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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.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • The generator meta tag’s content: MaxSite CMS

References

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Zero-Days to Shrink MTTR

1

Map your entire attack surface (continously)

IONIX uses multi-factor discovery methods, including DNS analysis, certificate mapping, metadata inspection, and more, to automatically map every internet-facing asset across your environment. This includes cloud instances, third-party platforms, shadow IT, and even forgotten infrastructure that traditional tools miss.

2

Monitor for new CVEs

Dozens of threat intel feeds using agentic technology are continuously analyzed to detect the appearance of proof-of-concept code, exploit kits, and indicators of active targeting. IONIX goes further by applying AI to proactively evaluate whether emerging vulnerabilities are likely to be exploited, even before PoCs go public.

3

Identify Potential External Exposures

Not all CVEs matter. IONIX filters vulnerabilities by asking attacker-centric questions: Can it be reached from the internet? Does it require authentication? Is it being exploited in the wild? This dramatically reduces noise and focuses teams on threats that can actually be weaponized.

4

Create Safe, Scalable Exploit Validations

IONIX transforms real-world PoCs into safe, non-intrusive test payloads that can be run in production environments without disruption. These simulations are precisely targeted to the systems that are vulnerable, ensuring rapid validation without unnecessary load.

5

Execute Exploit Validations

By combining context about software stack, versioning, exposure status, and reachability, IONIX ensures that only the right payloads are executed against the right assets, maximizing efficiency and minimizing risk.

6

Drive Fast and Actionable Remediation

Results are routed through integrations with ticketing, SOAR, and SIEM tools. Issues are written in plain language, bundled into remediation clusters, and prioritized based on asset criticality, exploitability, and blast radius. This shortens mean time to remediation (MTTR) and empowers teams to act with confidence.

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