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CVE Record

CVE-2021-37631: Circle can be accessed by non-Circle members in Nextcloud Deck

Deck is an open source kanban style organization tool aimed at personal planning and project organization for teams integrated with Nextcloud. In affected versions the Deck application didn't properly check membership of users in a Circle. This allowed other users in the instance to gain access to boards that have been shared with a Circle, even if the user was not a member of the circle. It is recommended that Nextcloud Deck is upgraded to 1.5.1, 1.4.4 or 1.2.9. If you are unable to update it is advised to disable the Deck plugin.

MediumCVSS 6.5Not KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

CVE-2021-37631 lets a logged-in user on the same Nextcloud instance view Deck boards shared with a Circle even when they are not a Circle member. The business risk is unauthorized disclosure of planning or project data stored in Deck, especially where boards contain customer, operational, or security information.

Executive priority

Treat as a near-term confidentiality issue, not an internet-wide emergency. Prioritize remediation for shared business, customer, security, or regulated project boards. The urgency increases if many internal users have accounts on the same Nextcloud instance.

Technical view

Nextcloud Deck failed to properly check Circle membership before allowing access to Circle-shared boards. The issue is an authorization weakness mapped to CWE-639, with network access, low attack complexity, low privileges, no user interaction, and high confidentiality impact under CVSS 3.1 score 6.5.

Likely exposure

Exposure is limited to Nextcloud instances running affected Deck versions: before 1.2.9, 1.3.0 through before 1.4.4, or 1.5.0 through before 1.5.1. Risk is most relevant where boards are shared with Circles and other local users exist.

Exploitation context

The source bundle does not indicate CISA KEV listing or active exploitation. Exploitation requires an authenticated user on the instance, but does not require user interaction according to the CVSS vector.

Researcher notes

Evidence supports an access-control failure in Deck Circle membership checks, fixed in specific Deck releases. The bundle names no exploit code, no active exploitation, and no workaround beyond disabling Deck. Keep validation non-invasive and focused on authorization behavior and version state.

Mitigation direction

  • Upgrade Nextcloud Deck to 1.5.1, 1.4.4, or 1.2.9 as applicable.
  • Disable the Deck plugin if immediate upgrade is not possible.
  • Review Circle-shared Deck boards for sensitive content exposure.
  • Check Nextcloud advisory guidance before applying compensating controls.

Validation and detection

  • Inventory Nextcloud instances and installed Deck plugin versions.
  • Identify boards shared with Circles on affected versions.
  • Confirm non-Circle users cannot access Circle-shared boards after remediation.
  • Review audit logs for unusual Deck board access where available.
Prepared
Confidence
high
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-639: 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-2021-37631 mapping review

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Vulnerability profileCVE Program record
Severity
Medium
CVSS
6.5 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N

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.

1CVSS vectors
0Timeline events
0ADP providers
5Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
6.5CVSS 3.1MediumCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N2.83.6Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

6.5Medium
CVSS 3.1 vector shape for CVE-2021-37631Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
nextcloudsecurity-advisories< 1.2.9, >= 1.3.0, < 1.4.4, >= 1.5.0, < 1.5.1Listed
Weakness

CWE details

CWE-639

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

CWE-639 · source CWE mapping

Authorization Bypass Through User-Controlled Key

Authorization Bypass Through User-Controlled Key represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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