CVE-2021-1884Network attack vector

Remote service interruption via Heimdal race condition

Published Sep 8, 2021 · Updated Aug 3, 2024

A Heimdal race condition in iOS, iPadOS, tvOS, watchOS, and macOS allows remote attackers to cause a service interruption. Apple identifies the locus only as Heimdal and states that insufficient locking permits the race; no function or protocol path is disclosed. Remote reachability is required; affected releases are before 14.5 on iOS, iPadOS, and tvOS, before 7.4 on watchOS, or before Big Sur 11.3, with Catalina and Mojave also affected.

CVSS severityUnavailable
Unscored
EPSS probability1.62%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

A Heimdal race condition in iOS, iPadOS, tvOS, watchOS, and macOS allows remote attackers to cause a service interruption. Apple identifies the locus only as Heimdal and states that insufficient locking permits the race; no function or protocol path is disclosed. Remote reachability is required; affected releases are before 14.5 on iOS, iPadOS, and tvOS, before 7.4 on watchOS, or before Big Sur 11.3, with Catalina and Mojave also affected.

The record

CVE
CVE-2021-1884
Published
Sep 8, 2021
Updated
Aug 3, 2024
Vendor
Apple
Product
iOS and iPadOS
Classifications
CWE-362, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 8, 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=unspecified <14.5

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Apple · iOS and iPadOSversion=unspecified <14.5
Apple · tvOSversion=unspecified <14.5
Apple · macOSversion=unspecified <11.3; version=unspecified <2021
Apple · watchOSversion=unspecified <7.4

Published CVSS scores

No CVSS assessment is available in this record.

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

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

Weakness, pattern, technique

CWE-362Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
T1499.004Application 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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