CVE-2026-58644: SharePoint unauthenticated deserialization RCE
Tech GuruQuick Answer
CISA added CVE-2026-58644 to its Known Exploited Vulnerabilities catalog on 2026-07-16, confirming that exploitation has occurred. If you operate Microsoft SharePoint, identify the exact version and relevant configuration now, preserve security evidence, and follow the live vendor remediation. A KEV match does not prove your system was compromised, and installing an update does not rule out earlier exploitation.
Fix Or Immediate Action
What to do now: Install the newest applicable cumulative SharePoint security update on every farm server and verify the installed builds.
Last verified: 2026-09-17 UTC. CISA KEV snapshot version
2026.09.16. Recheck the live advisory before changing production.
Context
Scope And Impact In Plain Language
Who should check: Anyone who manages Microsoft SharePoint.
When this applies: Only when the exact product, version, and required configuration match the live vendor advisory. Owning the product family alone does not mean the device is vulnerable.
What could happen: A successful attacker may be able to run attacker-chosen commands or software on the affected system, subject to the access and configuration requirements in the advisory.
Not sure? Find the product name and version in the device or application, then compare them with the affected-products section of the linked vendor notice.
Status At A Glance
| Product scope | Microsoft — SharePoint |
|---|---|
| CVE | CVE-2026-58644 |
| TechGeeks response tier | P0 — confirm an exact match and act now |
| CISA KEV added | 2026-07-16 |
| Vendor severity | Critical |
| CVSS | 9.8 (CVSS v3.1 base) |
| Ransomware field | Unknown — this is not the same as No; CISA has not marked the field Known. |
| Forensic-triage field | Yes — CISA's record calls for forensic triage. |
| CWE | CWE-502 |
| Federal remediation date | 2026-07-19 — applies to covered federal agencies; other organizations should set a risk-based deadline. |
What Happened
Microsoft SharePoint Deserialization of Untrusted Data Vulnerability. Microsoft SharePoint contains a deserialization of untrusted data vulnerability that allows an unauthorized attacker to execute code over a network. CISA's KEV listing confirms exploitation evidence, but it does not state how widespread the activity is or prove that any particular installation was compromised.
Affected Scope
On-premises SharePoint Server 2016, SharePoint Server 2019, and SharePoint Server Subscription Edition where listed by Microsoft; exact farm-server applicability is controlled by the live MSRC table.
Match the exact edition, hardware or virtual platform, release, patch level, enabled feature, and management exposure. A product-family name alone is not enough to establish applicability.
Steps
What To Do Now
- Confirm. Inventory every SharePoint asset and record its exact version, patch level, role, relevant configuration, and exposure.
- Preserve. Save UTC timestamps, off-device authentication, firewall, proxy, DNS, endpoint, application, and SIEM evidence before logs rotate or a disruptive change removes context.
- Contain. Restrict unnecessary internet and management-plane access using supported controls. Follow the live advisory: Open the live Microsoft advisory.
- Remediate. Install the newest applicable cumulative SharePoint security update on every farm server and verify the installed builds.
- Validate. Verify the resulting version or patch level, service health, logging, expected access, an expected denial, and representative dependent workflows.
- Escalate. If exposure overlapped the vulnerable period or evidence is suspicious, treat the system as a potential incident and follow vendor and incident-response recovery guidance.
Fixed Releases And Mitigation
Permanent Fix
Vendor-directed fix path: Install the newest applicable cumulative SharePoint security update on every farm server and verify the installed builds.
Verified fixed release, patch level, or update guidance captured at the last check:
A fixed build or KB was not safely preserved in the research record. Check the live vendor advisory before production changes; do not infer a fixed version from this summary.
Workaround Or Temporary Mitigation
No vendor workaround is safely captured in the current research notes. Reduce unnecessary external reachability until the farm is fully updated, but do not present exposure reduction as remediation.
Important boundary: A workaround, access restriction, or provider-side control may reduce risk while the update is prepared, but it does not necessarily remove vulnerable code or erase an earlier compromise. Install the vendor-approved update when one is available.
Investigation And Recovery
Microsoft and CISA identify exploitation in the wild. The flaw permits unauthenticated network code execution; preserve IIS, SharePoint, authentication, and endpoint evidence and treat suspicious activity as an incident.
When an attacker may have gained administrative, root, SYSTEM, identity, management-plane, or code-execution access, patching alone is not an eradication claim. Preserve evidence, scope affected accounts and downstream systems, rotate exposed credentials or certificates when justified, and rebuild from a trusted source when vendor or incident-response guidance requires it.
Notes
Exploitation Evidence
Vendor statement: Microsoft marked Exploited: Yes at original publication.
CISA statement: CVE-2026-58644 was added to KEV on 2026-07-16. The ransomware field is Unknown, and the forensic-triage field is Yes. These are separate signals.
Validation And Boundaries
This notice does not prove that every product carrying the family name is affected, that exploitation is widespread, that a clean indicator search rules out compromise, or that a successful update restored trust. TechGeeks has not reproduced the exploit and does not replace the vendor advisory, asset inventory, vulnerability scanner, or incident-response process.
Related TechGeeks Resources
- TechGeeks Security Notice Center
- All Security Notices
- What to Do When a Device Hits CISA KEV
- Router CVE Exposure, Version, and Mitigation Checklist
- IoT Firmware Update and End-of-Support Checklist
- Live Protect and Runtime Vulnerability Shielding
References
Wrap-Up
Record whether CVE-2026-58644 applies to your environment. If it does, assign an owner, preserve the change and investigation evidence, complete the vendor action, and document the validated result.
Correction policy: If a primary source changes affected versions, remediation, exploitation status, or recovery guidance, TechGeeks will update this page and its verification date.
