theregister.com - A week after Microsoft told the world that its July software updates didn't fully fix a couple of bugs, which allowed miscreants to take over on-premises SharePoint servers and remotely execute code, researchers have assembled much of the puzzle — with one big missing piece.

How did the attackers, who include Chinese government spies, data thieves, and ransomware operators, know how to exploit the SharePoint CVEs in such a way that would bypass the security fixes Microsoft released the following day?

"A leak happened here somewhere," Dustin Childs, head of threat awareness at Trend Micro's Zero Day Initiative (ZDI), told The Register. "And now you've got a zero-day exploit in the wild, and worse than that, you've got a zero-day exploit in the wild that bypasses the patch, which came out the next day."

Countdown to mass exploitation
It all began back in May, on stage at the Pwn2Own competition.

Pwn2Own is the hackers' equivalent of the World Series, and ZDI usually hosts these competitions twice a year.

The most recent contest occurred in Berlin, beginning May 15. On day 2 of the event, Vietnamese researcher Dinh Ho Anh Khoa combined an auth bypass and an insecure deserialization bug to exploit Microsoft SharePoint and win $100,000.

"What happens on the stage is just one part of Pwn2Own," Childs said.

After demonstrating a successful exploit, the bug hunter and vendor are whisked away into a private room where the researcher explains what they did and provides the technology company with a full write-up of the exploit. Assuming it's not a duplicate or already known vulnerability, the vendor then has 90 days to issue a fix before the bug and exploit are made public.

"So Microsoft received the working exploit in a white paper describing everything on that day," Childs said.

Less than two months later, on July 8, the software giant disclosed the two CVEs – CVE-2025-49704, which allows unauthenticated remote code execution, and CVE-2025-49706, a spoofing bug – and released software updates intended to patch the flaws. But mass exploitation had already started the day before, on July 7.

"Sixty days to fix really isn't a bad timeline for a bug that stays private and stays under coordinated disclosure rules," Childs said. "What is bad: a leak happened."

There's another key date that may shed light on when that leak happened.

Patch Tuesday happens the second Tuesday of every month – in July, that was the 8th. But two weeks before then, Microsoft provides early access to some security vendors via the Microsoft Active Protections Program (MAPP).

These vendors are required to sign a non-disclosure agreement about the soon-to-be-disclosed bugs, and Microsoft gives them early access to the vulnerability information so that they can provide updated protections to customers faster.

"The first MAPP drop occurs at what we call r minus 14, which is two weeks ahead of the [Patch Tuesday] release," Childs said – that is, beginning on June 24. "Then, on July 7, we started to see attacks. July 8, the patches were out and were almost immediately bypassed."

ZDI, along with other security providers, poked holes in the initial patches and determined that the authentication bypass piece was too narrow, and attackers could easily bypass this fix. In fact, anyone who received the early MAPP information about the CVEs and software updates "would be able to tell that this is an easy way to get past it," Childs said.

On July 18, Eye Security first sounded the alarm on "large-scale exploitation of a new SharePoint remote code execution (RCE) vulnerability chain in the wild."

A day later, Microsoft warned SharePoint server users that three on-prem versions of the product included a zero-day flaw that was under attack – and that its own failure to completely patch the holes was to blame.

By July 21, Redmond had issued software updates for all three versions. But by then, more than 400 organizations had been compromised by at least two Chinese state-sponsored crews, Linen Typhoon and Violet Typhoon, plus a gang Microsoft tracks as Storm-2603, which was abusing the vulnerabilities to deploy ransomware.

Microsoft declined to answer The Register's specific questions for this story. "As part of our standard process, we'll review this incident, find areas to improve, and apply those improvements broadly," a Microsoft spokesperson said in an emailed statement.

One researcher suggests a leak may not have been the only pathway to exploit. "Soroush Dalili was able to use Google's Gemini to help reproduce the exploit chain, so it's possible the threat actors did their own due diligence, or did something similar to Dalili, working with one of the frontier large language models like Google Gemini, o3 from OpenAI, or Claude Opus, or some other LLM, to help identify routes of exploitation," Tenable Research Special Operations team senior engineer Satnam Narang told The Register.

"It's difficult to say what domino had to fall in order for these threat actors to be able to leverage these flaws in the wild," Narang added.

microsoft.com - July 23, 2025 update – Expanded analysis and threat intelligence from our continued monitoring of exploitation activity by Storm-2603 leading to the deployment of Warlock ransomware. Based on new information, we have updated the Attribution, Indicators of compromise, extended and clarified Mitigation and protection guidance (including raising Step 6: Restart IIS for emphasis), Detections, and Hunting sections.

On July 19, 2025, Microsoft Security Response Center (MSRC) published a blog addressing active attacks against on-premises SharePoint servers that exploit CVE-2025-49706, a spoofing vulnerability, and CVE-2025-49704, a remote code execution vulnerability. These vulnerabilities affect on-premises SharePoint servers only and do not affect SharePoint Online in Microsoft 365. Microsoft has released new comprehensive security updates for all supported versions of SharePoint Server (Subscription Edition, 2019, and 2016) that protect customers against these new vulnerabilities. Customers should apply these updates immediately to ensure they are protected.

These comprehensive security updates address newly disclosed security vulnerabilities in CVE-2025-53770 that are related to the previously disclosed vulnerability CVE-2025-49704. The updates also address the security bypass vulnerability CVE-2025-53771 for the previously disclosed CVE-2025-49706.

As of this writing, Microsoft has observed two named Chinese nation-state actors, Linen Typhoon and Violet Typhoon exploiting these vulnerabilities targeting internet-facing SharePoint servers. In addition, we have observed another China-based threat actor, tracked as Storm-2603, exploiting these vulnerabilities to deploy ransomware. Investigations into other actors also using these exploits are still ongoing. With the rapid adoption of these exploits, Microsoft assesses with high confidence that threat actors will continue to integrate them into their attacks against unpatched on-premises SharePoint systems. This blog shares details of observed exploitation of CVE-2025-49706 and CVE-2025-49704 and the follow-on tactics, techniques, and procedures (TTPs) by threat actors. We will update this blog with more information as our investigation continues.

Microsoft recommends customers to use supported versions of on-premises SharePoint servers with the latest security updates. To stop unauthenticated attacks from exploiting this vulnerability, customers should also integrate and enable Antimalware Scan Interface (AMSI) and Microsoft Defender Antivirus (or equivalent solutions) for all on-premises SharePoint deployments and configure AMSI to enable Full Mode as detailed in Mitigations section below. Customers should also rotate SharePoint server ASP.NET machine keys, restart Internet Information Services (IIS), and deploy Microsoft Defender for Endpoint or equivalent solutions.