CVE-2021-42237
Sitecore XP Remote Command Execution Vulnerability
Description
CVE-2021-42237 is a critical insecure deserialization vulnerability in Sitecore Experience Platform (Sitecore XP) versions up to 9.3 Initial Release that allows unauthenticated remote code execution. The flaw exists in the Report.ashx handler, which deserializes untrusted data without proper validation, enabling an attacker to execute arbitrary commands on the server. With an EPSS score of 97.4% (99.9th percentile), this is one of the most heavily exploited CMS vulnerabilities. CISA has added CVE-2021-42237 to the Known Exploited Vulnerabilities catalog.
KEV Information
CVSS Score
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:HOpen in CalculatorAffected Products
| Vendor | Product | Version |
|---|---|---|
| sitecore | experience platform | 7.5; 8.0; 8.1; 8.2 |
Multiple CVSS Assessments
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
References
- http://packetstormsecurity.com/files/164988/Sitecore-Experience-Platform-XP-Remote-Code-Execution.html(Third Party Advisory, VDB Entry)
- https://blog.assetnote.io/2021/11/02/sitecore-rce/(Exploit, Third Party Advisory)
- https://support.sitecore.com/kb?id=kb_article_view&sysparm_article=KB1000776(Vendor Advisory)
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-42237(US Government Resource)
Weakness Type
CWE-502: Deserialization of Untrusted Data
CVE-2021-42237 is an insecure deserialization vulnerability in Sitecore XP's Report.ashx handler. The handler accepts serialized .NET objects from unauthenticated users and deserializes them without validating the object types, allowing an attacker to supply malicious serialized payloads that execute arbitrary code during the deserialization process.
Learn more: CWE-502 — Deserialization of Untrusted Data
Impact Analysis
CVE-2021-42237 has a CVSS v3.1 base score of 9.8 (Critical), reflecting the devastating impact of unauthenticated remote code execution. The vulnerability requires no authentication, no user interaction, and has low attack complexity — an attacker simply sends a crafted HTTP request to the Report.ashx endpoint. Successful exploitation grants full control of the Sitecore server, including access to the database, file system, and potentially the entire internal network. The EPSS score of 97.4% (99.9th percentile) confirms this is among the most exploited vulnerabilities globally. Sitecore XP is a widely used enterprise content management system, making this vulnerability particularly impactful for large organizations.
Exploit Maturity
Public exploit code for CVE-2021-42237 is readily available, including detailed proof-of-concept demonstrations and automated exploitation tools. CISA confirmed active exploitation by adding it to the KEV catalog. The vulnerability has been targeted by multiple threat actor groups for initial access to enterprise networks through Sitecore installations. The EPSS score of 97.4% (99.9th percentile) reflects the pervasive exploitation activity targeting exposed Sitecore instances.
Remediation
- Apply the Sitecore security hotfix that addresses CVE-2021-42237 immediately. Sitecore has released patches for all affected versions — download and install from the Sitecore support portal.
- Upgrade Sitecore XP to a version that is not affected by this vulnerability (9.3 Initial Release or later with the hotfix applied).
- Restrict access to the Report.ashx endpoint through web server configuration or a web application firewall (WAF) until patching is complete. Block unauthenticated access to this handler.
- Audit Sitecore installations for signs of compromise including unexpected files, modified configurations, new user accounts, web shells, and suspicious outbound connections from the Sitecore server.
- Implement defense in depth by running Sitecore with minimal privileges, isolating the CMS from critical internal resources, and monitoring all access to the Sitecore application.
Technical Details
CVE-2021-42237 is an insecure deserialization vulnerability (CWE-502) in Sitecore Experience Platform's Report.ashx handler. The handler is accessible without authentication and processes HTTP requests that include serialized .NET objects. The deserialization process uses the BinaryFormatter class, which is known to be unsafe for deserializing untrusted data, as it allows the instantiation of arbitrary .NET types during deserialization. An attacker crafts a serialized .NET object using gadget chains (such as those from the ysoserial.net tool) that execute arbitrary operating system commands when deserialized. The attack requires only a single HTTP POST request to the Report.ashx endpoint, making it trivially exploitable. The server executes the attacker's commands with the privileges of the Sitecore application pool identity, typically a high-privilege service account.
Frequently Asked Questions
Is CVE-2021-42237 being actively exploited?
Yes, CVE-2021-42237 is extensively exploited with public exploit code widely available. CISA confirmed active exploitation by adding it to the KEV catalog. The EPSS score of 97.4% places it among the most exploited vulnerabilities globally.
What products are affected by CVE-2021-42237?
Sitecore Experience Platform (Sitecore XP) versions up to and including 9.3 Initial Release are affected. The vulnerability is in the Report.ashx handler accessible without authentication.
How do I fix CVE-2021-42237?
Apply Sitecore's security hotfix immediately and restrict access to the Report.ashx endpoint. Audit the system for signs of compromise including web shells and unauthorized modifications.
How severe is CVE-2021-42237?
CVE-2021-42237 has a CVSS v3.1 score of 9.8 (Critical) and enables unauthenticated remote code execution through a single HTTP request. The EPSS score of 97.4% confirms it is one of the most actively exploited CMS vulnerabilities.
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