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CVE-2025-71389 – Unauthenticated RCE – cal.diy (Cal.com self-hosted) before 5.9.9

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Summary

CVE-2025-71389 is a critical unauthenticated remote code execution (RCE) vulnerability affecting cal.diy (calcom/cal.diy), the open-source self-hosted scheduling platform, in all versions prior to 5.9.9. The flaw stems from cal.diy bundling a vulnerable version of Next.js affected by the React Server Components deserialization vulnerability CVE-2025-55182, rated CVSS 10.0. A remote, unauthenticated attacker can send a single crafted HTTP request targeting React Server Component endpoints and trigger arbitrary code execution on the server — no authentication or user interaction is required.

Technical details

  • Root cause: cal.diy bundled a vulnerable version of Next.js that unsafely deserializes attacker-controlled payloads received at React Server Components (RSC) Server Function endpoints, propagating the upstream vulnerability CVE-2025-55182.
  • Trigger conditions: A single crafted HTTP request to an RSC Server Function endpoint is sufficient to trigger exploitation; no authentication, valid session, or user interaction is required.
  • Attack vector: Network — cal.diy is an internet-facing scheduling application, explicitly designed to be publicly accessible.
  • Impact: Full unauthenticated remote code execution on the host server, with critical impact across confidentiality, integrity, and availability of both the affected system and any connected systems.
  • Active exploitation context: The upstream vulnerability CVE-2025-55182 was added to the CISA Known Exploited Vulnerabilities (KEV) catalog on December 5, 2025, with a remediation due date of December 12, 2025. Public proof-of-concept exploit code targeting React Server Components is openly available. Multiple threat actors have been observed exploiting the underlying flaw, including for cryptominer delivery.

Affected software

  • cal.diy (calcom/cal.diy): All versions prior to 5.9.9

Severity

  • CVSS v4.0 Base Score: 10.0 (Critical)
    CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H
  • CVSS v3.1 Base Score: 10.0 (Critical)
    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Upgrade cal.diy to version 5.9.9 or later. This release resolves the vulnerability by updating the bundled Next.js dependency to a patched version.
  • If immediate patching is not feasible: Restrict network-level or reverse-proxy access to RSC Server Function endpoints, and limit public internet exposure of cal.diy instances until the patch can be applied.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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