Detect CVE-2026-45659 Microsoft SharePoint Server Deserialization RCE in Splunk
Detects exploitation of CVE-2026-45659, a critical deserialization of untrusted data vulnerability in Microsoft SharePoint Server. Successful exploitation allows remote code execution by sending crafted serialized .NET objects to SharePoint web endpoints. This CVE is listed in CISA KEV indicating active exploitation in the wild.
MITRE ATT&CK
- Tactic
- Initial Access Execution Persistence
SPL Detection Query
index=iis OR index=wineventlog OR index=sysmon
| eval src_category=case(
sourcetype="iis", "web",
sourcetype="WinEventLog:Security", "process",
sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational", "sysmon",
true(), "unknown"
)
| eval is_sharepoint_process=if(match(ParentImage, "(?i)(w3wp|owstimer|microsoft\.sharepoint)"), 1, 0)
| eval is_suspicious_child=if(match(Image, "(?i)(cmd\.exe|powershell\.exe|wscript\.exe|cscript\.exe|mshta\.exe|rundll32\.exe|certutil\.exe|bitsadmin\.exe)"), 1, 0)
| eval is_sharepoint_endpoint=if(match(cs_uri_stem, "(?i)(_vti_bin|_layouts|viewlsts\.aspx|upload\.aspx|_api)"), 1, 0)
| eval is_large_post=if(cs_method="POST" AND cs_bytes > 5000, 1, 0)
| where (src_category="web" AND is_sharepoint_endpoint=1 AND is_large_post=1)
OR (src_category IN ("process", "sysmon") AND is_sharepoint_process=1 AND is_suspicious_child=1)
| eval alert_type=case(
src_category="web", "SharePoint Suspicious HTTP Request",
src_category IN ("process", "sysmon"), "SharePoint Worker Process Spawned Suspicious Child",
true(), "Unknown"
)
| table _time, host, src_ip, cs_uri_stem, cs_method, sc_status, cs_bytes, ParentImage, Image, CommandLine, alert_type
| sort -_timeDetects CVE-2026-45659 exploitation via IIS logs for anomalous POSTs to SharePoint endpoints and Sysmon/Security events for suspicious child processes spawned by SharePoint worker processes.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Legitimate third-party SharePoint integrations making large API calls to _vti_bin or _layouts endpoints
- SharePoint farm configuration scripts that invoke PowerShell from administrative contexts tied to w3wp.exe
- Antivirus or EDR agents spawning child processes for scanning operations initiated via SharePoint worker processes
Other platforms for CVE-2026-45659
Testing Methodology
Validate this detection against 3 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1CVE-2026-45659 SharePoint Deserialization Simulation via ysoserial.net
Expected signal: IIS logs should show POST request to /_vti_bin/UserProfileService.asmx with large cs-bytes; Sysmon Event ID 1 should capture cmd.exe spawned by w3wp.exe with CommandLine containing whoami
- Test 2SharePoint Worker Process Suspicious Child Simulation
Expected signal: Sysmon Event ID 1 showing cmd.exe execution with Network Service account; Windows Security Event ID 4688 capturing process creation with suspicious command line arguments
- Test 3SharePoint Endpoint Fuzzing and Web Shell Upload Detection Test
Expected signal: Sysmon Event ID 11 (FileCreate) capturing .aspx file creation in SharePoint web directory path; Windows Security Event ID 4663 (file access) if object access auditing is enabled
References (4)
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-45659
- https://www.cisa.gov/news-events/directives/bod-26-04-prioritizing-security-updates-based-risk
- https://www.cisa.gov/news-events/directives/bod-26-04-implementation-guidance-prioritizing-security-updates-based-risk
- https://nvd.nist.gov/vuln/detail/CVE-2026-45659
Unlock Pro Content
Get the full detection package for CVE-2026-45659 including response playbook, investigation guide, and atomic red team tests.