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Fencer Vulns
high 8.8

CVE-2021-29988 json

Firefox incorrectly treated an inline list-item element as a block element, resulting in an out of bounds read or memory corruption, and a…

published 2021-08-17 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-16447

cvss
8.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.4%
EPSS 1.4% chance of exploitation in 30 days, percentile 72%
public exploits
0
none known

Summary

Firefox incorrectly treated an inline list-item element as a block element, resulting in an out of bounds read or memory corruption, and a potentially exploitable crash. This vulnerability affects Thunderbird < 78.13, Thunderbird < 91, Firefox ESR < 78.13, and Firefox < 91.

Patch / fix

  • Mozilla Thunderbird 78.13, 91
  • Mozilla Firefox ESR 78.13
  • Mozilla Firefox 91
  • firefox-esr Debian 14, Debian 13, Debian 12 78.13.0esr-1
  • thunderbird Debian 14, Debian 13, Debian 12 1:78.13.0-1

debian advisories DEBIAN-CVE-2021-29988

  • firefox Ubuntu 18.04 LTS 91.0+build2-0ubuntu0.18.04.1
  • firefox Ubuntu 20.04 LTS 91.0+build2-0ubuntu0.20.04.1
  • thunderbird Ubuntu 18.04 LTS 1:78.13.0+build1-0ubuntu0.18.04.1
  • thunderbird Ubuntu 20.04 LTS 1:78.13.0+build1-0ubuntu0.20.04.2

ubuntu advisories UBUNTU-CVE-2021-29988

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

  • thunderbird RHEL 7 78.13.0-1.el7_9
  • thunderbird RHEL 8 78.13.0-1.el8_4
  • firefox RHEL 8 78.13.0-2.el8_4
  • firefox RHEL 7 78.13.0-2.el7_9
  • firefox RHEL 8.2 EUS 78.13.0-2.el8_2
  • firefox RHEL 8.1 EUS 78.13.0-2.el8_1
  • thunderbird RHEL 8.2 EUS 78.13.0-1.el8_2
  • thunderbird RHEL 8.1 EUS 78.13.0-1.el8_1

red hat advisories RHSA-2021:3156RHSA-2021:3159RHSA-2021:3162RHSA-2021:3160RHSA-2021:3155RHSA-2021:3161

  • MozillaFirefox SLES for SAP 12 SP5, SLES for SAP 12 SP4, SLES for SAP 12 SP3 +11 more 91.9.0-112.104.1
  • MozillaFirefox SLES for SAP 15 SP1, SLES for SAP 15, SLES 15-LTSS +7 more 91.9.0-150000.150.34.1
  • MozillaFirefox SLE Module for Desktop Applications 15 SP3, SLE Module for Desktop Applications 15 SP2 91.2.0-8.54.1
  • MozillaFirefox-branding-SLE SLE Module for Desktop Applications 15 SP3, SLE Module for Desktop Applications 15 SP2 91-9.5.1
  • MozillaFirefox SLES for SAP 15 SP1, SLES for SAP 15, SLES 15-LTSS +7 more 91.2.0-3.155.2
  • MozillaFirefox-branding-SLE SLES for SAP 15 SP1, SLES for SAP 15, SLES 15-LTSS +7 more 91-4.19.1
  • MozillaFirefox SLES for SAP 12 SP5, SLES for SAP 12 SP4, SLES for SAP 12 SP3 +11 more 91.1.0-112.71.1
  • MozillaFirefox-branding-SLE SLES for SAP 12 SP5, SLES for SAP 12 SP4, SLES for SAP 12 SP3 +10 more 91-35.6.6

suse advisories SUSE-SU-2022:1582-1SUSE-SU-2022:1577-1SUSE-SU-2021:14826-1SUSE-SU-2021:3451-1SUSE-SU-2021:3331-1SUSE-SU-2021:14821-1

  • firefox-esr openSUSE Tumbleweed 128.5.1-1.1
  • MozillaFirefox openSUSE Tumbleweed 92.0-1.2
  • MozillaThunderbird openSUSE Tumbleweed 91.1.1-1.1
  • MozillaFirefox openSUSE Leap 15.2 91.2.0-lp152.2.67.1
  • rust-cbindgen openSUSE Leap 15.2 0.19.0-lp152.2.7.1
  • MozillaFirefox openSUSE Leap 15.3 91.2.0-8.54.1
  • MozillaFirefox-branding-SLE openSUSE Leap 15.3 91-9.5.1
  • rust-cbindgen openSUSE Leap 15.3 0.19.0-1.9.1

opensuse advisories openSUSE-SU-2024:14572-1openSUSE-SU-2024:10600-1openSUSE-SU-2024:10601-1openSUSE-SU-2021:1367-1openSUSE-SU-2021:3451-1openSUSE-SU-2021:3331-1

  • firefox Rocky Linux 8 78.13.0-2.el8_4
  • firefox Rocky Linux 8 78.13.0-2.el8
  • thunderbird Rocky Linux 8 78.13.0-1.el8_4
  • thunderbird Rocky Linux 8 78.13.0-1.el8

rocky linux advisories RLSA-2021:3157RLSA-2021:3155

  • firefox Alpine 3.23, Alpine 3.22, Alpine 3.21 +6 more 92.0.1-r0
  • librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 92.0.1-r0
  • firefox-esr Alpine 3.23, Alpine 3.22, Alpine 3.21 +8 more 78.13.0-r0
  • thunderbird Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 91.3.2-r0

alpine advisories ALPINE-CVE-2021-29988

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 91.0-1
  • thunderbird Arch Linux 78.13.0-1

arch linux advisories AVG-2269AVG-2270

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-29988 updated cvss 8.8

Mozilla: Memory corruption as a result of incorrect style treatment

  • Red Hat Enterprise Linux 6 firefox, thunderbird
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux AppStream (v. 8) firefox 78.13.0-2.el8_4, thunderbird 78.13.0-1.el8_4
    fixed: vendor fix RHSA-2021:3157
  • Red Hat Enterprise Linux AppStream EUS (v. 8.1) firefox 78.13.0-2.el8_1, thunderbird 78.13.0-1.el8_1
    fixed: vendor fix RHSA-2021:3159
  • Red Hat Enterprise Linux AppStream EUS (v. 8.2) firefox 78.13.0-2.el8_2, thunderbird 78.13.0-1.el8_2
    fixed: vendor fix RHSA-2021:3156
  • Red Hat Enterprise Linux Client (v. 7) firefox 78.13.0-2.el7_9, thunderbird 78.13.0-1.el7_9 Red Hat Enterprise Linux Client Optional (v. 7) firefox 78.13.0-2.el7_9 Red Hat Enterprise Linux Server (v. 7) firefox 78.13.0-2.el7_9 Red Hat Enterprise Linux Server Optional (v. 7) firefox 78.13.0-2.el7_9, thunderbird 78.13.0-1.el7_9 Red Hat Enterprise Linux Workstation (v. 7) firefox 78.13.0-2.el7_9, thunderbird 78.13.0-1.el7_9 Red Hat Enterprise Linux Workstation Optional (v. 7) firefox 78.13.0-2.el7_9
    fixed: vendor fix RHSA-2021:3154

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

1.38% chance of exploitation in the next 30 days, higher than 72% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • bugzilla.mozilla.org https://bugzilla.mozilla.org/show_bug.cgi?id=1717922 referenced PoC, tagged "exploit" by a source, unverified

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 Thunderbird
  • Mozilla Firefox ESR
  • Mozilla Firefox

Classification

Weakness

References

Sources

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.