The Mozilla Maintenance Service does not guard against files being hardlinked to another file in the updates directory, allowing for the replacement of local files, including the Maintenance Service executable, which is run with privileged access. Additionally, there was a race condition during checks for junctions and symbolic links by the Maintenance Service, allowing for potential local file and directory manipulation to be undetected in some circumstances. This allows for potential privilege escalation by a user with unprivileged local access.
Note: These attacks requires local system access and only affects Windows. Other operating systems are not affected.. This vulnerability affects Firefox < 69 and Firefox ESR < 68.1.

References

• https://nvd.nist.gov/vuln/detail/CVE-2019-11736
• https://bugzilla.mozilla.org/show_bug.cgi?id=1551913
• https://bugzilla.mozilla.org/show_bug.cgi?id=1552206
• https://www.mozilla.org/security/advisories/mfsa2019-25
• https://www.mozilla.org/security/advisories/mfsa2019-26
• http://lists.opensuse.org/opensuse-security-announce/2019-10/msg00011.html
• http://lists.opensuse.org/opensuse-security-announce/2019-10/msg00017.html