The Threat

On June 18th, 2019, Mozilla released a critical advisory (CVE-2019-11707) for a 'type confusion' vulnerability that is being actively exploited in the wild [1]. If successful, exploitation enables remote malicious actors to take full control of any vulnerable system(s).

The researcher that discovered this flaw indicates the bug can be exploited for UXSS (Universal Cross-Site Scripting) or RCE (Remote Code Execution) [2]. While both methods are serious, it is worth noting RCE would require additional complexities for successful exploitation (e.g. sandbox escape), while UXSS is a type of attack that could necessitate the execution of malicious code via client-side vulnerabilities. 

Mozilla has indicated the following versions are fixed and protected against this critical flaw: 67.0.3 and Firefox ESR 60.7.1 [3]. It is highly recommended to apply the security patches after performing a business impact review, as attacks exploiting CVE-2019-11707 are already occurring.  

What we’re doing about it

  • MVS (formerly esRECON) local plugins identify this vulnerability
  • The Threat Intelligence team is monitoring this topic for additional information

What you should do about it

  • After performing a business impact review, apply the latest update for Firefox (Firefox 67.0.3 / Firefox ESR 60.7.1)

Additional information

The vulnerability resides in the Array.pop file which is an array method is used by Firefox in JavaScript objects. There are not currently any workarounds for this issue outside of applying security patches. If not already enabled, it is recommended to turn on auto-updates for Firefox [4]. 

Attacks in the wild are speculatively believed to be financially motivated and targeting cryptocurrency exchanges.  

References: 

[1] https://www.mozilla.org/en-US/security/advisories/mfsa2019-18/ 

[2] https://twitter.com/5aelo/status/1141273396958900224 

[3] https://www.securityfocus.com/bid/108810/info 

[4] https://www.technipages.com/enable-disable-automatic-updates-in-firefox 

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Articles and reports written by eSentire staff and our Threat Intelligence Research Group.

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