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CVE-2025-27218: Sitecore Unlocked: The Tale of the Toxic Thumbnail

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Sitecore Unlocked: The Tale of the Toxic Thumbnail

Vulnerability ID: CVE-2025-27218
CVSS Score: 5.3
Published: 2025-02-20

A critical insecure deserialization vulnerability in Sitecore Experience Manager (XM) and Experience Platform (XP) 10.4 allows unauthenticated remote attackers to execute arbitrary code via the 'ThumbnailsAccessToken' HTTP header. Despite a confusingly low initial CVSS score, this is a textbook RCE leading to full system compromise.

TL;DR

Unauthenticated RCE in Sitecore 10.4. Attackers send a malicious serialized .NET object in the 'ThumbnailsAccessToken' header. The server deserializes it using the banned BinaryFormatter, granting the attacker a shell. Patch immediately with KB1002844.

⚠️ Exploit Status: WEAPONIZED

Technical Details

  • CVE ID: CVE-2025-27218
  • CWE: CWE-502 (Insecure Deserialization)
  • CVSS v3.1: 5.3 (Official) / 9.8 (Real World Impact)
  • EPSS Score: 0.576 (98th Percentile)
  • Attack Vector: Network (HTTP Header)
  • Exploit Status: Weaponized (Metasploit Available)

Affected Systems

  • Sitecore Experience Manager (XM) 10.4
  • Sitecore Experience Platform (XP) 10.4
  • Sitecore Experience Manager (XM): 10.4.0 < KB1002844 (Fixed in: 10.4 + KB1002844)
  • Sitecore Experience Platform (XP): 10.4.0 < KB1002844 (Fixed in: 10.4 + KB1002844)

Exploit Details

  • Metasploit: Ruby module that generates a WindowsIdentity gadget payload to achieve RCE.
  • Nuclei: Detection template using OAST to verify the deserialization trigger.

Mitigation Strategies

  • Apply Vendor Hotfix immediately.
  • Implement WAF rules to block the 'ThumbnailsAccessToken' header.
  • Restrict public access to Sitecore management interfaces.

Remediation Steps:

  1. Download KB1002844 from the Sitecore Support Portal.
  2. Back up your Sitecore configuration and databases.
  3. Apply the package to all Sitecore XP and XM 10.4 instances.
  4. Restart the IIS services to ensure new binaries are loaded.
  5. Verify the patch by attempting a benign request with the header (should be rejected or ignored).

References

Read the full report for CVE-2025-27218 on our website for more details including interactive diagrams and full exploit analysis.

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