Details for this vulnerability have not been published by NIST at this point. Descriptions from software vendor advisories for this issue are provided below.
USN-4637-1 fixed vulnerabilities in Firefox. This update provides the corresponding updates for Ubuntu 16.04 LTS.
Original advisory details:
Multiple security issues were discovered in Firefox. If a user were tricked in to opening a specially crafted website, an attacker could potentially exploit these to cause a denial of service, obtain sensitive information across origins, bypass security restrictions, conduct phishing attacks, conduct cross-site scripting (XSS) attacks, bypass Content Security Policy (CSP) restrictions, conduct DNS rebinding attacks, or execute arbitrary code.