SharePoint Under Siege: Deep Dive into CVE-2025-53770 and CVE-2025-53771 Exploits

Widespread Exploits Raise Alarm Across SharePoint Environments

In a growing storm of cyber intrusions, the SharePoint platform has recently become a primary target for threat actors exploiting two critical vulnerabilities: CVE-2025-53770 and CVE-2025-53771. These flaws, particularly dangerous due to their authentication bypass and deserialization attack vectors, have been weaponized in the wild. Attacks began increasing rapidly just days after public disclosure, with even basic honeypots showing a notable surge in probing activity. Exploitation efforts are already leveraging variations of the ToolPane.aspx endpoint, along with carefully crafted payloads that unravel SharePoint’s security framework from the inside out.

Security researchers have flagged the key identifier for CVE-2025-53771: a forged referer header linked to SignOut.aspx, indicating an authentication bypass trick. Attackers are sending POST requests to ToolPane.aspx with URL-encoded payloads containing embedded .NET elements and base64-encoded data blobs. Once decoded and decompressed using tools like CyberChef, these payloads reveal a malicious structure that triggers .NET deserialization through the “CompressedDataTable” attribute. Buried within is a PowerShell command string that ultimately leads to the creation of the notorious spinstall0.aspx backdoor on the server.

Once executed, the backdoor page runs .NET code to extract and leak the MachineKey, a critical component of SharePoint’s ViewState integrity system. If compromised, this key allows attackers to forge signed ViewState tokens—essentially enabling remote code execution or session hijacking without detection. The article emphasizes the serious consequences of these exploits and the urgent need for rotating MachineKeys and locking down vulnerable endpoints. Security analysts warn that failure to remediate could lead to persistent and hard-to-detect compromises of enterprise SharePoint environments.

What Undercode Say:

Anatomy of the Exploit and Its Devastating Impact

The SharePoint exploit chain discussed in this case is a textbook example of how layered encoding and familiar administrative URLs can be used to veil powerful attacks. By embedding .NET deserialization payloads within URL-encoded POST requests, the attackers circumvent typical input validation mechanisms. The abuse of SharePoint’s ToolPane.aspx endpoint reflects how legacy web interfaces and customizable .aspx pages continue to be ripe targets when misconfigured or left unpatched.

The pivotal moment in the exploit is the decoding of the CompressedDataTable property, which acts as a payload container. Within it, a structured PowerShell command initiates the drop of a malicious script file—spinstall0.aspx—into a known SharePoint directory. This payload isn’t just a simple web shell. Instead, it strategically exploits .NET reflection to access the MachineKeySection, a powerful configuration element typically locked down within the system’s internals.

Exfiltration of the MachineKey is devastating. It opens the door for forging ViewState tokens, allowing attackers to issue authenticated requests or inject executable code into server memory. Since many .NET applications rely heavily on ViewState for maintaining state and user session data, this is equivalent to full control over the web application’s logic. Such control can be leveraged silently, making detection extremely difficult for defenders.

Another layer of concern is the sheer automation behind these attacks. Scanners and bots are already cycling through variations of the exploit, hitting even non-configured or test SharePoint instances. This mass probing increases the risk for organizations unaware of the flaw or slow to patch. Furthermore, the use of open tools like CyberChef and GitHub-hosted payloads shows how easily these techniques are being shared across underground and public cybersecurity circles.

This vulnerability also underscores a critical design flaw: exposing administrative or debug endpoints like ToolPane.aspx without sufficient access controls. Developers and administrators often leave such paths accessible under the assumption of obscurity or internal use. But attackers have become increasingly proficient in identifying and abusing these routes.

The final payload’s sophistication indicates a deep understanding of both SharePoint’s internal workings and .NET architecture. It cleverly invokes methods using reflection, bypasses standard security guards, and ensures minimal detection by using compressed, encoded payloads. This technique is not new but remains effective because it outpaces traditional signature-based intrusion detection systems.

Lastly, this breach reminds defenders of the importance of not just patching software but rotating sensitive keys, monitoring for unauthorized ViewState activity, and blocking access to all non-essential .aspx endpoints. It’s also a call to incorporate behavior-based analysis into SharePoint environments, as static rule-based defenses are increasingly inadequate.

🔍 Fact Checker Results:

✅ CVE-2025-53771 is confirmed as an authentication bypass using forged referer headers
✅ The decoded payload results in the deployment of spinstall0.aspx, leaking MachineKeys

✅ ViewState manipulation post-exploit provides remote execution without authentication

📊 Prediction:

🔐 Expect mass exploitation of unpatched SharePoint servers within the next 2–4 weeks
💣 Toolkits exploiting CVE-2025-53770/53771 will become mainstream in pentesting circles
🛡️ Microsoft will likely issue emergency Mitigation Guidance & Patches before August 2025