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[TZO-16-2020] – F-SECURE Generic Malformed Container bypass (GZIP)

From: Thierry Zoller <thierry () zoller lu>
Date: Mon, 24 Feb 2020 10:38:50 +0100

________________________________________________________________________ From the low-hanging-fruit-department F-SECURE Generic Malformed Container bypass (GZIP)

Ref : [TZO-16-2020] - F-SECURE Generic Malformed Container bypass (GZIP)

Vendor : F-SECURE
Status : Patched
CVE : CVE-2020-9342

Blog : https://blog.zoller.lu/p/tzo-16-2020-f-secure-generic-malformed.html

Vulnerability Dislosure Policy: https://caravelahq.com/b/policy/20949 Affected Products
F-Secure Email and Server Security
F-Secure Internet GateKeeper
F-SECURE CLOUD PROTECTION FOR SALESFORCE Linux below 17.0.605.474 I. Background

Quote: "Unprecedented challenges threaten to undermine the very survival of society. Only unprecedented innovation can prevent irreversible disasters. This is only possible if we

trust the technology that can bring us together. 

From our humble beginnings more than thirty years ago, F-Secure has grown into a reliable cyber security leader, earning the trust of organizations and people around the world."

 II. Description

The parsing engine supports the GZIP Archive. The parsing engine can be bypassed by manipulating a GZIP archive (Compression Method). This way the User can extract the file but the AV Engine cannot giving the file a clean pass.

 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

- For the unix version (IGK) the fix was released on 20th January with library version 17.0.605.474 - For Windows the version was already out in December 2019. There were also avira's fixes as well.


"We can conclude that January 20th date, since that is when the final fixes went out to production."

 V. Disclosure timeline

We would like to thank F-Secure for their customer oriented way of handling this vulnerability.

 - NOV 14 2019
Initiated Vulnerability coordination - FEB 05 2020
F-Secure notifies me that they have patched the flaw. 

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  • [TZO-16-2020] – F-SECURE Generic Malformed Container bypass (GZIP) Thierry Zoller (Feb 24)

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