020] - Bitdefender Malformed Archive bypass (RAR Uncompressed Size)
Vendor          : Bitdefender
Status          : Patched (amsiscan.dll >24.0.14.74)
CVE             : Reserved 3 CVEs then pulled them back (Although patching the vulnerability)
Dislosure Policy: https://caravelahq.com/b/policy/20949
Vendor Advisory : No Advisory issued
Patch release   : https://www.bitdefender.com/consumer/support/answer/10690/


Affected Products
All Bitdefender Products and Vendors that have licensed the Engine before Dec 12 2019. Exact version is unknown as Bitdefender has not made this public.

Quoting Bitdefender :
"All Bitdefender endpoint solutions (including but not limited to Bitdefender Total Security,
Bitdefender Antivirus Free Edition, Bitdefender GravityZone) as well as all products
using our engines."

Consumer: 
Bitdefender Premium Security
Bitdefender Total Security 2020
Bitdefender Internet Security 2020
Bitdefender Antivirus Plus 2020
Bitdefender Family Pack 2020
Bitdefender Antivirus for Mac
Bitdefender Mobile Security for Android
Bitdefender Mobile Security for iOS

Enterprise:
Bitdefender Small Office Security
GravityZone Business Security
GravityZone Advanced Business Security
Bitdefender Security for AWS
GravityZone Ultra Security
GravityZone Managed EDR
GravityZone Elite Security
GravityZone Enterprise Security
Security for Virtualized Environments
Security for Endpoints
Security for Mobiles
Security for Exchange
GravityZone Security for Storage

Vulnerable OEM Partners (According to AV-TEST):
Adaware
Bullguard
Vipr
Total360
eScan
emiSoft
G-DATA
Qihoo 360
Quick Heal
TotalDefense
Tencent

I. Background
"Since 2001, Bitdefender innovation has consistently delivered award-winning security products and threat intelligence for people, homes, businesses and their devices, networks and cloud services. Today, Bitdefender is also the provider of choice, used in over 38% of the world’s security solutions.
Recognized by industry, respected by vendors and evangelized by our customers, Bitdefender is  the cybersecurity company you can trust and rely on."

II. Description
The parsing engine supports the RAR archive format. The parsing engine can be bypassed by specifically manipulating an RAR Archive (Compressed Size) so that it can be accessed by an end-user  but not the Anti-Virus software. The AV engine is unable to scan the archive and issues the file a "clean" rating.

I may release further details after all known vulnerable vendors have patched their products.


III. Impact
Impacts depends on the contextual use of the product and engine within the organisation of a customer. Gateway Products (Email, HTTP Proxy etc) may allow the file through unscanned and give it a clean bill of health. Server side AV software will not be able to discover  any code or sample contained within this ISO file and it will not raise suspicion even  if you know exactly what you are looking for (Which is for example great to hide your implants or Exfiltration/Pivot Server).

There is a lot more to be said about this bug class, so rather than bore you with it in this advisory I provide a link to my 2009 blog post
http://blog.zoller.lu/2009/04/case-for-av-bypassesevasions.html

IV. Patch / Advisory
If you are an enterprise customer I would suggest to reach out to Bitdefender to discuss how you can be notified about patched vulnerabilities within their products. Some releases may requires binary updates that cant be pulled from the auto-update.

amsiscan.dll >24.0.14.74

For Users of the OEM Partners (G-Data, Vipr, etc) I would suggest to get in contact to ensure these vulnerabilities are patched or not present in their offering. I would also suggest discussing how you can be made aware of future patches.

V. Disclosure Timeline
See Previous Bitdefender disclosures :
https://blog.zoller.lu/p/tzo-04-2019-bitdefender-malformed.html