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CVE-2026-70554 – Unauthenticated PHP Object Injection (RCE) – MaxSite CMS 0.78 through 109.5

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Summary

CVE-2026-70554 is an unauthenticated PHP Object Injection (deserialization of untrusted data, CWE-502) vulnerability in MaxSite CMS. The application passes attacker-controlled serialized data from the maxsite_comuser cookie directly to unserialize(), allowing a remote attacker to inject crafted PHP objects and achieve remote code execution. It is rated Critical.

Technical details

  • Root cause: Serialized data taken from the maxsite_comuser cookie is passed directly to PHP’s unserialize() without validation or class allowlisting.
  • Trigger conditions: A single HTTP request carrying a crafted serialized PHP object in the maxsite_comuser cookie; magic methods are invoked during reconstruction of the object graph.
  • Attack vector: Network-based, requiring no authentication and no user interaction.
  • Impact: Property-oriented programming (gadget-chain) attacks leading to remote code execution, with high impact to confidentiality, integrity, and availability.

Affected software

  • MaxSite CMS versions 0.78 through 109.5.

Severity

  • CVSS v3.1 base score: 9.8 (Critical) — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  • CVSS v4.0 base score: 9.3 (Critical) — CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H

Mitigation and recommended actions

  • Immediate: Upgrade to MaxSite CMS 109.6 or later, which remediates the issue.
  • If no patch can be applied: Restrict or filter untrusted maxsite_comuser cookie values at a web application firewall or reverse proxy, and limit network exposure of the affected application until the upgrade is completed.

References

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