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CVE Record

CVE-2021-22915: Nextcloud server before 19.0.11, 20.0.10, 21.0.2 is vulnerable to brute force attacks due to lack of inclus...

Nextcloud 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.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

Nextcloud Server had a weakness in brute-force protection for IPv6 traffic. Attackers could potentially evade rate limits by spreading login attempts across IPv6 subnets, increasing the chance of password-guessing success against exposed accounts.

Executive priority

Prioritize remediation for externally exposed Nextcloud systems because this weakens a control meant to slow account compromise. Treat as moderate urgency unless sensitive accounts lack MFA or show suspicious login activity.

Technical view

CVE-2021-22915 is a CWE-307 improper restriction of excessive authentication attempts issue. Nextcloud Server before 19.0.11, 20.0.10, and 21.0.2 did not include IPv6 subnets in rate-limiting considerations, potentially bypassing brute-force controls.

Likely exposure

Organizations running affected Nextcloud Server versions with internet-accessible login endpoints, especially over IPv6, are the primary exposure. Risk depends on account password strength, MFA usage, logging, and whether brute-force protection is relied on as a key control.

Exploitation context

The sources describe a public vulnerability report and vendor advisory, but the bundle does not provide evidence of active exploitation. The CVE is not listed as KEV in the provided data.

Researcher notes

The key condition is IPv6 subnet handling in rate limiting. Evidence confirms fixed versions, but the provided bundle lacks CVSS scoring, detailed exploit telemetry, and proof of exploitation in the wild.

Mitigation direction

  • Upgrade Nextcloud Server to 19.0.11, 20.0.10, 21.0.2, or later.
  • Review Nextcloud advisory NC-SA-2021-009 for vendor-specific guidance.
  • Confirm brute-force protection remains enabled after upgrade.
  • Use MFA and strong password policy for exposed accounts.
  • Apply relevant Fedora package updates if using Fedora packages.

Validation and detection

  • Inventory all Nextcloud Server instances and record exact versions.
  • Confirm whether login endpoints are reachable from the internet over IPv6.
  • Review authentication logs for repeated failures across related IPv6 ranges.
  • Verify upgraded systems report fixed Nextcloud versions.
  • Check package manager history for relevant Fedora updates where applicable.
Prepared
Confidence
medium
Sources
6

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · low confidence lookup

CWE-307: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

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cve · low confidence lookup

CVE-2021-22915 mapping review

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
0Timeline events
0ADP providers
5Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
n/aNextcloud ServerFixed in 19.0.11, 20.0.10, 21.0.2Listed
Weakness

CWE details

CWE-307

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-307 · source CWE mapping

Improper Restriction of Excessive Authentication Attempts

Improper Restriction of Excessive Authentication Attempts represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs