CVE-2021-20219

System hang via incorrect TTY buffer comparison

Published Mar 23, 2021 · Updated Aug 3, 2024

Denial of service in Linux Kernel 4.18 allows local users to hang the system through the N_TTY line discipline. In n_tty_receive_char_special(), an unmasked comparison reads ldata->read_head while it can change, allowing the receive loop to stall indefinitely. A normal local account is sufficient; exploitation affects availability by holding the kernel loop rather than reading or altering data.

CVSS severity5.5
Medium
EPSS probability0.40%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in Linux Kernel 4.18 allows local users to hang the system through the N_TTY line discipline. In n_tty_receive_char_special(), an unmasked comparison reads ldata->read_head while it can change, allowing the receive loop to stall indefinitely. A normal local account is sufficient; exploitation affects availability by holding the kernel loop rather than reading or altering data.

The record

CVE
CVE-2021-20219
Published
Mar 23, 2021
Updated
Aug 3, 2024
Vendor
The Linux Kernel Organization
Product
Linux Kernel
Classifications
CWE-697, T1499.004
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Mar 23, 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=kernel 4.18

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

The Linux Kernel Organization · Linux Kernelversion=kernel 4.18

Published CVSS scores

2.1NIST NVDAV:L/AC:L/Au:N/C:N/I:N/A:P
5.5CanonicalCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-697Incorrect Comparison
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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