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SharePoint’s Latest Threat Exposes Enterprises to Silent Takeover
A newly discovered remote code execution (RCE) vulnerability in Microsoft SharePoint has ignited major concern across the cybersecurity landscape. This flaw, found in SharePoint’s WebPart properties deserialization process, allows authenticated attackers to inject and execute arbitrary code directly on vulnerable servers. Although Microsoft has silently patched the vulnerability, the absence of a public CVE identifier has left many enterprises scrambling for clarity. Originally uncovered by security researcher Khoadha during routine analysis, the vulnerability is rooted in unsafe deserialization within SharePoint’s SPObjectStateFormatter class. Exploiting this flaw could enable full system compromise via crafted XML content embedded in SharePoint WebPart controls. Here’s everything you need to know.
Dangerous Deserialization: SharePoint’s Hidden Exploit Window
A critical vulnerability has been uncovered in Microsoft SharePoint that could allow authenticated users to execute arbitrary code on affected servers. The issue lies in the way SharePoint handles the deserialization of WebPart properties—specifically through the SPObjectStateFormatter class. This insecure mechanism enables attackers to inject serialized payloads into SharePoint’s XML-based WebPart controls, leading to binary deserialization via BinaryFormatter, which can result in remote code execution. The flaw affects multiple SharePoint versions, including build 15.0.5145.1000.
Security researcher Khoadha stumbled upon the bug during an unrelated analysis of SharePoint’s WebPart control parsing routine. He discovered that injecting HTML or XML content into WebPart controls could trigger a chain of method calls—from WebPart.AddParsedSubObject() to Utility.DeserializeStringToObject()—that eventually leads to unsafe deserialization. The attack is executed through malicious XML content embedded within the <WebPartPages:XmlWebPart> element.
The exploitation chain begins with XML parsing, where ParseXml() and DoPostDeserializationTasks() pave the way to the vulnerable GetAttachedProperties() method. This function deserializes the _serializedAttachedPropertiesShared field using SPObjectStateFormatter, which in turn uses BinaryFormatter. Critically, the method IsAllowedType() permits deserialization of any class listed in SharePoint’s SafeControls, including SPThemes—a class that inherits from DataSet and supports serialization-based execution, making it an ideal RCE vector.
Attackers can generate malicious payloads using tools like ysoserial, leveraging the SPThemes class. These payloads are then encoded in Base64 and inserted into the <AttachedPropertiesShared> tag within the WebPart XML. Once uploaded through SharePoint’s webpartpages.asmx endpoint via the ConvertWebPartFormat SOAP action, the server processes the payload and executes the embedded code.
Microsoft has issued a silent patch to address the vulnerability, although details such as the CVE number or exact fix remain undisclosed. Until organizations apply the latest security updates and audit WebPart configurations, their SharePoint environments remain at high risk of exploitation.
What Undercode Say:
Enterprise Threat at the Core of Collaboration
This SharePoint flaw underscores a broader trend of serialization vulnerabilities that have plagued enterprise platforms for over a decade. While RCE bugs in deserialization aren’t new, what makes this discovery particularly alarming is its presence in such a widely deployed product. SharePoint is a cornerstone of business collaboration, meaning that an exploit like this could have catastrophic consequences, including full internal network compromise.
SafeControls: A False Sense of Security
The vulnerability highlights a critical oversight in SharePoint’s SafeControls list. By trusting classes like SPThemes, which can be serialized and contain execution paths, SharePoint inadvertently extends trust to user input—an age-old security blunder. The IsAllowedType() check should have acted as a safeguard but ended up facilitating exploitation.
Silent Patching Raises Transparency Concerns
Microsoft’s decision to patch the issue without releasing a CVE or detailed disclosure presents challenges for administrators. Without clear documentation, many organizations remain unaware of the gravity of the threat or whether their specific configurations are impacted. This lack of transparency hampers vulnerability management and slows remediation efforts.
Weaponizing XML in SharePoint Environments
The attack vector—crafting malicious WebPart XML—is both simple and potent. Any authenticated user with permission to modify WebParts can deploy the exploit. This lowers the bar for potential attackers, especially insider threats or compromised user accounts. Unlike zero-days that require complex delivery mechanisms, this bug exploits a default SharePoint feature used every day in many organizations.
ysoserial: The Hacker’s Swiss Army Knife
The use of ysoserial for crafting payloads once again proves how powerful this tool remains in deserialization attacks. Modifying payloads to suit SharePoint’s internal classes like SPThemes makes the process efficient and reproducible. This increases the likelihood of seeing this exploit used in the wild, especially before all vulnerable systems are patched.
The BinaryFormatter Problem
Despite years of warnings from the security community, BinaryFormatter continues to be a common denominator in many .NET deserialization vulnerabilities. Microsoft itself has advised against its use, yet it persists in legacy systems like SharePoint. As long as legacy code remains in production environments, so will these threats.
Developer and Admin Oversight
This flaw reflects the long-standing gap between developer intentions and real-world application use. Admins trust the platform to sandbox actions like WebPart customization, but back-end deserialization logic doesn’t always account for malicious input. Better secure-by-default practices are essential to avoid these oversights.
Organizations Need Rapid Response Protocols
Given the delay in full disclosure and the silent nature of the patch, organizations must proactively monitor their SharePoint deployments for suspicious XML content, audit WebPart usage, and enforce strict user permission policies. Waiting for a public CVE is no longer a viable option in this age of fast-moving threats.
The Bigger Picture: A Call to Modernize
Enterprises using legacy SharePoint installations are now facing the consequences of outdated architecture. Deserialization bugs will continue to surface until platforms like SharePoint undergo deep architectural revisions that replace insecure design patterns. Until then, security teams must stay vigilant and reactive.
🔍 Fact Checker Results:
✅ The vulnerability exists in the SPObjectStateFormatter class
✅ Exploitation involves XML payloads using WebPart controls
❌ Microsoft has not disclosed a CVE or full patch details
📊 Prediction:
This vulnerability is likely to become a hot target for automated exploitation in the next wave of SharePoint-based attacks, especially in large organizations that delay patching due to internal red tape. As PoCs circulate, attackers will increasingly weaponize this flaw in phishing campaigns or lateral movement strategies within enterprise networks. Patch adoption rates and Microsoft’s future transparency will determine the exploit’s lifespan.
References:
Reported By: cyberpress.org
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