yuvalpery presents strategies to defend on-premises SharePoint Servers against CVE-2025-53770 by using Azure Web Application Firewall, detailing the attack vector, Microsoft’s update guidance, and actionable custom WAF rule configurations.

Mitigating SharePoint CVE-2025-53770 Using Azure Web Application Firewall (WAF)

Author: yuvalpery

Summary

Microsoft has disclosed CVE-2025-53770, a critical remote code execution (RCE) vulnerability affecting on-premises SharePoint Server versions 2016, 2019, 2010, 2013, and Subscription Edition (SE). The vulnerability is exploited by chaining two issues—authentication bypass (CVE-2025-49706) and a deserialization flaw (CVE-2025-49704)—allowing unauthenticated attackers to execute arbitrary C# code on affected servers.

Impacted Versions and Patch Status

  • Patched: SharePoint 2016, 2019, Subscription Edition (SE) — security updates available
  • Not Patched: SharePoint 2010, 2013 — unsupported, remains vulnerable

Attack Chain in Brief

  1. CVE-2025-49706 – Authentication Bypass: Attackers send a specially crafted POST request to /\_layouts/15/ToolPane.aspx?DisplayMode=Edit&a=/ToolPane.aspx using a manipulated Referer header (/\_layouts/SignOut.aspx), tricking SharePoint into authorizing the request.
  2. CVE-2025-49704 – Deserialization: The attacker sends another POST to the same endpoint, this time with a serialized payload that extracts cryptographic keys from web.config. They then use these keys to craft a VIEWSTATE payload containing malicious C# code, which SharePoint executes.
  • Apply security updates for supported versions immediately.
  • Isolate or upgrade legacy systems (2010/2013) as they will not receive patches. Restrict network access if upgrades are not possible.
  • Consult Microsoft’s official blog for full mitigation steps: Customer guidance for SharePoint vulnerability CVE-2025-53770.

Defending with Azure Web Application Firewall (WAF)

To help detect and block exploitation attempts on SharePoint Servers, create a custom WAF rule that matches known attack patterns used for CVE-2025-53770.

Example WAF Custom Rule Logic

  • Condition 1: Request URI contains /\_layouts/15/ToolPane.aspx or /\_layouts/15/spinstall0.aspx
  • Condition 2: Referer header contains /\_layouts/SignOut.aspx or /\_layouts/15/SignOut.aspx

Sample JSON Rule

{
  "customRules": [
    {
      "name": "CVE202553770",
      "priority": 100,
      "ruleType": "MatchRule",
      "action": "Block",
      "matchConditions": [
        {
          "matchVariables": [
            { "variableName": "RequestUri" }
          ],
          "operator": "Regex",
          "negationConditon": false,
          "matchValues": ["(?i)/_layouts(?:/\\d+)?/(SignOut|spinstall0|ToolPane)\\.aspx"],
          "transforms": []
        },
        {
          "matchVariables": [
            { "variableName": "RequestHeaders", "selector": "Referer" }
          ],
          "operator": "Regex",
          "negationConditon": false,
          "matchValues": ["(?i)/_layouts(?:/\\d+)?/(SignOut|spinstall0|ToolPane)\\.aspx"],
          "transforms": []
        }
      ],
      "skippedManagedRuleSets": [],
      "state": "Enabled"
    }
  ]
}

References:

Next Steps

  • Patch immediately: Update SharePoint 2016/2019/SE to the latest security releases.
  • Review WAF configuration: Implement and test the provided Azure WAF custom rules to protect vulnerable endpoints.
  • Restrict legacy SharePoint server access: Limit network exposure and plan migration away from unsupported versions.

Additional Information

  • Stay informed on the latest SharePoint vulnerabilities via Microsoft’s Security Response Center.
  • Regularly audit and update network security appliances to respond to emerging threats.

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