Is CVE-2021-38507 in the CISA KEV catalog?

No. CVE-2021-38507 is not currently on the CISA Known Exploited Vulnerabilities list.

What is the CVSS severity of CVE-2021-38507?

CVE-2021-38507 has a CVSS v3 base score of 6.5 (MEDIUM).

What is the EPSS score for CVE-2021-38507?

CVE-2021-38507 has an EPSS score of 0.80% (52th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2021-38507?

Upgrade firefox on debian unstable to 94.0-1 or later. Fixes are available for 7 affected products.

CVE & CISA-KEV Catalog

CVE-2021-38507

MEDIUM

6.5

CVSS v3

NVD

Description

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.

CWE-346

How to fix

Remediation Available

firefoxDebian

Fixed in:94.0-1CVE-2021-38507

firefoxUbuntu

Fixed in:94.0+build3-0ubuntu0.18.04.1USN-5131-1

Fixed in:94.0+build3-0ubuntu0.20.04.1USN-5131-1

thunderbirdUbuntu

Fixed in:1:91.5.0+build1-0ubuntu0.18.04.1USN-5248-1

Fixed in:1:91.5.0+build1-0ubuntu0.20.04.1USN-5248-1

DeveloperEditionWindows application

Affected:94.0Fixed in:94.0Mozilla

Mozilla Firefox (en-US)Windows application

Affected:94.0Fixed in:94.0Mozilla

Mozilla Firefox ESR (en-US)Windows application

Affected:91.3.0Fixed in:91.3.0Mozilla

Mozilla Thunderbird (en-US)Windows application

Affected:91.3.0Fixed in:91.3.0Mozilla

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityLow

Privileges RequiredNone

User InteractionRequired

ScopeUnchanged

Impact

ConfidentialityNone

IntegrityHigh

AvailabilityNone

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

Exploit Intelligence

0.80%probability of exploitation in 30 days

52ndpercentile

Moderate risk: more likely to be exploited than 52% of all known CVEs.

References

Vendor Advisory4

Third-Party Advisory6

Related Vulnerabilities

Other CWE-346 vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2009-1185	High	7.2	82%	-	Fix
CVE-2025-34291	High	8.8	79%	KEV	-
CVE-2020-16952	High	8.6	71%	-	-
CVE-2023-29711	Critical	9.8	70%	-	-
CVE-2015-4495	High	8.8	70%	KEV	Fix
CVE-2024-23898	High	8.8	67%	-	Fix

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2024-11-21.