CVE-2026-55640 cbcoutinho CVE debrief

Who should care

Defenders responsible for Nextcloud MCP Server deployments should assess exposure and apply the patch. Network administrators and security teams should verify that the server is updated to version 0.117.2 or later to prevent potential attacks.

Why it matters

CVE-2026-55640 allows unauthenticated network attackers to manipulate vector embeddings and potentially destroy the semantic search index in Nextcloud MCP Server versions prior to 0.117.2. Defenders should verify exposure, apply the patch, and monitor for suspicious activity.

• Potential deletion or re-indexing of vector embeddings for any user
• Possible destruction of the semantic search index
• Verification of user data integrity and search functionality required
• Patch application and network exposure assessment necessary

Technical summary

The POST /webhooks/nextcloud endpoint in nextcloud_mcp_server/vector/webhook_receiver.py has no authentication by default due to WEBHOOK_SECRET defaulting to None. This allows an attacker to delete or trigger re-indexing of vector embeddings for any user by manipulating the payload. The vulnerability is fixed in version 0.117.2, and defenders should prioritize verifying exposure and applying the patch to prevent significant impact on user data and search functionality. The payload[“user”][“uid”] field parsed in nextcloud_mcp_server/vector/webhook_parser.py is attacker-controlled and is used without an authenticated-session cross-check for Qdrant operations.

Defensive priority

Defenders should prioritize verifying exposure and applying the patch, as the vulnerability allows for significant impact on user data and search functionality.

Recommended defensive actions

• Verify exposure by checking the Nextcloud MCP Server version and reviewing network exposure.
• Apply the patch by updating to version 0.117.2 or later.
• Monitor for suspicious activity related to vector embeddings and semantic search index usage.
• Review compensating controls for exposed systems while remediation is scheduled and verified.
• Check relevant monitoring, detection, and logs for exposed assets that need extra review.
• Track exceptions, retest remediated assets, and close the item only after evidence is documented.
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.

Evidence notes

The vulnerability is confirmed in Nextcloud MCP Server versions prior to 0.117.2. The issue is fixed in version 0.117.2. Details on exploitation or specific impacts on user data are limited. Defenders should verify exposure, apply the patch, and monitor for suspicious activity related to vector embeddings and semantic search index usage. The CVE record was published on 2026-08-25T16:16:55.810Z and has not been modified since then.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.