CVE-2021-22915Network attack vector

Unauthenticated login throttling bypass via IPv6 rotation

Published Jun 11, 2021 · Updated Aug 3, 2024

Improper brute-force protection in Nextcloud Server before 19.0.11, 20.0.10, and 21.0.2 allows remote attackers to evade login throttling. The brute-force throttler groups IPv4 requests by subnet but fails to include IPv6 subnets when calculating rate-limit buckets, allowing address rotation within an IPv6 allocation to avoid accumulated delays. An unauthenticated network position with multiple IPv6 addresses is required, and successful password guessing grants access only to accounts whose credentials are discovered.

CVSS severity9.8
Critical
EPSS probability1.74%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper brute-force protection in Nextcloud Server before 19.0.11, 20.0.10, and 21.0.2 allows remote attackers to evade login throttling. The brute-force throttler groups IPv4 requests by subnet but fails to include IPv6 subnets when calculating rate-limit buckets, allowing address rotation within an IPv6 allocation to avoid accumulated delays. An unauthenticated network position with multiple IPv6 addresses is required, and successful password guessing grants access only to accounts whose credentials are discovered.

The record

CVE
CVE-2021-22915
Published
Jun 11, 2021
Updated
Aug 3, 2024
Vendor
Nextcloud GmbH
Product
Nextcloud Server
Classifications
CWE-307, CAPEC-49, T1110
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 11, 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=Fixed in 19.0.11, 20.0.10, 21.0.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Nextcloud GmbH · Nextcloud Serverversion=Fixed in 19.0.11, 20.0.10, 21.0.2

Published CVSS scores

9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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.

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-307Improper Restriction of Excessive Authentication Attempts
CAPEC-49Password Brute Forcing
T1110Brute Force

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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