CVE-2021-38507

Name of the Vulnerable Software and Affected Versions Firefox versions prior to 94 Thunderbird versions prior to 91.3 Firefox ESR versions prior to 91.3

Description The issue is related to the Opportunistic Encryption feature of HTTP2, which allows a connection to be upgraded to TLS while retaining the visual properties of an HTTP connection. A network attacker could exploit this by forwarding a connection from the browser to a different port, causing the browser to treat the content as same-origin with HTTP. This could allow a remote attacker to bypass security restrictions.

Recommendations For Firefox versions prior to 94, disable the Opportunistic Encryption feature. For Thunderbird versions prior to 91.3, disable the Opportunistic Encryption feature. For Firefox ESR versions prior to 91.3, disable the Opportunistic Encryption feature.