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Hackers are actively exploiting a new zero-day bug in Microsoft’s SharePoint Server software. The same software is used by key U.S. government agencies, including those tied to national security.
The vulnerability affects on-premise versions of SharePoint, allowing attackers to break into systems, steal data and quietly move through connected services. While the cloud version is unaffected, the on-premise version is widely used by major U.S. agencies, universities and private companies. That puts far more than just internal systems at risk.
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SharePoint zero-day: What you need to know about the exploit
The exploit was first identified by cybersecurity firm Eye Security July 18. Researchers say it stems from a previously unknown vulnerability chain that can give attackers full control of vulnerable SharePoint servers without needing any credentials. The flaw lets them steal machine keys used to sign authentication tokens, meaning attackers can impersonate legitimate users or services even after a system is patched or rebooted.
According to Eye Security, the vulnerability appears to be based on two bugs demonstrated at the Pwn2Own security conference earlier this year. While those exploits were initially shared as proof-of-concept research, attackers have now weaponized the technique to target real-world organizations. The exploit chain has been dubbed “ToolShell.”
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How the SharePoint vulnerability lets hackers access Microsoft services
Once inside a compromised SharePoint server, hackers can access connected Microsoft services. These include Outlook, Teams and OneDrive. This puts a wide range of corporate data at risk. The attack also allows hackers to maintain long-term access. They can do this by stealing cryptographic material that signs authentication tokens. The U.S. Cybersecurity and Infrastructure Security Agency (CISA) is urging organizations to act. It recommends checking systems for signs of compromise and isolating vulnerable servers from the internet.
Early reports confirmed about 100 victims. Now, researchers believe attackers have compromised more than 400 SharePoint servers worldwide. However, this number refers to servers, not necessarily organizations. According to reports, the number of affected groups is growing rapidly. One of the highest-profile targets is the National Nuclear Security Administration (NNSA). Microsoft confirmed it was targeted but has not confirmed a successful breach.
Other affected agencies include the Department of Education, Florida’s Department of Revenue and the Rhode Island General Assembly.
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Microsoft confirms SharePoint exploit and releases patches
Microsoft confirmed the issue, disclosing that it was aware of “active attacks” exploiting the vulnerability. The company has released patches for SharePoint Server 2016, SharePoint Server 2019 and SharePoint Subscription Edition. Patches for all supported on-prem versions were issued as of July 21.
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What you should do about the SharePoint security risk
If you’re part of a business or organization that runs its own SharePoint servers, especially older on-premise versions, your IT or security team should take this seriously. Even if a system is patched, it could still be at risk if machine keys were stolen. Administrators should also rotate cryptographic keys and audit authentication tokens. For the general public, there’s no action needed right now since this issue doesn’t affect cloud-based Microsoft accounts like Outlook.com, OneDrive or Microsoft 365. But it’s a good reminder to stay cautious online.
Microsoft’s name and logo on a building (Kurt “CyberGuy” Knutsson)
What you should do about the SharePoint security risk
If your organization uses on-premise SharePoint servers, take the following steps right away to reduce risk and limit potential damage:
1. Disconnect vulnerable servers: Take unpatched SharePoint servers offline immediately to prevent active exploitation.
2. Install available updates: Apply Microsoft’s emergency patches for SharePoint Server 2016, 2019 and Subscription Edition without delay.
3. Rotate authentication keys: Replace all machine keys used to sign authentication tokens. These may have been stolen and can allow ongoing access even after patching.
4. Scan for compromise: Check systems for signs of unauthorized access. Look for abnormal login behavior, token misuse or lateral movement within the network.
5. Enable security logging: Turn on detailed logging and monitoring tools to help detect suspicious activity going forward.
6. Review connected services: Audit access to Outlook, Teams and OneDrive for signs of suspicious behavior linked to the SharePoint breach.
7. Subscribe to threat alerts: Sign up for advisories from CISA and Microsoft to stay updated on patches and future exploits.
8. Consider migration to the cloud: If possible, transition to SharePoint Online, which offers built-in security protection and automatic patching.
9. Strengthen passwords and use two-factor authentication: Encourage employees to stay vigilant. Even though this exploit targets organizations, it’s a good reminder to enable two-factor authentication (2FA) and use strong passwords. Create strong passwords for all your accounts and devices, and avoid using the same password for multiple online accounts. Consider using a password manager, which securely stores and generates complex passwords, reducing the risk of password reuse. Check out the best expert-reviewed password managers of 2025 at Cyberguy.com/Passwords
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Kurt’s key takeaway
This SharePoint zero-day shows how fast research can turn into real attacks. What started as a proof-of-concept is now hitting hundreds of real systems, including major government agencies. The scariest part isn’t just the access it gives but how it lets hackers stay hidden even after you patch.
Should there be stricter rules around using secure software in government? Let us know by writing to us at Cyberguy.com/Contact
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