Nextcloud ServerApplication · Nextcloud
CVE-2021-22915
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedNextcloud server before 19.0.11, 20.0.10, 21.0.2 is vulnerable to brute force attacks due to lack of inclusion of IPv6 subnets in rate-limiting considerations. This could potentially result in an attacker bypassing rate-limit controls such as the Nextcloud brute-force protection.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceNextcloud's brute-force protection system fails to account for IPv6 subnets when enforcing rate limits. An attacker can bypass authentication rate limiting by rotating through different IPv6 addresses within a subnet, since the system only considers individual IPv4 addresses or lacks proper IPv6 subnet aggregation.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
Nextcloud ServerApplication
< 19.0.11>= 20.0.0, < 20.0.10>= 21.0.0, < 21.0.2
FedoraOperating system
= 33= 34CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check Nextcloud Server versionLocate the Nextcloud version file (version.php in the root directory) or run: sudo -u www-data php occ --versionAffected if The installed version is less than 19.0.11, OR between 20.0.0 and 20.0.10, OR between 21.0.0 and 21.0.2
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Verify brute-force protection is activeInspect the Nextcloud config.php file (usually at /var/www/nextcloud/config/config.php) for 'auth.bruteforce.threshold' and 'auth.bruteforce.lockout.minutes' settings, or check via the admin web interface under Security settingsAffected if Brute-force protection settings exist and are configured (indicating the feature is active)
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Confirm IPv6 is enabled in the environmentCheck system network configuration with: ip -6 addr show OR check if the server has IPv6 addresses assigned. Also verify Nextcloud's trusted_proxies configuration includes IPv6 subnets if a reverse proxy is in useAffected if The server or network environment supports IPv6 connectivity and the vulnerable Nextcloud version is running
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Inspect rate-limiting behaviorReview Nextcloud server logs (/var/log/nextcloud/nextcloud.log) for authentication attempts from multiple IPv6 addresses in short succession, or test by sending login attempts from different IPv6 addresses within the same /64 subnetAffected if Multiple authentication attempts from different IPv6 addresses within a subnet are NOT being rate-limited (indicating the bypass is present)
You are affected if you run a vulnerable Nextcloud Server version (pre-19.0.11, 20.0.0-20.0.10, or 21.0.0-21.0.2) and your environment uses IPv6, since the brute-force protection fails to aggregate IPv6 subnet addresses.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped19.0.1120.0.1021.0.2
Update Nextcloud server to version 19.0.11, 20.0.10, or 21.0.2 or later to receive the patch that implements proper IPv6 subnet handling in rate-limiting logic. Consider supplementary network-level rate limiting as an additional control.
19.0.11, 20.0.10, or 21.0.2 (depending on your major version branch)
- Identify your current Nextcloud Server version by checking the admin settings or config/config.php
- Determine which major version branch you are on (19.x, 20.x, or 21.x)
- For Nextcloud 19.x: Upgrade to version 19.0.11 or later
- For Nextcloud 20.x: Upgrade to version 20.0.10 or later
- For Nextcloud 21.x: Upgrade to version 21.0.2 or later
- After upgrade, verify the rate-limiting now properly includes IPv6 subnets by testing from an IPv6 address
- Review the Nextcloud administration manual for brute-force protection settings to confirm protection is working
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Nextcloud Server
Scoped from the published advisory
- Consultation4.0 h
- Implementation16.0 h
- Testing8.0 h
- Review / QA4.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $8,960.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-22915 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-22915 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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