Microsoft SharePoint Authentication Bypass CVE-2026-55040 Enables Chained RCE Exploits
Rapid7 Labs has disclosed CVE-2026-55040, a medium-severity authentication bypass vulnerability in Microsoft SharePoint that, when chained with another RCE flaw, allows unauthenticated attackers to achieve remote code execution.
Rapid7 Labs has uncovered a critical exploit chain targeting Microsoft SharePoint, beginning with the authentication bypass vulnerability CVE-2026-55040. This flaw, discovered during zero-day research, allows unauthenticated attackers to bypass SharePoint's security mechanisms and assume the identity of a specific user, provided the attacker knows the user's Security Identifier (SID) or User Principal Name (UPN). While CVE-2026-55040 itself carries a CVSSv3.1 score of 5.3 (Medium), its true danger lies in its ability to be chained with other vulnerabilities, leading to unauthenticated remote code execution (RCE).
Microsoft SharePoint, a cornerstone of enterprise collaboration and document management within the Microsoft 365 ecosystem, handles vast amounts of sensitive data. Its deep integration with Active Directory and cloud infrastructure makes it a high-value target for attackers. Vulnerabilities within SharePoint can therefore have a significant impact on organizations that rely on it for internal file sharing, workflow automation, and managing corporate intranets.
The impact of exploiting CVE-2026-55040 is substantial. An attacker can impersonate any user on a targeted SharePoint site, including administrators. This impersonation allows the attacker to perform actions as that user. The prerequisite for this impersonation is prior knowledge of the target user's SID or UPN, which can be obtained through various enumeration techniques. Rapid7 Labs demonstrated this capability with a proof-of-concept script that identified potential users via SID enumeration and then leveraged the vulnerability to bypass authentication.
Crucially, the authentication bypass provided by CVE-2026-55040 serves as a gateway to further exploitation. Rapid7 Labs successfully chained this vulnerability with a separate RCE vulnerability, achieving unauthenticated RCE against a vulnerable SharePoint server. This chaining highlights a common and dangerous trend in cybersecurity: medium-severity vulnerabilities, when combined, can lead to critical outcomes. Patching CVE-2026-55040 is essential as it breaks this specific exploit chain.
Microsoft has acknowledged the vulnerability and is expected to address the RCE component in its August 2026 update cycle. While the exact timeline for a patch specifically for CVE-2026-55040 is not detailed, the company stated it thanks Rapid7 for responsible disclosure. Rapid7 plans to publish full technical details for CVE-2026-55040 within 30 days of this disclosure, providing further insight into the vulnerability's mechanics.
This research was partly enabled by artificial intelligence. Rapid7 Labs utilized an agentic AI system during two research sprints, with the second sprint in March 2026 yielding the successful exploit chain. The AI was prompted extensively, performing approximately 80,000 tool calls over 24 active days. This demonstrates the growing role of AI in vulnerability discovery and the rapid evolution of AI models and research workflows.
Organizations using Microsoft SharePoint are advised to apply the latest available updates to protect themselves. Rapid7 customers using Exposure Command, InsightVM, and Nexpose can assess their exposure through authenticated vulnerability checks that were made available in the July 14 content release. The discovery and disclosure of this vulnerability underscore the importance of proactive vulnerability research and timely patching, especially for widely used enterprise platforms.