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CVE-2026-73389 – Unauthenticated PHP Object Injection – Kalles Addons WordPress Plugin (≤ 1.0.6)

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Summary

CVE-2026-73389 is a critical, unauthenticated PHP Object Injection (Insecure Deserialization) vulnerability affecting the Kalles Addons WordPress plugin developed by The4, in versions up to and including 1.0.6. The plugin passes attacker-controllable input into a PHP unserialize() call without proper validation, allowing a remote, unauthenticated attacker to submit crafted serialized data to the target site. Depending on other classes ("gadget chains") available in the WordPress installation, this can lead to arbitrary object instantiation, data tampering, denial of service, or remote code execution, and the flaw carries a CVSS v3.1 base score of 9.8 (Critical).

Technical details

  • Root cause: The plugin deserializes untrusted, user-supplied data via PHP’s native unserialize() (or an equivalent unsafe deserialization routine) without validating or sanitizing the input, classified as CWE-502 (Deserialization of Untrusted Data).
  • Trigger conditions: No authentication or user interaction is required; an attacker only needs network access to a site running an affected version of the Kalles Addons plugin.
  • Attack vector: Network (AV:N) — the vulnerable parameter is reachable through a standard HTTP request to the WordPress site, with low attack complexity (AC:L) and no privileges (PR:N) or user interaction (UI:N) needed.
  • Impact: Successful exploitation can result in PHP Object Injection, which — combined with exploitable "gadget chains" elsewhere in the WordPress core, theme, or other installed plugins — can escalate to arbitrary file operations, data manipulation, denial of service, or remote code execution. The CVSS vector reflects high impact to confidentiality, integrity, and availability (C:H/I:H/A:H).

Affected software

  • Kalles Addons WordPress plugin (developed by The4) — all versions up to and including 1.0.6.

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Update the Kalles Addons plugin to a version newer than 1.0.6 as soon as the vendor (The4) releases a fixed build that removes or properly secures the unsafe deserialization logic. Site owners should check their WordPress admin dashboard and the theme/plugin vendor’s distribution channel for available updates and apply them immediately.
  • If no patch is yet available:
    • Temporarily disable or remove the Kalles Addons plugin if it is not business-critical, until a fixed version is confirmed and installed.
    • Deploy a Web Application Firewall (WAF) rule to block requests containing suspicious serialized PHP object payloads (e.g., strings beginning with O:, a:, or similar serialization markers) targeting plugin endpoints.
    • Restrict direct external access to non-essential plugin endpoints where feasible, and monitor server and application logs for anomalous POST requests or unexpected PHP errors related to deserialization.
    • Conduct an inventory check of all internet-facing WordPress assets to confirm whether the Kalles Addons plugin is installed and at what version, prioritizing remediation for externally exposed instances.

References

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