SharePoint RCE: AI-Discovered Exploit Chain (CVE-2026-55040, CVE-2026-63520)

A JWT authentication bypass and a remote code execution vulnerability in Microsoft SharePoint, chained together, give an attacker full control of a vulnerable server with zero credentials.

Tracked as CVE-2026-55040 and CVE-2026-63520, the pair was discovered and weaponized by Rapid7 Labs. The goal was to test whether an AI agent, guided by human researchers, could find and weaponize a real exploit chain. It could.

The chain was built as an entry for Pwn2Own Berlin and disclosed to Microsoft through a coordinated process; CVE-2026-55040 was patched in July 2026, CVE-2026-63520 in August 2026. A proof-of-concept for the first vulnerability is already public, shortening the time before real-world attackers catch up.

Risk Analysis: CVE-2026-55040

Risk Analysis: CVE-2026-63520

Inside the CVE-2026-55040 / CVE-2026-63520 Exploit Chain

CVE-2026-55040 targets the JWT authentication validation pipeline, while CVE-2026-63520 affects the vulnerable code path used for remote code execution. That makes it a natural target: it is present on every SharePoint farm, it processes external input by design, and it sits close to the identity layer.

CVE-2026-55040 breaks the first barrier. SharePoint’s JWT validation pipeline has multiple vulnerabilities that allow an unauthenticated attacker to bypass JWT authentication and impersonate a SharePoint user.

CVE-2026-63520 breaks the second. With a valid-looking identity in hand, the attacker reaches an unsafe .NET type instantiation path in Business Connectivity Services. Improper input validation (CWE-20) lets the attacker’s payload get instantiated as a live .NET object. That instantiation is what ultimately turns into arbitrary code execution, running as the SharePoint site’s service account.

Neither vulnerability alone reaches unauthenticated RCE. The authentication bypass without the RCE gets an attacker inside as a user. The RCE without the authentication bypass still needs some form of access to trigger. Chain the two together and both barriers disappear. An attacker needs no credentials, and no user interaction, to execute code on the server.

What CVE-2026-55040 and CVE-2026-63520 Affect

Both vulnerabilities affect SharePoint Server Subscription Edition, 2019, and 2016. CVE-2026-63520 also reaches certain versions of Project Server and Office Web Apps Server. SharePoint Online is not affected; this is an on-premises vulnerability only.

No confirmed in-the-wild exploitation of this exploit chain has been publicly reported at the time of writing. Full technical detail and a working proof-of-concept for CVE-2026-55040 are public, and CVE-2026-63520’s root cause is documented alongside it. That combination, public detail plus a demonstrated chain, is usually what shortens the runway before independent exploitation attempts appear.

CVE-2026-55040 and CVE-2026-63520: Patch and Remediation Steps

Blocking this exploit chain takes more than a single update. Here is what to check and patch, in order:

AppTrana Coverage for CVE-2026-55040 and CVE-2026-63520

AppTrana blocks exploitation attempts targeting this vulnerability chain. Customers running SharePoint behind AppTrana stay protected while patching is completed across the farm. AppTrana WAAP inspects incoming requests to internet-facing SharePoint instances and blocks traffic that matches known malicious patterns, including forged authentication tokens and attempts to abuse Business Connectivity Services endpoints.