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Fencer Vulns
high 7.5

CVE-2021-20181 json

A race condition flaw was found in the 9pfs server implementation of QEMU up to and including 5.2.0.

published 2021-05-13 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-7636

cvss
7.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 26%
public exploits
0
none known

Summary

A race condition flaw was found in the 9pfs server implementation of QEMU up to and including 5.2.0. This flaw allows a malicious 9p client to cause a use-after-free error, potentially escalating their privileges on the system. The highest threat from this vulnerability is to confidentiality, integrity as well as system availability.

Patch / fix

  • qemu Debian 14, Debian 13, Debian 12 1:5.2+dfsg-4

debian advisories DEBIAN-CVE-2021-20181

  • qemu Ubuntu 16.04 LTS 1:2.5+dfsg-5ubuntu10.49
  • qemu Ubuntu 18.04 LTS 1:2.11+dfsg-1ubuntu7.35
  • qemu Ubuntu 20.04 LTS 1:4.2-3ubuntu6.12
  • qemu Ubuntu Pro 14.04 LTS 2.0.0+dfsg-2ubuntu1.47+esm6

ubuntu advisories UBUNTU-CVE-2021-20181

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

  • kvm SLE Point of Sale 11 SP3 1.4.2-53.38.1
  • qemu SLES 12 SP2-BCL 2.6.2-41.62.1
  • qemu SLES for SAP 15 SP1, SLES 15 SP1-LTSS, SLES 15 SP1-BCL +6 more 3.1.1.1-9.24.3
  • qemu SLES for SAP 15, SLES 15-LTSS, SLE High Performance Computing 15-LTSS +1 more 2.11.2-9.43.1
  • qemu SLES for SAP 12 SP5, SLES 12 SP5 3.1.1.1-48.2
  • qemu SLES for SAP 12 SP4, SLES 12 SP4-LTSS, OpenStack Cloud Crowbar 9 +1 more 2.11.2-5.29.1
  • qemu SLES for SAP 12 SP3, SLES 12 SP3-LTSS, SLES 12 SP3-BCL +3 more 2.9.1-6.47.1
  • qemu SLE Module for Server Applications 15 SP2, SLE Module for Basesystem 15 SP2 4.2.1-11.13.1

suse advisories SUSE-SU-2021:14706-1SUSE-SU-2021:1305-1SUSE-SU-2021:14704-1SUSE-SU-2021:1245-1SUSE-SU-2021:1244-1SUSE-SU-2021:1242-1

  • qemu openSUSE Tumbleweed 6.1.0-32.1
  • qemu openSUSE Leap 15.2 4.2.1-lp152.9.9.2
  • qemu-testsuite openSUSE Leap 15.2 4.2.1-lp152.9.9.5
  • qemu-linux-user openSUSE Leap 15.2 4.2.1-lp152.9.9.3

opensuse advisories openSUSE-SU-2024:11287-1openSUSE-SU-2021:0363-1

  • qemu Alpine 3.24, Alpine 3.23, Alpine 3.22 +6 more 6.0.0-r1

alpine advisories ALPINE-CVE-2021-20181

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

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-2021-20181 updated cvss 7.5

qemu: 9pfs: TOCTOU privilege escalation vulnerability

  • Red Hat Enterprise Linux 6 qemu-img, qemu-kvm, qemu-kvm-tools, qemu-guest-agent, qemu-guest-agent-win32 Red Hat Enterprise Linux 7 qemu-img, qemu-kvm, libcacard, qemu-img-ma, qemu-kvm-ma, qemu-kvm-tools, +5 more Red Hat Enterprise Linux 8 qemu-img, qemu-kvm, qemu-kvm-core, qemu-kvm-tests, qemu-kvm-common, qemu-guest-agent, +8 more Red Hat Enterprise Linux 8 Advanced Virtualization qemu-img, qemu-kvm, qemu-kiwi, qemu-kvm-core, qemu-kvm-tests, qemu-kvm-common, +9 more Red Hat Enterprise Linux 9 qemu-img, qemu-kvm, qemu-kvm-core, qemu-kvm-tools, qemu-pr-helper, qemu-kvm-common, +16 more Red Hat OpenStack Platform 10 (Newton) qemu-img-rhev, qemu-kvm-rhev, qemu-kvm-tools-rhev, qemu-kvm-common-rhev
    +2 more products
    Red Hat OpenStack Platform 13 (Queens) qemu-img-rhev, qemu-kvm-rhev, qemu-kvm-tools-rhev, qemu-kvm-common-rhev Red Hat Virtualization 4 qemu-img-rhev, qemu-kvm-rhev, qemu-kvm-tools-rhev, qemu-kvm-common-rhev
    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

Exploit prediction

0.33% chance of exploitation in the next 30 days, higher than 26% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 7.5 CVSS:3.1/AV:L/AC:H/PR:H/UI:N/S:C/C:H/I:H/A:H
NVD 2.0 6.9 AV:L/AC:M/Au:N/C:C/I:C/A:C

Affected products

  • n/a qemu

Classification

Weakness
  • CWE-367 Time-of-check Time-of-use (TOCTOU) Race Condition
  • CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization

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.