CVE-2019-9874: Sitecore CMS and Experience Platform (XP) Deserialization Vulnerability

CVE-2019-9874 is a deserialization vulnerability affecting Sitecore CMS and Experience Platform (XP). It exists in the Sitecore.Security.AntiCSRF module and allows an unauthenticated attacker to execute arbitrary code by sending a serialized .NET object in the HTTP POST parameter __CSRFTOKEN.

This matters because remote code execution on a content-management or digital-experience platform can give an attacker control of the host, access to content and configuration data, and a foothold for further movement. Teams running Sitecore should treat the issue as high priority and confirm every technical detail against the official vendor advisory.

How it works

The weakness is CWE-502: deserialization of untrusted data. The AntiCSRF module accepts the __CSRFTOKEN value from an HTTP POST request and deserializes it as a .NET object without sufficient validation. An attacker who can reach the vulnerable endpoint crafts a malicious serialized payload; when the application deserializes that object, the payload can trigger arbitrary code execution under the privileges of the Sitecore process.

No authentication is required. Exact payload construction, gadget chains, or request formats are not detailed here; defenders must obtain those specifics from the vendor advisory rather than inventing them.

Am I affected? How to find it in your systems

Sitecore CMS and XP are typically deployed on Windows servers running IIS and the .NET Framework, often as internet-facing or partner-facing web applications. Inventory every instance of Sitecore CMS or Experience Platform in your environment.

• Identify installed product editions and build numbers, then compare them with the affected versions listed in the vendor advisory; public detail on exact version ranges is limited in this record, so confirmation against Sitecore’s guidance is required.
• Check whether the Sitecore.Security.AntiCSRF module is present and active.
• Review web-server and application logs for POST requests that include the __CSRFTOKEN parameter, especially those carrying unusually large or binary-looking values.
• Look for secondary indicators of compromise on the host: unexpected child processes spawned by the IIS worker process, anomalous outbound network connections, or sudden changes to Sitecore configuration or content.
• If the platform is hosted in a cloud or managed service, apply the same inventory and log-review steps and follow any BOD 22-01 guidance that applies to cloud services.

How to remediate

Patch first. Apply the update or mitigation package supplied by Sitecore for CVE-2019-9874 exactly as described in the vendor advisory. CISA’s required action is to apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

• Test the vendor fix in a non-production environment that mirrors production configuration before broad deployment.
• After patching, verify that the AntiCSRF handling no longer deserializes untrusted objects.
• As a longer-term hardening measure for this class of flaw, ensure that .NET applications never deserialize data from untrusted sources, prefer type allow-lists where deserialization cannot be avoided, and keep the underlying framework and libraries current.

If you can't patch immediately

Until the vendor update can be applied, reduce exposure with compensating controls.

• Place Sitecore hosts behind strict network segmentation; allow inbound traffic only from trusted networks or reverse proxies and block direct internet access where possible.
• Deploy or tune a web-application firewall to inspect the __CSRFTOKEN parameter and block requests that contain serialized .NET objects or other anomalous payloads.
• If operationally feasible and confirmed safe with the vendor, disable or restrict the AntiCSRF feature until the patch is installed.
• Run the Sitecore application pool under the least privileges necessary and monitor process creation, file-system changes, and outbound connections from those hosts.
• Increase logging verbosity for the relevant modules and forward logs to a central SIEM for rapid alerting.

If your data may have been exposed

Vulnerabilities that permit unauthenticated code execution are frequently used to establish persistence and exfiltrate data. Ransomware use is not documented for this specific CVE, yet any successful exploitation can still result in a breach. Conduct a thorough forensic review of affected systems, rotate credentials that may have been accessible, and assess whether content, customer, or configuration data left the environment. As an additional check, you can run a free exposure scan of your email addresses against known breach data sets to see whether related accounts have appeared in prior incidents.