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Fencer Intel
medium 6.8

CVE-2021-33656 json

When setting font with malicous data by ioctl cmd PIO_FONT,kernel will write memory out of bounds.

published 4y ago modified last mo. also EUVD-2021-20333

cvss
6.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.6%
EPSS 0.6% chance of exploitation in 30 days, percentile 49%
public exploits
0
none known

Summary

When setting font with malicous data by ioctl cmd PIO_FONT,kernel will write memory out of bounds.

Patch / fix

  • linux linux_kernel 5.10.127
  • linux Debian 14, Debian 13, Debian 12 5.14.6-1

debian advisories DEBIAN-CVE-2021-33656

  • linux Ubuntu 18.04 LTS 4.15.0-192.203
  • linux Ubuntu 20.04 LTS 5.4.0-125.141
  • linux-aws Ubuntu 18.04 LTS 4.15.0-1140.151
  • linux-aws Ubuntu 20.04 LTS 5.4.0-1084.91
  • linux-gcp Ubuntu 20.04 LTS 5.4.0-1087.95
  • linux-gke Ubuntu 20.04 LTS 5.4.0-1081.87
  • linux-ibm Ubuntu 20.04 LTS 5.4.0-1032.36
  • linux-iot Ubuntu 20.04 LTS 5.4.0-1004.6

ubuntu advisories UBUNTU-CVE-2021-33656

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

  • perf rhev hypervisor 4.4, RHEL 8.6 EUS 4.18.0-372.70.1.el8_6
  • kernel-rt RHEL 8 4.18.0-477.10.1.rt7.274.el8_8
  • kernel-rt-kvm RHEL 8 4.18.0-477.10.1.rt7.274.el8_8
  • kernel-rt-core RHEL 8 4.18.0-477.10.1.rt7.274.el8_8
  • kernel-rt-modules RHEL 8 4.18.0-477.10.1.rt7.274.el8_8
  • kernel-rt-debug-kvm RHEL 8 4.18.0-477.10.1.rt7.274.el8_8
  • kernel-rt-debug-core RHEL 8 4.18.0-477.10.1.rt7.274.el8_8
  • kernel-rt-debug-modules RHEL 8 4.18.0-477.10.1.rt7.274.el8_8

red hat advisories RHSA-2023:4789RHSA-2023:2736RHSA-2023:2951

  • kernel-rt SLE Real Time 12 SP5 4.12.14-10.97.1
  • kernel-syms-rt SLE Real Time 12 SP5 4.12.14-10.97.1
  • kernel-rt_debug SLE Real Time 12 SP5 4.12.14-10.97.1
  • kernel-source-rt SLE Real Time 12 SP5 4.12.14-10.97.1
  • kernel-rt SLE Micro 5.2, SLE Micro 5.1, Real Time Module 15 SP3 +1 more 5.3.18-150300.99.1
  • kernel-syms-rt Real Time Module 15 SP3 5.3.18-150300.99.1
  • kernel-rt_debug Real Time Module 15 SP3 5.3.18-150300.99.1
  • kernel-source-rt Real Time Module 15 SP3 5.3.18-150300.99.1

suse advisories SUSE-SU-2023:0416-1SUSE-SU-2022:2910-1SUSE-SU-2022:2892-2SUSE-SU-2022:2892-1SUSE-SU-2022:2875-2SUSE-SU-2022:2875-1

  • kernel-rt AlmaLinux 8 4.18.0-477.10.1.rt7.274.el8_8
  • kernel-rt-kvm AlmaLinux 8 4.18.0-477.10.1.rt7.274.el8_8
  • kernel-rt-core AlmaLinux 8 4.18.0-477.10.1.rt7.274.el8_8
  • kernel-rt-modules AlmaLinux 8 4.18.0-477.10.1.rt7.274.el8_8
  • kernel-rt-debug-kvm AlmaLinux 8 4.18.0-477.10.1.rt7.274.el8_8
  • kernel-rt-debug-core AlmaLinux 8 4.18.0-477.10.1.rt7.274.el8_8
  • kernel-rt-debug-modules AlmaLinux 8 4.18.0-477.10.1.rt7.274.el8_8
  • kernel-rt-modules-extra AlmaLinux 8 4.18.0-477.10.1.rt7.274.el8_8

almalinux advisories ALSA-2023:2736ALSA-2023:2951

Is CVE-2021-33656 in your stack? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

Check with Fencer

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-2021-33656 updated cvss 6.8

kernel: when setting font with malicious data by ioctl PIO_FONT, kernel will write memory out of bounds

  • Red Hat Enterprise Linux 6 perf, kernel, python-perf, kernel-kdump, kernel-firmware, kernel-bootwrapper, +1 more Red Hat Enterprise Linux 7 perf, kernel, bpftool, kernel-rt, python-perf, kernel-kdump, +8 more
    affected: workaround, no fix planned “Out of support scope”
  • Red Hat Enterprise Linux 9 rv, perf, rtla, kernel, bpftool, libperf, +82 more
    affected: workaround, no fix available
  • Red Hat CodeReady Linux Builder EUS (v.8.6) kernel-debuginfo-common-x86_64 4.18.0-372.70.1.el8_6, kernel-debuginfo-common-aarch64 4.18.0-372.70.1.el8_6, kernel-debuginfo-common-ppc64le 4.18.0-372.70.1.el8_6
    fixed: vendor fix RHSA-2023:4789
  • Red Hat Enterprise Linux BaseOS (v. 8) perf 4.18.0-477.10.1.el8_8, kernel 4.18.0-477.10.1.el8_8, bpftool 4.18.0-477.10.1.el8_8, kernel-core 4.18.0-477.10.1.el8_8, kernel-tools 4.18.0-477.10.1.el8_8, python3-perf 4.18.0-477.10.1.el8_8, +15 more
    fixed: vendor fix RHSA-2023:2951
  • Red Hat Enterprise Linux BaseOS EUS (v.8.6) perf 4.18.0-372.70.1.el8_6, kernel 4.18.0-372.70.1.el8_6, bpftool 4.18.0-372.70.1.el8_6, kernel-core 4.18.0-372.70.1.el8_6, kernel-tools 4.18.0-372.70.1.el8_6, python3-perf 4.18.0-372.70.1.el8_6, +15 more
    fixed: vendor fix RHSA-2023:4789
  • Red Hat Enterprise Linux CRB (v. 8) kernel-debuginfo-common-x86_64 4.18.0-477.10.1.el8_8, kernel-debuginfo-common-aarch64 4.18.0-477.10.1.el8_8, kernel-debuginfo-common-ppc64le 4.18.0-477.10.1.el8_8
    fixed: vendor fix RHSA-2023:2951
  • Red Hat Enterprise Linux NFV (v. 8) kernel-rt 4.18.0-477.10.1.rt7.274.el8_8, kernel-rt-kvm 4.18.0-477.10.1.rt7.274.el8_8, kernel-rt-core 4.18.0-477.10.1.rt7.274.el8_8, kernel-rt-modules 4.18.0-477.10.1.rt7.274.el8_8, kernel-rt-debug-kvm 4.18.0-477.10.1.rt7.274.el8_8, kernel-rt-debug-core 4.18.0-477.10.1.rt7.274.el8_8, +4 more Red Hat Enterprise Linux RT (v. 8) kernel-rt 4.18.0-477.10.1.rt7.274.el8_8, kernel-rt-core 4.18.0-477.10.1.rt7.274.el8_8, kernel-rt-modules 4.18.0-477.10.1.rt7.274.el8_8, kernel-rt-debug-core 4.18.0-477.10.1.rt7.274.el8_8, kernel-rt-debug-modules 4.18.0-477.10.1.rt7.274.el8_8, kernel-rt-modules-extra 4.18.0-477.10.1.rt7.274.el8_8, +2 more
    fixed: vendor fix RHSA-2023:2736
  • Red Hat Virtualization 4 Hypervisor for RHEL 8 perf 4.18.0-372.70.1.el8_6, kernel-debuginfo-common-x86_64 4.18.0-372.70.1.el8_6
    fixed: vendor fix RHSA-2023:4789
show 1 more
  • Red Hat Enterprise Linux 9 kernel-rt, kernel-rt-selftests-internal
    not affected: vulnerable code not present

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

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
total
Assessed
last mo.

Exploit prediction

0.61% chance of exploitation in the next 30 days, higher than 49% of all scored CVEs (FIRST EPSS, 2026-11-09).

CVSS scores

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

Affected products

  • n/a openEuler:kernel

Classification

Weakness
Technology feeds

References

Sources

Fencer Intel 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.