CVE-2025-53691

Executive Summary

CVE-2025-53691 is a critical deserialization vulnerability affecting Sitecore Experience Manager (XM) and Experience Platform (XP). The flaw allows an unauthenticated attacker to achieve Remote Code Execution (RCE) by sending a crafted serialized object to the vulnerable application. The vulnerability impacts all versions from 9.0 through 9.3 and 10.0 through 10.4 for both XM and XP. As of this report, no official patch has been released, and no public proof-of-concept (PoC) code is available. The CVSS score is 8.8 (High), indicating a severe risk to confidentiality, integrity, and availability.

Supply Chain & Package Security

Sitecore is a proprietary enterprise CMS platform. The vulnerability is not related to open-source packages (NPM, PyPI, Go, Ruby). However, organizations using Sitecore should treat this as a critical supply chain risk due to the lack of a patch and the potential for exploitation in enterprise environments.

CVSS Breakdown & Threat Level

• CVSS Score: 8.8 (High)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: High (C:H)
• Integrity Impact: High (I:H)
• Availability Impact: High (A:H)

The high CVSS score reflects the ease of exploitation (no authentication required) and the full impact on system security. An attacker can execute arbitrary code on the server, leading to complete compromise.

Deep Dive & Mechanics

The vulnerability lies in the deserialization of untrusted data within Sitecore's application logic. Deserialization is the process of converting a byte stream back into an object. When an application deserializes data from an untrusted source without proper validation, an attacker can craft a malicious serialized object that, when deserialized, executes arbitrary code.

In Sitecore, the vulnerable component likely processes serialized objects from HTTP requests (e.g., in session state, view state, or custom APIs). The attacker sends a specially crafted serialized payload that triggers the execution of system commands or loads malicious classes. The exact vulnerable class or method is not publicly disclosed, but the pattern is consistent with known .NET deserialization attacks (e.g., using BinaryFormatter or SoapFormatter).

The attack flow:

1. Attacker identifies a Sitecore endpoint that accepts serialized data.
2. Attacker crafts a malicious serialized object (e.g., using ysoserial.net for .NET gadgets).
3. Attacker sends the payload to the vulnerable endpoint.
4. Sitecore deserializes the object, triggering code execution.
5. Attacker gains remote control of the server.

Vulnerable vs. Patched Code

Since no patch is available, a direct code comparison is not possible. However, the vulnerable code pattern is likely:

Vulnerable Code (C#):

// Vulnerable: Deserializing without type validation
using (var stream = new MemoryStream(serializedData))
{
    var formatter = new BinaryFormatter();
    var obj = formatter.Deserialize(stream); // Unsafe
}

Patched Code (Hypothetical):

// Patched: Use safe deserialization with type validation
using (var stream = new MemoryStream(serializedData))
{
    var settings = new JsonSerializerSettings
    {
        TypeNameHandling = TypeNameHandling.None // Disable polymorphic deserialization
    };
    var obj = JsonConvert.DeserializeObject(stream, settings);
}

The fix would involve replacing insecure deserializers (e.g., BinaryFormatter) with safer alternatives like Newtonsoft.Json with TypeNameHandling.None, or implementing a custom SerializationBinder to restrict allowed types.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

• Target: Sitecore XP 10.3 running on Windows Server 2019.
• Attacker: Kali Linux with ysoserial.net and curl.

Exploitation Steps:

1. Identify the vulnerable endpoint (e.g., /sitecore/shell/WebService/...).
2. Generate a malicious payload using ysoserial.net:

   # Generate a .NET deserialization payload for command execution
   ysoserial.exe -f BinaryFormatter -g TextFormattingRunProperties -c "powershell -e <base64_encoded_command>"
   

3. Send the payload to the target:

   curl -X POST -H "Content-Type: application/octet-stream" --data-binary @payload.bin http://target/sitecore/shell/WebService/Service.asmx
   

4. The server executes the command, and the attacker gains a reverse shell.

Note: No public PoC is available; this is a hypothetical exploitation based on common deserialization techniques.

Blue Team (Defensive)

Detection:

• Monitor for unusual serialized data in HTTP requests (e.g., base64-encoded binary data in POST bodies).
• Look for processes spawning from w3wp.exe (IIS worker process) that are not typical (e.g., cmd.exe, powershell.exe).
• Use network monitoring to detect outbound connections from the Sitecore server to unknown IPs.

Mitigation:

• If possible, disable deserialization endpoints or restrict access via firewall.
• Implement Web Application Firewall (WAF) rules to block known deserialization payloads.
• Apply the principle of least privilege to the Sitecore application pool account.

Indicators of Compromise (IOCs)

• Network:
  • POST requests to /sitecore/shell/WebService/*.asmx with Content-Type: application/octet-stream.
  • Outbound connections from the Sitecore server to external IPs on ports 80, 443, or 4444.
• File System:
  • Unexpected files in C:\inetpub\wwwroot\sitecore\ (e.g., .ps1, .exe, .dll).
  • Modified or new .config files in the Sitecore web root.
• Process:
  • w3wp.exe spawning cmd.exe, powershell.exe, or rundll32.exe.
• Registry:
  • Unauthorized changes to HKLM\SOFTWARE\Sitecore\ keys.

Detection Engineering

YARA Rule

rule CVE_2025_53691_Deserialization {
    meta:
        description = "Detects potential deserialization payloads targeting Sitecore CVE-2025-53691"
        author = "Valters IT"
        date = "2025-04-10"
        hash = "unknown"
    strings:
        $binary_formatter = { 00 01 00 00 00 FF FF FF FF 01 00 00 00 00 00 00 00 }
        $soap_formatter = { 3C 3F 78 6D 6C 20 76 65 72 73 69 6F 6E 3D 22 31 2E 30 22 }
        $sitecore_path = /sitecore/shell/WebService/
    condition:
        any of ($binary_formatter, $soap_formatter) and $sitecore_path
}

Sigma Rule

title: Sitecore Deserialization RCE Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects HTTP POST requests with binary content to Sitecore WebService endpoints
author: Valters IT
date: 2025/04/10
logsource:
    category: webserver
    product: iis
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/sitecore/shell/WebService/*'
        cs-content-type: 'application/octet-stream'
    condition: selection
falsepositives:
    - Legitimate binary uploads (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-53691 Sitecore Deserialization RCE Attempt"; flow:to_server; content:"POST"; http_method; content:"/sitecore/shell/WebService/"; http_uri; content:"application/octet-stream"; http_content_type; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected Sitecore servers from the internet if possible.
2. Monitor logs for exploitation attempts (see IOCs and detection rules).
3. Apply virtual patching via WAF to block deserialization payloads.

Long-term Fix:

• Await an official patch from Sitecore. Until then, consider upgrading to a non-vulnerable version (if available) or applying vendor-supplied workarounds.
• Review and harden deserialization code in custom Sitecore modules.

Workarounds:

• Disable the vulnerable WebService endpoints by removing or renaming the .asmx files.
• Implement a custom SerializationBinder to restrict deserialized types.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert