CVE-2021-41177Network attack vector

Two-factor code brute-force via skipped rate limiting

Published Oct 25, 2021 · Updated Aug 4, 2024

Improper rate limiting in Nextcloud Server before 20.0.13, 21.0.5, and 22.2.0 allows remote authenticated users to brute-force two-factor codes. When no distributed memory cache is configured, the rate-limit check is skipped instead of falling back to persistent storage. A default installation leaves two-factor-code attempts unthrottled, allowing repeated guesses but not directly granting access without a valid code.

CVSS severity8.1
High
EPSS probability1.55%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper rate limiting in Nextcloud Server before 20.0.13, 21.0.5, and 22.2.0 allows remote authenticated users to brute-force two-factor codes. When no distributed memory cache is configured, the rate-limit check is skipped instead of falling back to persistent storage. A default installation leaves two-factor-code attempts unthrottled, allowing repeated guesses but not directly granting access without a valid code.

The record

CVE
CVE-2021-41177
Published
Oct 25, 2021
Updated
Aug 4, 2024
Vendor
Nextcloud GmbH
Product
Nextcloud Security Advisories
Classifications
CWE-799, T1110
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Oct 25, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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=< 20.0.13
  2. Affected versionversion=>= 21.0.0, < 21.0.5
  3. Affected versionversion=< 22.2.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Nextcloud GmbH · Nextcloud Security Advisoriesversion=< 20.0.13; version=>= 21.0.0, < 21.0.5; version=< 22.2.0

Published CVSS scores

8.1NISTCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/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.

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
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-799Improper Control of Interaction Frequency
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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