CVE-2026-55040 Sumo Logic CSE · Sumo

Detect Microsoft SharePoint Weak Authentication Exploitation (CVE-2026-55040) in Sumo Logic CSE

Detects exploitation of CVE-2026-55040, a weak authentication vulnerability (CWE-1390) in Microsoft SharePoint that allows attackers to bypass or abuse insufficient authentication controls to gain unauthorized access to SharePoint content and administrative surfaces. This CVE is listed in the CISA KEV catalog with active in-the-wild exploitation observed as of August 2026. Detection focuses on anomalous authentication patterns against SharePoint (_layouts, _api, _vti_bin endpoints), unauthenticated or under-authenticated access to protected resources, suspicious ToolPane/ViewState activity, and post-exploitation web shell or process spawning from the SharePoint w3wp.exe worker process.

MITRE ATT&CK

Tactic
Initial Access Privilege Escalation Credential Access

Sumo Detection Query

Sumo Logic CSE (Sumo)
sql 
_sourceCategory=*iis* ("/_layouts/" OR "/_api/" OR "/_vti_bin/" OR "ToolPane.aspx")
| parse regex "(?<cs_method>\S+)\s+(?<cs_uri_stem>\S+)\s+(?<cs_uri_query>\S+)" nodrop
| parse regex "\s(?<cs_username>\S+)\s(?<c_ip>\d+\.\d+\.\d+\.\d+)" nodrop
| parse regex "\s(?<sc_status>\d{3})\s" nodrop
| where cs_method = "POST" and (sc_status = "200" or sc_status = "302") and (cs_username = "-" or isEmpty(cs_username))
| count as request_count by c_ip, cs_uri_stem
| where request_count > 3
| sort by request_count desc
high severity medium confidence

Sumo Logic detection of unauthenticated success POSTs to sensitive SharePoint endpoints, indicating CVE-2026-55040 weak-authentication exploitation.

Data Sources

Sumo LogicIIS Access Logs

Required Tables

_sourceCategory=iis

False Positives & Tuning

  • Intentional anonymous access on public document libraries.
  • Uptime monitoring hitting SharePoint without auth.
  • Log format variance causing username field misparse.

Other platforms for CVE-2026-55040

Microsoft Sentinel (KQL) Splunk (SPL) Elastic Security (Elastic) IBM QRadar (QRadar) Google Chronicle (YARA-L) CrowdStrike LogScale (LogScale) All platforms (combined) →

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.

  1. Test 1Simulate unauthenticated POST to SharePoint ToolPane endpoint

    Expected signal: IIS W3C log entry showing a POST to /_layouts/15/ToolPane.aspx with an empty/'-' username and a 200/302 status code.

  2. Test 2Simulate unauthenticated SharePoint _api access burst

    Expected signal: Five IIS log entries for POST /_api/web/lists with no authenticated username.

  3. Test 3Simulate w3wp.exe spawning suspicious child process

    Expected signal: Process-creation event (Sysmon EID 1 / EDR ProcessRollup) showing cmd.exe with a parent of w3wp.exe (when run under the SharePoint app pool context).

Last updated: 2026-08-19 Research depth: standard
References (6)

Response Playbook

Triage

  1. Confirm the affected host is running Microsoft SharePoint and identify its patch level against the MSRC advisory for CVE-2026-55040; determine whether the applicable security update is installed.
  2. Review the source IPs from the detection: enrich against threat intelligence and determine whether they are internal, VPN, or external/untrusted, and whether they appear in the CISA KEV-linked exploitation reporting.
  3. Examine the IIS logs for the flagged host to confirm unauthenticated POSTs to /_layouts/, /_api/, or /_vti_bin/ returned 200/302, and correlate with any subsequent w3wp.exe child-process activity or new .aspx files under the SharePoint LAYOUTS/wwwroot directories.
  4. Check for indicators of web shell deployment (new/modified .aspx, .ashx, .asmx files) and unexpected changes to SharePoint machine keys or ViewState configuration.

Containment

  1. Isolate the affected SharePoint server from the network if web shell or hands-on-keyboard activity is confirmed, preserving volatile state for forensics.
  2. Apply the Microsoft security update for CVE-2026-55040 immediately per CISA BOD 26-04 prioritization; if patching is not immediately possible, restrict inbound access to SharePoint _layouts/_api/_vti_bin endpoints at the WAF/reverse proxy.
  3. Rotate SharePoint ASP.NET machine keys and invalidate existing sessions to defeat forged/replayed authentication material.
  4. Reset credentials for any service and administrative accounts that may have been exposed through the weak-authentication bypass.

Evidence Collection

  1. Preserve full IIS W3C logs, SharePoint ULS logs, and Windows Security/Sysmon event logs from the affected host for the exploitation window.
  2. Capture a forensic image or at minimum the SharePoint wwwroot and LAYOUTS directories to retain any dropped web shells and timestamps.
  3. Export the process-creation telemetry showing w3wp.exe child processes and their full command lines.
  4. Collect the SharePoint web.config and machine key configuration as evidence of pre/post exploitation state.

