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CVE-2021-38507

Date: December 8, 2021

The Opportunistic Encryption feature of HTTP2 (RFC 8164) allows a connection to be transparently upgraded to TLS while retaining the visual properties of an HTTP connection, including being same-origin with unencrypted connections on port 80. However, if a second encrypted port on the same IP address (e.g. port 8443) did not opt-in to opportunistic encryption; a network attacker could forward a connection from the browser to port 443 to port 8443, causing the browser to treat the content of port 8443 as same-origin with HTTP. This was resolved by disabling the Opportunistic Encryption feature, which had low usage. This vulnerability affects Firefox < 94, Thunderbird < 91.3, and Firefox ESR < 91.3.

Severity Score

Severity Score

Weakness Type (CWE)

Origin Validation Error

CWE-346

CVSS v3.1

Base Score:
Attack Vector (AV): NETWORK
Attack Complexity (AC): LOW
Privileges Required (PR): NONE
User Interaction (UI): REQUIRED
Scope (S): UNCHANGED
Confidentiality (C): NONE
Integrity (I): HIGH
Availability (A): NONE

CVSS v2

Base Score:
Access Vector (AV): NETWORK
Access Complexity (AC): MEDIUM
Authentication (AU): NONE
Confidentiality (C): NONE
Integrity (I): PARTIAL
Availability (A): NONE
Additional information:

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