What is CVE-2020-12399?

CVE-2020-12399 is a medium-severity vulnerability in Mozilla Firefox, classified under Observable Discrepancy. CVSS score: 4.4/10. Published 2020-07-09.

How severe is CVE-2020-12399?

Medium severity. CVSS v3 base score is 4.4 out of 10.

Is CVE-2020-12399 known to be exploited?

1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Vulnerability in Mozilla Firefox

CVE-2020-12399

NSS has shown timing differences when performing DSA signatures, which was exploitable and could eventually leak private keys. This vulnerability affects Thunderbird < 68.9.0, Firefox < 77, and Firefox ESR < 68.9.

Published 2020-07-09 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 4.4 (Medium). Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:R/S:U/C:H/I:N/A:N.

EPSS exploit prediction

EPSS: 0.007 (49.3th percentile), scored 2026-10-01.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 49th percentile — 49.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.007 · newest: 0.007 · change: 0.000

Affected products

Mozilla Firefox — versions unspecified
Mozilla Firefox Esr — versions unspecified
Mozilla Firefox Esr
Mozilla Thunderbird — versions unspecified
Debian Linux — versions 9.0

Weakness classification (CWE)

CWE-203 — Observable Discrepancy

Public proof-of-concept exploits

ARPSyndicate/cvemon

References

www.mozilla.org/security/advisories/mfsa2020-20/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2020-21/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2020-22/ (Vendor Advisory)
bugzilla.mozilla.org/show_bug.cgi (Permissions Required, Issue Tracking, Vendor Advisory)
www.debian.org/security/2020/dsa-4726 (vendor-advisory, Third Party Advisory)
usn.ubuntu.com/4421-1/ (vendor-advisory, Third Party Advisory)
security.gentoo.org/glsa/202007-49 (vendor-advisory, Third Party Advisory)
lists.debian.org/debian-lts-announce/2020/09/msg00029.html (mailing-list, Mailing List, Third Party Advisory)

Frequently asked questions

What is CVE-2020-12399?: CVE-2020-12399 is a medium-severity vulnerability in Mozilla Firefox, classified under Observable Discrepancy. CVSS score: 4.4/10. Published 2020-07-09.
How severe is CVE-2020-12399?: Medium severity. CVSS v3 base score is 4.4 out of 10.
Is CVE-2020-12399 known to be exploited?: 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.