Last month, we covered a SharePoint exploit chain that lets attackers take over a server without a password. This time, let’s zoom in on the vulnerability that makes the entire chain possible in the first place: CVE-2026-55040, an authentication bypass that’s now confirmed under active attack.
If you read our earlier piece on the SharePoint exploit chain, this is the piece that fits right before it. If you didn’t, here’s what you need to know either way.
What CVE-2026-55040 Actually Does
Every legitimate user on your network has an identity — something that proves they are who they say they are. CVE-2026-55040 sits in the part of SharePoint responsible for validating that identity. Researchers found several issues in this process that let an attacker skip validation entirely and act as a chosen user, without ever knowing that user’s password.
The one thing an attacker needs is a way to identify which account they want to impersonate — something as simple as a username formatted like an email address. In practice, that’s a far lower bar than most business owners would assume, since usernames and email formats are often easy to find or guess.
Why This Is the Piece That Matters Most
On its own, an authentication bypass is serious. Combined with the remote-code-execution flaw covered in our previous post, it becomes something far more dangerous: an attacker can impersonate a real user, then use that access to run their own code on your server. No credentials stolen. No employee tricked. Just a gap in the validation logic itself.
This is also the vulnerability the U.S. Cybersecurity and Infrastructure Security Agency has confirmed is being actively exploited — meaning this isn’t a theoretical risk. Real attackers are using it against real organizations right now.
What to Check
Microsoft addressed this vulnerability in its July 2026 security updates. If your organization runs an on-premises SharePoint environment, the priority action is confirming — not assuming — that those updates have been fully applied across every server. Given that this vulnerability is under active exploitation, verification matters more than usual here.
If you want a clear, documented answer to “are we exposed,” rather than a best guess, a vulnerability assessment gives you exactly that: a prioritized view of what’s actually exposed in your environment, not a general checklist.
The Pattern Behind Both Vulnerabilities
Taken together, these two SharePoint stories point to a bigger trend worth paying attention to: attackers are increasingly chaining multiple vulnerabilities together rather than relying on a single flaw. Each individual piece might look moderate in isolation. Combined, they add up to full compromise. That’s a harder pattern to catch with a one-time check, and it’s exactly the kind of ongoing oversight a vCISO is positioned to provide — someone tracking how new vulnerabilities interact with your specific environment, not just reading headlines after the fact.
Frequently Asked Questions
What is an authentication bypass vulnerability?
An authentication bypass lets an attacker act as a legitimate user on a system without providing valid login credentials. It skips the identity-verification step that normally stands between an outsider and your systems.
Is CVE-2026-55040 being actively exploited?
Yes. Federal cybersecurity officials have confirmed active exploitation of this vulnerability, which makes timely patch verification especially important for affected organizations.
How is this different from the SharePoint remote code execution vulnerability covered previously?
CVE-2026-55040 is the authentication bypass that lets an attacker impersonate a user. When combined with a separate remote-code-execution flaw, the two together allow an attacker to both gain access and run commands on the server — with no credentials required at any point.
What should my business do first?
Confirm that the July 2026 SharePoint security updates have been applied across all on-premises servers. If you’re not confident in that answer, a vulnerability assessment can verify your actual exposure.