CVE-2021-20257

Host CPU exhaustion via transmit-descriptor underflow

Published Mar 16, 2022 · Updated Aug 3, 2024

An infinite loop in QEMU's e1000 NIC emulator allows guest OS users to monopolize host CPU through crafted transmit descriptors. In process_tx_desc, chaining a data descriptor to a legacy descriptor can make msh smaller than tp->size, so subtracting tp->size underflows the byte count and prevents loop progress. The path requires control of an emulated e1000 transmit ring; the affected QEMU process spins and consumes host CPU without reading or altering host data.

CVSS severity6.5
Medium
EPSS probability0.37%
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's e1000 NIC emulator allows guest OS users to monopolize host CPU through crafted transmit descriptors. In process_tx_desc, chaining a data descriptor to a legacy descriptor can make msh smaller than tp->size, so subtracting tp->size underflows the byte count and prevents loop progress. The path requires control of an emulated e1000 transmit ring; the affected QEMU process spins and consumes host CPU without reading or altering host data.

The record

CVE
CVE-2021-20257
Published
Mar 16, 2022
Updated
Aug 3, 2024
Vendor
QEMU Project
Product
QEMU
Classifications
CWE-835, T1499.004
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Mar 16, 2022CVE 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?

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  1. Affected versionversion=Fixed-In v6.2.0

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

QEMU Project · QEMUversion=Fixed-In v6.2.0

Published CVSS scores

3.2Red HatCVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:C/C:N/I:N/A:L
6.5NVDCVSS:3.1/AV:L/AC:L/PR:L/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
Privilegesauthenticated
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.

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Technologies

Your stack

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Your stack

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