CVE-2021-3416

Host process hang or crash via NIC loopback

Published Mar 18, 2021 · Updated Aug 3, 2024

An infinite loop in QEMU 5.2.0 and earlier allows guest OS users to consume host CPU or crash the host QEMU process. NIC loopback handlers call receive callbacks directly, bypassing the reentrancy check in qemu_net_queue_send and recursively reentering packet reception. The guest must control an emulated NIC configured for loopback; impact is limited to a hang or crash of that virtual machine's QEMU process.

CVSS severity6.0
Medium
EPSS probability0.46%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

An infinite loop in QEMU 5.2.0 and earlier allows guest OS users to consume host CPU or crash the host QEMU process. NIC loopback handlers call receive callbacks directly, bypassing the reentrancy check in qemu_net_queue_send and recursively reentering packet reception. The guest must control an emulated NIC configured for loopback; impact is limited to a hang or crash of that virtual machine's QEMU process.

The record

CVE
CVE-2021-3416
Published
Mar 18, 2021
Updated
Aug 3, 2024
Vendor
QEMU Project
Product
QEMU
Classifications
CWE-835, T1499.004
Attack vector
local
Privileges
admin

Timeline

How it unfolded

  1. Mar 18, 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=versions up to and including 5.2.0

What conditions does exploitation require?

Attack vectorlocal
Required privilegesadmin

What is affected?

QEMU Project · QEMUversion=versions up to and including 5.2.0

Published CVSS scores

6.0UbuntuCVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:C/C:N/I:N/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

CWE-835Loop with Unreachable Exit Condition ('Infinite Loop')
T1499.004Endpoint Denial of Service: Application or System Exploitation

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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