SharePoint CVE-2026-55040 + CVE-2026-63520: The RCE Chain Found by an AI Agent

> TL;DR: CVE-2026-55040 (JWT authentication bypass, CVSS 9.1) and CVE-2026-63520 (RCE via unsafe .NET type instantiation, CVSS 8.1) combine into a full, unauthenticated takeover of on-premise SharePoint Server (2016, 2019, Subscription Edition). Over 8,500 internet-facing servers were exposed at disclosure time. Notably, the exploit chain was discovered with the help of an AI agent.

The attack chain, step by step

1. JWT bypass (CVE-2026-55040) — SharePoint's JWT validation pipeline contains several distinct flaws: the "alg":"none" algorithm disables signature verification, a forged signing key thumbprint tricks key resolution, and issuer validation is insufficient. The result: an attacker can craft a JWT that impersonates any user, including a site administrator — provided they know a valid identifier (SID or UPN), typically obtainable from Active Directory.

2. RCE via Business Connectivity Services (CVE-2026-63520) — Once fraudulently authenticated with sufficient privileges, the attacker exploits insufficient input validation in the Business Connectivity Services API: a malicious payload is instantiated as a .NET object, leading to arbitrary code execution in the context of the SharePoint service account.

On its own, the second flaw requires authentication. Chained with the first, it becomes a complete pre-authentication RCE.

Why an AI agent found this chain

According to published technical write-ups, discovery of this exploit chain was heavily assisted by an AI agent capable of quickly analyzing the JWT validation pipeline and identifying exploitable combinations of flaws. It's one more signal echoing several of our recent posts: AI tools are accelerating vulnerability discovery on both the defensive and offensive sides — disclosure and exploitation are both speeding up.

Scope of exposure

More than 8,500 internet-facing SharePoint servers were vulnerable at initial disclosure. A Python PoC was published on August 11, with active scanning and exploitation reported by threat intelligence organizations within 24 hours.

Important: SharePoint Online (the Microsoft 365 SaaS product) is not affected. Only on-premise instances (SharePoint Server 2016, 2019, Subscription Edition) are impacted.

What to do

The lesson for any system using JWTs

This chain is a reminder of the basic JWT validation rules covered in our guide to JWT and OAuth authentication flaws: always explicitly reject the none algorithm, verify the signature against a known, trusted key (never one extracted from the token itself), and strictly validate the issuer. These are not secondary implementation details — they are the three controls that were all bypassed simultaneously here.

What CleanIssue checks for

During an application audit, we systematically test the robustness of JWT validation (algorithm, signature, issuer, expiration) as well as alternate authentication paths — exactly the type of flaw that made this chain possible.

Key takeaways