Rapid7 and Microsoft Patch AI-Assisted SharePoint RCE Chain

The chain begins with CVE-2026-55040, a SharePoint authentication bypass disclosed and patched in July. Rapid7 found that the first flaw could let a remote unauthenticated attacker assume the identity of a known SharePoint user, including an administrator, using an Active Directory identifier such as a SID or user principal name. Microsoft now rates that bypass Critical at 9.1.

CVE-2026-63520 supplies the code-execution step. Microsoft says it requires no privileges or user interaction, although its CVSS vector marks attack complexity as high. Chaining the July bypass with the newly disclosed RCE removes the normal authentication barrier and produces the unauthenticated code-execution path Rapid7 originally reported to Microsoft in May.

Microsoft lists SharePoint Enterprise Server 2016, SharePoint Server 2019 and SharePoint Server Subscription Edition as affected below the fixed August builds. The vendor record marked exploit-code maturity as unproven when published, so the disclosure establishes a working researcher-developed chain, not confirmed exploitation in the wild.

Rapid7 says its project mixed manual code review and reverse engineering with an AI agent. Across two research sprints, the team logged 24 active days of agentic work, 96 sessions, 256 prompts and about 80,000 tool calls. The January sprint produced no usable finding; a second sprint in March yielded the two-bug chain after researchers changed models and workflows.

That sequence matters for security teams evaluating AI-assisted vulnerability research: automation expanded the search effort, but coordinated disclosure, reproducible exploitation and vendor validation still established the result. For SharePoint operators, the immediate issue is simpler: verify that every supported on-premises server has both the July authentication-bypass fix and the August build that addresses CVE-2026-63520.