CVE-2021-30716Public exploitNetwork attack vector

SMB service interruption via infinite lock loop

Published Sep 8, 2021 · Updated Aug 3, 2024

Resource exhaustion in Apple macOS Big Sur before 11.4 allows remote authenticated users to interrupt SMB service with crafted lock requests. The smbx server retries ntvfs::lock_range after a lock failure inside a loop with no exit, leaving each client thread busy and consuming CPU. Reachability requires SMB access from a privileged network position; repeated requests can exhaust all handler threads and make file sharing unavailable.

CVSS severity6.5
Medium
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

Resource exhaustion in Apple macOS Big Sur before 11.4 allows remote authenticated users to interrupt SMB service with crafted lock requests. The smbx server retries ntvfs::lock_range after a lock failure inside a loop with no exit, leaving each client thread busy and consuming CPU. Reachability requires SMB access from a privileged network position; repeated requests can exhaust all handler threads and make file sharing unavailable.

The record

CVE
CVE-2021-30716
Published
Sep 8, 2021
Updated
Aug 3, 2024
Vendor
Apple
Product
macOS
Classifications
CWE-835, T1499.004
Attack vector
network
Privileges
authenticated

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 <11.4
  2. Affected versionversion=unspecified <2021

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Apple · macOSversion=unspecified <11.4; version=unspecified <2021

Published CVSS scores

6.5Cisco TalosCVSS:3.0/AV:N/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)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-835Loop with Unreachable Exit Condition ('Infinite Loop')
T1499.004Application or System Exploitation

Public exploit references

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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