[TZO-21-2020] - Sophos Generic Archive Bypass (ZIP)

Release mode    : Coordinated Disclosure
Ref             : [TZO-21-2020] - Sophos Generic Archive Bypass (ZIP)
Vendor          : Sophos
Status          : Patched / Silent Fix
CVE             : CVE-2020-9363
Dislosure Policy: https://caravelahq.com/b/policy/20949
Bug Class Info  : http://blog.zoller.lu/2009/04/case-for-av-bypassesevasions.html
Vendor Advisory : No Advisory issued, customers not informed.


Introduction
============
10 years ago I took a look at ways to evade AV/DLP Engine detection by using various techniques and released a metric ton of Advisories. 10 years 
later after multiple CISO type roles I wanted to deep dive again and see how far (or not) the AV  industry has reacted to this class of vulnerabilities. 

These types of evasions are now actively being used in offensive operations [1]. To my surprise with a few exceptions most AV Vendors haven't, 
in some cases I found the very same vulnerabilities that were patched and disclosed years ago.

Worse than that is the fact that some vendors that were very collaborative in 2008/2009 have now  started to ignore submissions (until I threaten disclosure)
or are trying to argue that generically evading AV detection is not a vulnerability. 

A lot of exchanges took place on this matter, for instance one vendor argued that this could not be called a vulnerability because it would not impact Integrity, 
Availability or Confidentiality so it can't possible be a vulnerability.

Even more bothering to me is how the bu bounty platform have created a distorted Reporter/Vendor relationship and mostly are executed to the detriment of the customers.
I am collecting my experiences and will write a blog post about this phenomnon.

There will by many more advisories, hoping that I can finally erradicate this bug class and I don't have to come back to this 10 years from now again.

[1] https://www.bleepingcomputer.com/news/security/specially-crafted-zip-files-used-to-bypass-secure-email-gateways/
https://www.techradar.com/news/zip-files-are-being-used-to-bypass-security-gateways

Affected Products
=================
Sophos Endpoint Protection
Sophos Cloud Optix
Sophos Mobile
Sophos Intercept X Endpoint
Sophos Intercept X  for Server
Sophos Secure Web Gateway

I. Background
----------------------------
"Sophos evolves to meet every new challenge, protecting more than 400,000 organizations of all sizes in more than 150 countries from today’s most advanced cyber threats. Powered by SophosLabs, our cloud-native and AI-enhanced solutions are able to adapt and evolve to secure endpoints and networks against never-before-seen cybercriminal tactics and techniques."

II. Description
----------------------------
The parsing engine supports the ZIP archive format. The parsing engine can be bypassed  by specifically manipulating an ZIP Archve so that it can be accessed by an end-user but not the Anti-Virus software. The AV engine is unable to scan the container and gives  the file a "clean" rating. 

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


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
----------------------------
13.10.2019 Submitted sample
14.01.2020 Patch deployed across customer base
24.02.2020 Release of this Advisory