Unauthenticated Deserialization RCE in Microsoft SharePoint Server
CISA: Microsoft SharePoint Deserialization of Untrusted Data Vulnerability
CVSS 3.1
9.8critical
EPSS
16%p97
Published
()
KEV added
AI analysis
CVE-2026-58644 is a critical (CVSS 9.8) deserialization-of-untrusted-data flaw (CWE-502) in Microsoft SharePoint Server that allows an unauthorized attacker to execute code over a network. An attacker triggers it by sending crafted serialized data to the server, with no privileges or user interaction required, gaining code execution in the context of the SharePoint service. Any organization running on-premises SharePoint Server is affected, with highest risk for instances reachable from the internet or by untrusted network users. CISA added the flaw to its Known Exploited Vulnerabilities catalog on 2026-07-16, and contemporaneous headlines describe an exploited SharePoint zero-day, indicating active in-the-wild exploitation; EPSS assigns a roughly 16% probability of exploitation within 30 days (97th percentile). Fixes shipped in Microsoft's July 2026 Patch Tuesday release (621 CVEs total), while no public proof-of-concept for this specific CVE is documented.
What to do: Apply Microsoft's July 2026 Patch Tuesday security updates for SharePoint Server immediately, prioritizing internet-facing instances, as required for federal agencies under CISA BOD 26-04 and the KEV listing. If patching must be delayed, restrict network exposure to SharePoint per Microsoft/CISA mitigation guidance and assess each asset's internet exposure. Review SharePoint and web-server logs for signs of exploitation and enumerate all SharePoint Server installations in the environment to confirm coverage.
Affected
Microsoft SharePoint Server (on-premises)
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Estimated exposure
masson the order of 100,000+ on-prem SharePoint Server deployments, with tens of thousands likely internet-exposed (estimate) — SharePoint Server remains broadly deployed in enterprise and government intranets, and public internet-scan counts during earlier SharePoint exploitation waves indexed on the order of 100,000 endpoints; no authoritative installed-base…
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.