Active Exploitations: Multiple Vulnerabilities in Mozilla Firefox and Thunderbird Could Allow for Arbitrary Code Execution - PATCH NOW

Posting ID 130655
Date Added: 02/24/2021 4:33 PM EST
Date Modified: 02/25/2021 1:46 PM EST
Orlando Stevenson | Watch Floor



Mozilla has reported multiple vulnerabilities in Mozilla Firefox, Firefox Extended Support Release (ESR), and Mozilla Thunderbird, the most severe of which could allow for arbitrary code execution. Depending on the user's privileges, an attacker could then install programs, view, change, or delete data; or create new accounts with full user rights. Users whose accounts having fewer user rights on the system could be less impacted than those who operate with administrative user rights.

  • There are reports of adversarial exploitation of these vulnerabilities in the wild.
  • Software security updates are available.

Systems affected:

  • Mozilla Firefox versions prior to 86
  • Firefox ESR versions prior to 78.8
  • Mozilla Thunderbird versions prior to 78.8


With National Vulnerability Database details at pending, specific vulnerabilities addressed in available software updates include the following:

  • CVE-2021-23968 - If Content Security Policy blocked frame navigation, the full destination of a redirect served in the frame was reported in the violation report; as opposed to the original frame URI. This could be used to leak sensitive information contained in such URIs.

  • CVE-2021-23969 - As specified in the W3C Content Security Policy draft, when creating a violation report, "User agents need to ensure that the source file is the URL requested by the page, pre-redirects. If that’s not possible, user agents need to strip the URL down to an origin to avoid unintentional leakage." Under certain types of redirects, Firefox incorrectly set the source file to be the destination of the redirects. This was fixed to be the redirect destination's origin.

  • CVE-2021-23970 - Context-specific code was included in a shared jump table; resulting in assertions being triggered in multithreaded Wasm (WebAssembly) code.

  • CVE-2021-23971 - When processing a redirect with a conflicting Referrer-Policy, Firefox would have adopted the redirect's Referrer-Policy. This would have potentially resulted in more information than intended by the original origin being provided to the destination of the redirect.

  • CVE-2021-23972 - One phishing tactic on the web is to provide a link with HTTP Auth. For example To mitigate this type of attack, Firefox will display a warning dialog; however, this warning dialog would not have been displayed if used a redirect that was cached by the browser.

  • CVE-2021-23973 - When trying to load a cross-origin resource in an audio/video context a decoding error may have resulted, and the content of that error may have revealed information about the resource.

  • CVE-2021-23974 - The DOMParser API did not properly process <noscript> elements for escaping. This could be used as an mXSS vector to bypass an HTML Sanitizer.

  • CVE-2021-23975 - The developer page about:memory has a Measure function for exploring what object types the browser has allocated and their sizes. When this function was invoked we incorrectly called the sizeof function, instead of using the API method that checks for invalid pointers.

  • CVE-2021-23976 - When accepting a malicious intent from other installed apps, Firefox for Android accepted manifests from arbitrary file paths and allowed declaring webapp manifests for other origins. This could be used to gain fullscreen access for UI spoofing and could also lead to cross-origin attacks on targeted websites.
    Note: This issue is a different issue from CVE-2020-26954 and only affected Firefox for Android. Other operating systems are unaffected.

  • CVE-2021-23977 - Firefox for Android suffered from a time-of-check-time-of-use vulnerability that allowed a malicious application to read sensitive data from application directories.
    Note: This issue is only affected Firefox for Android. Other operating systems are unaffected.

  • CVE-2021-23978 - Mozilla developers Alexis Beingessner, Tyson Smith, Nika Layzell, and Mats Palmgren reported memory safety bugs present in Firefox 85 and Firefox ESR 78.7. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code.

  • CVE-2021-23979 - Mozilla developers Tyson Smith, Lars T Hansen, Valentin Gosu, and Sebastian Hengst reported memory safety bugs present in Firefox 85. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code.


What to Do:

Many home users choose to receive updates automatically, and most organizations orchestrate regular software security updates to maintain end-user software, e.g., Patch Tuesday monthly. Given that there are active adversarial exploits of these issues reported, there is an opportunity to reduce risk with timely software update deployment. 

General recommendations include:

  • Apply appropriate updates provided by Mozilla to vulnerable systems immediately after appropriate testing.

  • Run all software as a non-privileged user (one without administrative privileges) to diminish the effects of a successful attack.

  • Remind users not to visit un-trusted websites or follow links provided by unknown or un-trusted sources.

  • Inform and educate users regarding the threats posed by hypertext links contained in emails or attachments especially from un-trusted sources.

  • Apply the Principle of Least Privilege to all systems and services.



The E-ISAC thanks the MS-ISAC and other partners for advising of this development, and is providing this information for member awareness. The E-ISAC has not established any specified threat to the electricity community based upon these vulnerabilities. We will continue to monitor this situation and provide relevant updates when necessary. Your feedback helps us improve our products for the industry. If you have comments, questions, or if your organization experiences malicious activity, please create a Portal post for rapid community awareness or directly contact E-ISAC Watch Operations at or 202-790-6000.




Category Type:
Cyber Security
TLP - White
Shared Count (21)
  • ANL
  • Canadian CERTs
  • CRISP - Cyber Risk Info Sharing Program
  • DOD
  • DOE Complex
  • E-ISAC AOO Members
  • E-ISAC Staff
  • ESCC, including SEWG
  • FBI, LE Fusion
  • FERC - OEIS, etc
  • International (other ISACs, CERTs)
  • International AOOs
  • Other (inc. local/state commissions)
  • Trade Organizations
  • Watch Floor
Change History
  • Admin, 02/25/2021
  • Orlando Stevenson, 02/24/2021