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.
Published 2021-12-08 22:15:09
Updated 2022-12-09 15:54:39
View at NVD,   CVE.org

Products affected by CVE-2021-38507

Exploit prediction scoring system (EPSS) score for CVE-2021-38507

0.71%
Probability of exploitation activity in the next 30 days EPSS Score History
~ 81 %
Percentile, the proportion of vulnerabilities that are scored at or less

CVSS scores for CVE-2021-38507

Base Score Base Severity CVSS Vector Exploitability Score Impact Score Score Source First Seen
4.3
MEDIUM AV:N/AC:M/Au:N/C:N/I:P/A:N
8.6
2.9
NIST
6.5
MEDIUM CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
2.8
3.6
NIST

CWE ids for CVE-2021-38507

  • The product does not properly verify that the source of data or communication is valid.
    Assigned by: nvd@nist.gov (Primary)

References for CVE-2021-38507

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