CVE-2021-20181

Guest-to-host privilege escalation via stale fid reference

Published May 13, 2021 · Updated Aug 3, 2024

Race condition in QEMU 9pfs through 5.2.0 allows guest OS users to escalate privileges through a malicious 9p client. The v9fs_mark_fids_unreclaim loop retains the first fid across a coroutine yield, while a concurrent clunk request can free that fid and leave the restart reference stale. Exploitation requires control of a guest using the 9pfs backend and can compromise host confidentiality, integrity, and availability.

CVSS severity7.5
High
EPSS probability0.32%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Race condition in QEMU 9pfs through 5.2.0 allows guest OS users to escalate privileges through a malicious 9p client. The v9fs_mark_fids_unreclaim loop retains the first fid across a coroutine yield, while a concurrent clunk request can free that fid and leave the restart reference stale. Exploitation requires control of a guest using the 9pfs backend and can compromise host confidentiality, integrity, and availability.

The record

CVE
CVE-2021-20181
Published
May 13, 2021
Updated
Aug 3, 2024
Vendor
QEMU Project
Product
QEMU
Classifications
T1068
Attack vector
local
Privileges
admin

Timeline

How it unfolded

  1. May 13, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=up to, including 5.2.0

What conditions does exploitation require?

Attack vectorlocal
Required privilegesadmin

What is affected?

QEMU Project · QEMUversion=up to, including 5.2.0

Published CVSS scores

7.5UbuntuCVSS:3.1/AV:L/AC:H/PR:H/UI:N/S:C/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesadmin
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1068Exploitation for Privilege Escalation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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