Escalation Criteria

  • !Escalate to incident response and management if a web shell, unauthorized administrative account, or interactive command execution under the SharePoint app pool is confirmed.
  • !Escalate if lateral movement, credential dumping, or data exfiltration from SharePoint content databases is observed following the initial access.
  • !Notify the CISA-required reporting chain given this CVE is in the KEV catalog and subject to BOD 26-04 remediation timelines.
  • !Escalate if the affected server is internet-facing and exploitation success (200/302 to sensitive endpoints without auth) is confirmed.

Investigation Guide

Related Techniques

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Forensic Artifacts

  • >IIS W3C logs showing unauthenticated POSTs to /_layouts/, /_api/, or /_vti_bin/ with 200/302 responses.
  • >New or modified .aspx/.ashx/.asmx files under the SharePoint LAYOUTS or wwwroot directories.
  • >SharePoint ULS logs and Windows event logs showing anomalous authentication or process creation from w3wp.exe.
  • >Modified machine keys or ViewState/web.config settings in the SharePoint site configuration.

Tuning Guidance

Baseline which SharePoint sites in your environment legitimately allow anonymous access and exclude those specific site collections/paths from the unauthenticated-POST logic. Confirm your IIS logging captures the cs-username field reliably (some reverse-proxy or NTLM configurations may leave it blank for legitimate authenticated traffic); if so, pivot the detection toward the w3wp.exe child-process and web-shell file-creation signals, which have far lower false-positive rates. Tune the request-count threshold to your traffic volume.

Hunting Queries

Hunts for repeated unauthenticated POSTs to the highest-signal SharePoint exploitation endpoints across IIS logs.

Hunting — KQL
kql 
W3CIISLog | where csUriStem has_any ("/_layouts/15/ToolPane.aspx", "/_vti_bin/", "/_api/web") | where csMethod == "POST" | where isempty(csUsername) or csUsername == "-" | summarize count() by cIP, csUriStem, sComputerName | where count_ > 1
Hunting — SPL
spl 
index=iis (cs_uri_stem="*ToolPane.aspx*" OR cs_uri_stem="*/_vti_bin/*" OR cs_uri_stem="*/_api/web*") cs_method=POST (cs_username="-" OR cs_username="") | stats count BY c_ip cs_uri_stem s_computername | where count>1

Atomic Red Team Tests

Test 1 Simulate unauthenticated POST to SharePoint ToolPane endpoint
linux

Sends an unauthenticated POST request to the SharePoint ToolPane.aspx endpoint to generate the IIS log signature associated with CVE-2026-55040 exploitation attempts.

Command

bash 
curl -s -k -X POST "https://sharepoint.lab.local/_layouts/15/ToolPane.aspx?DisplayMode=Edit" -H "Referer: /_layouts/SignOut.aspx" -H "Content-Type: application/x-www-form-urlencoded" --data "MSOTlPn_Uri=test" -o /tmp/sp_toolpane_resp.txt

Cleanup

bash 
rm -f /tmp/sp_toolpane_resp.txt

Expected Telemetry

IIS W3C log entry showing a POST to /_layouts/15/ToolPane.aspx with an empty/'-' username and a 200/302 status code.

Expected Detection

The kql/spl unauthenticated-POST detections fire when request count exceeds the threshold from the source IP.

Test 2 Simulate unauthenticated SharePoint _api access burst
linux

Issues multiple unauthenticated POSTs to the SharePoint REST _api endpoint to simulate the weak-authentication access pattern.

Command

bash 
for i in $(seq 1 5); do curl -s -k -X POST "https://sharepoint.lab.local/_api/web/lists" -H "Content-Type: application/json;odata=verbose" --data '{}' -o /dev/null; done

Cleanup

bash 
echo "no artifacts to clean"

Expected Telemetry

Five IIS log entries for POST /_api/web/lists with no authenticated username.

Expected Detection

KQL/SPL/QRadar/Sumo/Chronicle detections aggregate the burst and alert on request_count > 3.

Test 3 Simulate w3wp.exe spawning suspicious child process
windows

Simulates the post-exploitation behavior of the SharePoint IIS worker process spawning a command shell, matching the endpoint-based detections.

Command

powershell 
powershell.exe -NoProfile -Command "Start-Process -FilePath cmd.exe -ArgumentList '/c whoami'"

Cleanup

powershell 
powershell.exe -NoProfile -Command "Get-Process cmd -ErrorAction SilentlyContinue | Stop-Process -Force"

Expected Telemetry

Process-creation event (Sysmon EID 1 / EDR ProcessRollup) showing cmd.exe with a parent of w3wp.exe (when run under the SharePoint app pool context).

Expected Detection

The elastic_eql and crowdstrike_cql detections fire on w3wp.exe spawning cmd.exe/powershell.exe.

Related Detections

Tactic Hubs

TA0001Initial AccessTA0004Privilege EscalationTA0006Credential Access

Related Techniques

T1190Exploit Public-Facing ApplicationT1505.003Web ShellT1078Valid Accounts

Same Tactic: Initial Access

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