When using certain blank characters in a URL, they where incorrectly rendered as spaces instead of an encoded URL.

published 2020-07-09 6 y ago modified 2026-06-17 4 mo ago also EUVD-2020-4721

cvss: 8.8; version 3.1, NVD
cisa kev: Not listed; not in CISA's catalog
epss: 1.2%; EPSS 1.2% chance of exploitation in 30 days, percentile 67%
public exploits: 0; none known

When using certain blank characters in a URL, they where incorrectly rendered as spaces instead of an encoded URL. This vulnerability affects Firefox < 77.

Patch / fix

fixed in 1 distros 4

Mozilla Firefox 77

ubuntu opensuse alpine arch linux

firefox Ubuntu 16.04 LTS 77.0.1+build1-0ubuntu0.16.04.1
firefox Ubuntu 18.04 LTS 77.0.1+build1-0ubuntu0.18.04.1
firefox Ubuntu 20.04 LTS 77.0.1+build1-0ubuntu0.20.04.1

ubuntu advisories UBUNTU-CVE-2020-12409

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

firefox-esr openSUSE Tumbleweed 128.5.1-1.1
MozillaFirefox openSUSE Tumbleweed 92.0-1.2

opensuse advisories openSUSE-SU-2024:14572-1 openSUSE-SU-2024:10600-1

firefox Alpine 3.23, Alpine 3.22, Alpine 3.21 +9 more 77.0-r0
librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 77.0-r0

alpine advisories ALPINE-CVE-2020-12409

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

firefox Arch Linux 77.0-1

arch linux advisories AVG-1173

Exploit prediction

1.18% chance of exploitation in the next 30 days, higher than 67% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
NVD	3.1	8.8	CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
NVD	2.0	6.8	AV:N/AC:M/Au:N/C:P/I:P/A:P

Affected products

Mozilla Firefox

References

mozilla.org /security/advisories/mfsa2020-20/
vendor-advisory
bugzilla.mozilla.org /show_bug.cgi?id=1629506
issue-tracking, permissions-required, vendor-advisory

Sources

CVE List CVE-2020-12409
NVD CVE-2020-12409

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary