Unauthenticated Deserialization RCE in Microsoft SharePoint Server
CISA: Microsoft SharePoint Deserialization of Untrusted Data Vulnerability
CVSS 3.1
9.8critical
EPSS
85%p100
Published
()
KEV added
AI analysis
CVE-2026-50522 is a deserialization of untrusted data flaw (CWE-502) in Microsoft SharePoint Server that allows an unauthenticated attacker to send maliciously crafted serialized data over the network and execute code on the server, reflected in its 9.8 critical CVSS score with no privileges or user interaction required. Successful exploitation yields full remote code execution with high impact on confidentiality, integrity, and availability, giving attackers a foothold for follow-on actions such as data theft, lateral movement, or ransomware. Any organization running on-premises SharePoint Server is in scope, particularly deployments reachable from untrusted networks; the required action notes stakeholders must evaluate each asset's internet exposure under CISA BOD 26-04. The flaw is under active exploitation: CISA added it to the Known Exploited Vulnerabilities catalog on 2026-07-22 and security news headlines describe it as a critical RCE exploited in the wild, with some reports referencing exploitation after a public proof-of-concept release and an authentication bypass. The structured record lists no public PoC as confirmed, but an EPSS of 84.6% (100th percentile) underscores a very high near-term exploitation likelihood.
What to do: Apply Microsoft's security updates for SharePoint Server immediately per the vendor advisory, as required under CISA's KEV listing and BOD 26-04, and prioritize any SharePoint deployments that are internet-facing (federal/critical-infrastructure operators must follow BOD 26-04 timelines or discontinue unmitigated use). Until patched, restrict public access to SharePoint endpoints (VPN, firewall rules, or reverse proxy) and review IIS/application logs and running processes for signs of unauthenticated deserialization abuse. Because some reports reference an authentication bypass being chained, also verify authentication paths and monitor for follow-on attacker activity after patching.
Affected
Microsoft SharePoint Server (on-premises)
—
Estimated exposure
massorder of 100,000+ on-prem SharePoint Server deployments worldwide, with tens of thousands plausibly internet-exposed — SharePoint Server is Microsoft's widely deployed on-premises collaboration platform used across enterprises and governments, and internet-wide scans during comparable SharePoint RCE events have identified tens to hundreds of thousands of…
Order-of-magnitude estimate by the model from install counts, market share and public scan data it knows; verify before quoting.
Description
Deserialization of untrusted data in Microsoft Office SharePoint allows an unauthorized attacker to execute code over a network.
CISA Known Exploited Vulnerability
Affected
Microsoft SharePoint
Required action
Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.
WatchTowr observed active exploitation of SharePoint RCE CVE-2026-50522 (CVSS 9.8), part of an ongoing wave of exploited on-prem SharePoint flaws patched by Microsoft.
On 14 July 2026 Microsoft released updates for critical SharePoint Server RCE flaws, including CVE-2026-50522 (CVSS 9.8), a deserialisation vulnerability possibly exploitable without authentication. On 20 July 2026 WatchTowr published PoC exploit code and observed active exploitation of CVE-2026-50522, prompting CISA to urge SharePoint hardening. The advisory also covers recently fixed SharePoint flaws: CVE-2026-32201 (6.5, spoofing), CVE-2026-45659 (8.8, authenticated RCE), CVE-2026-56164 (9.8, unauthenticated privilege escalation), and CVE-2026-58644 (9.8, unauthenticated RCE). Affected products include SharePoint Server Subscription Edition, 2019, and Enterprise Server 2016; CERT-EU recommends immediate patching, credential rotation, and compromise assessments.
Attackers exploit SharePoint authentication bypass CVE-2026-55040 (CVSS 9.1) after Rapid7's PoC release, with 12 exploitation attempts recorded since July 19, 2026.
Microsoft patched CVE-2026-55040 in its July 2026 Patch Tuesday; the 9.1-severity flaw chains four weaknesses in SharePoint's JWT token validation pipeline, letting unauthenticated attackers forge valid JWTs and impersonate any SharePoint site user or administrator. Threat actors began exploiting the bug after Rapid7 released a Python PoC that forges tokens and enumerates domain users by SID. KEVIntel telemetry shows 12 exploitation attempts since July 19, 2026, eight of them on August 12-13 following the PoC release, originating from eight IPs across Hong Kong, Japan, the Netherlands, Taiwan, and the U.S. This is the fifth SharePoint vulnerability exploited this year, alongside CVE-2026-45659, CVE-2026-56164, CVE-2026-58644, and CVE-2026-50522.
CVE-2026-50522 is an unauthenticated .NET deserialization RCE in SharePoint Server Subscription, 2019, and 2016; Microsoft patched it July 14, 2026.
CVE-2026-50522 (CWE-502) lets unauthenticated attackers send crafted serialized .NET payloads to network-accessible SharePoint endpoints, triggering gadget-chain deserialization and remote code execution under the privileged SharePoint service account. It affects SharePoint Server Subscription Edition, SharePoint Server 2019, and SharePoint Server 2016 Enterprise; Microsoft published the advisory on July 14, 2026. The EPSS score is 20.346%, but no public PoC exists and the flaw is not in CISA's KEV catalog at publication. Mitigations include applying Microsoft updates, rotating SharePoint machine keys, enabling AMSI, and restricting internet exposure.
Microsoft patched CVE-2026-50522 (CVSS 9.8), an unauthenticated deserialization RCE in on-premises SharePoint servers, in its July 2026 updates; no exploitation confirmed at disclosure.
CVE-2026-50522 is a critical (CVSS 9.8) deserialization of untrusted data flaw (CWE-502) allowing unauthenticated remote code execution on on-premises Microsoft SharePoint servers. It affects SharePoint Enterprise Server 2016, SharePoint Server 2019, and SharePoint Server Subscription Edition, with fixes shipped in Microsoft's July 14, 2026 security updates; SharePoint Online is not listed as affected. No public evidence of active exploitation of this specific CVE existed at disclosure, though CISA's SSVC judged the attack automatable with total technical impact.
August 2026 Patch Tuesday forecast urges risk-based triage as AI inflates CVE volume, flags actively exploited SharePoint RCE CVE-2026-50522 and LegacyHive Windows flaw.
July 2026 Patch Tuesday set records with over 600 CVEs, including 405 for Windows 11 and Server 2025, though only two were exploited zero-days and one was publicly disclosed. The author recommends prioritizing known-exploited and internet-facing vulnerabilities, matching patches to high-risk systems, and accelerating ring-based deployment toward Microsoft's suggested three-day turnaround. Notable items include actively exploited SharePoint RCE CVE-2026-50522 enabling machine key theft, the unassigned LegacyHive Windows User Profile Service flaw, and several end-of-servicing dates for Windows and Exchange.