CVE-2017-7821: A vulnerability where WebExtensions can download and attempt to open a file of some non-executable file types.

Security readout for executives and security teams

A malicious or compromised Firefox WebExtension could cause Firefox versions before 56 to download and try to open certain document-like files without specific user action. The business risk is indirect: it may expose users to separate vulnerabilities in applications registered to open those file types. Exposure is most likely on legacy endpoints running Firefox before version 56, especially where users can install or run WebExtensions. Current supported Firefox deployments are unlikely to be affected based on the stated version range. Treat this as legacy browser hygiene with moderate urgency. It is not shown as actively exploited, but old Firefox plus unmanaged extensions can create a path into other vulnerable desktop applications. Mitigation focus: Upgrade Firefox to version 56 or later, preferably a current supported release.; Remove or disable untrusted WebExtensions on legacy Firefox systems.; Review Mozilla advisory guidance for any deployment-specific remediation details..

Prepared

Jun 7, 2026, 06:44 UTC (UTC+00:00)

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.