CVE-2021-38507: debian debian_linux Origin Validation Error
Vulnerability summary
CVE-2021-38507 is a medium (CVSS 6.5) Origin Validation Error vulnerability in debian debian_linux. EPSS estimates a 0.83% chance of exploitation in the next 30 days, higher than 56.8% of scored vulnerabilities.
Advisory lineage
Upstream 0 · Downstream 26 · Related 12Timeline
- 08-12-2021PublishedVulnerability first disclosed.
- 04-08-2024Last modifiedVulnerability information updated.
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.
CVSS metrics
4 scores · 3 sources · 2 versions- OSV · UbuntuEcosystem advisory · calculated6.5MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
- OSV · DebianEcosystem advisory · calculated6.5MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
EPSS trend
Probability
0.83%
Chance of exploitation in the next 30 days
Percentile
57%
Versus all other CVEs
30-day change
+0.03 pp
From 0.80% on 19-09-2026
Known exploited
Compare KEV catalogsNot in any of the 3 tracked catalogs.
Public exploits
0 foundNo public exploits indexed.
Weaknesses
CWE · root causeAffected systems
Ubuntu6 products
- firefox< 94.0+build3-0ubuntu0.18.04.1 | < 94.0+build3-0ubuntu0.20.04.1 | < 94.0+build3-0ubuntu1
- mozjs38All versions (No fix recorded)
- mozjs52All versions (No fix recorded)
- mozjs68All versions (No fix recorded)
- mozjs78All versions (No fix recorded)
Debian3 products
- firefox-esr< 91.3.0esr-1
- thunderbird< 1:91.3.0-1
- debian_linuxEOL9.0 | 10.0 | 11.0
Mozilla3 products
- firefox≥ unspecified, < 94 | < 94.0
- firefox_esr≥ unspecified, < 91.3 | < 91.3.0
- thunderbird≥ unspecified, < 91.3 | < 91.3.0
3 vendors · 12 products
References
18 links- bugzilla.mozilla.org/show_bug.cgi?id=1730935X Refsource MISCIssue TrackingPermissions RequiredVendor Advisory
- lists.debian.org/debian-lts-announce/2021/12/msg00030.htmlMailing ListX Refsource MLISTThird Party Advisory
- lists.debian.org/debian-lts-announce/2022/01/msg00001.htmlMailing ListX Refsource MLISTThird Party Advisory