Given a compromised sandboxed content process due to a separate vulnerability, it is possible to escape that sandbox by loading accounts.firefox.com in that process and forcing a log-in to a malicious Firefox Sync account. Preference settings that disable the sandbox are then synchronized to the local machine and the compromised browser would restart without the sandbox if a crash is triggered. This vulnerability affects Firefox ESR < 60.9, Firefox ESR < 68.1, and Firefox < 69.

References

• https://nvd.nist.gov/vuln/detail/CVE-2019-9812
• https://bugzilla.mozilla.org/show_bug.cgi?id=1538008
• https://bugzilla.mozilla.org/show_bug.cgi?id=1538015
• https://www.mozilla.org/security/advisories/mfsa2019-25
• https://www.mozilla.org/security/advisories/mfsa2019-26
• https://www.mozilla.org/security/advisories/mfsa2019-27