April 2026 Patch Tuesday: SharePoint Zero-Day & 'BlueHammer' Blues
Wow, 167 CVEs in one go is a headache. Everyone is likely talking about the SharePoint Server zero-day (CVE-2026-3047), but I'm actually more concerned about the publicly disclosed 'BlueHammer' weakness in Windows Defender. Having a known flaw in the primary line of defense for most Windows endpoints is a nightmare scenario, even if there isn't an exploit out yet.
On top of that, we have Chrome zero-day #4 (CVE-2026-3124) and an emergency update for Adobe Reader (CVE-2026-3098) fixing an actively exploited RCE flaw. It feels like the threat actors are speeding up faster than we can patch.
If you are managing SharePoint, you should prioritize checking your build versions immediately. We are using this quick PowerShell snippet to identify instances that haven't pulled the update yet:
Get-SPFarm | Select-Object BuildVersion
For the Adobe Reader issue, since it is actively exploited, I recommend pushing the update via SCCM or Intune as soon as possible. If you are hunting for IOCs related to the Adobe flaw, check for suspicious child processes spawning from `AcroRd32.exe`:
DeviceProcessEvents
| where InitiatingProcessFileName == 'AcroRd32.exe'
| where Timestamp > ago(7d)
| where ProcessCommandLine contains "powershell" or ProcessCommandLine contains "cmd"
How is everyone prioritizing the SharePoint zero-day? Are you patching immediately regardless of testing, or do you have a mitigation in place?
We're treating the SharePoint patch as critical because of the authentication bypass nature of the zero-day, but we're sandboxing the update first. Last year's cumulative update broke our search indexing, so we are gun-shy. However, the Adobe update is going out to everyone immediately—active RCE in reader software is too big a risk to wait.
The 'BlueHammer' disclosure is worrying from a blue team perspective. It targets the AMSI/Defender inspection logic. We are seeing a lot of false positives today, but the real concern is the bypass potential. We have updated our Sentinel rules to look for specific Defender event logs indicating engine failures:
DeviceEvents
| where ActionType contains "AntivirusDetection"
| where AdditionalFields contains "BlueHammer" or AdditionalFields contains "InspectionBypass"
As a pentester, I love Patch Tuesday—it resets the playing field. The Chrome zero-day is already being integrated into exploit kits, so if you haven't updated Chrome extensions across your fleet, do it now. Don't rely on the browser auto-update; enforce it via Group Policy.
While patching the Defender engine is key for 'BlueHammer', we should also validate that AMSI providers are still functioning correctly on our internet-facing servers. If the bypass logic was triggered prior to the patch, simply updating might not restore inspection capabilities.
Run this to check provider status:
Get-AmsiProvider | Where-Object { $_.IsReady -eq $false }
If anything returns, you might need to restart the service or investigate further.
Building on Quinn's concern about validating AMSI providers, we shouldn't assume the 'BlueHammer' fix didn't inadvertently break the inspection pipeline. Before pushing the update broadly, we run this quick smoke test to ensure the scanning logic is still actively catching heuristics:
try { Invoke-Expression 'Invoke-Mimikatz' } catch { Write-Host 'AMSI Blocked Execution' }
If this executes without triggering a Defender block or alert, you have a regression in your protection stack that needs immediate attention.
Building on whatahey’s warning about exploit kits, we need to hunt for IOCs associated with CVE-2026-3124 immediately. We are filtering our proxy logs for the specific malicious User-Agent strings seen in the wild. If you are using Sentinel, this KQL query helps narrow down suspicious traffic patterns before your fleet is fully patched:
DeviceNetworkEvents
| where Timestamp > ago(48h)
| where RemoteUrl has_any ("suspicious-domain", "payload-path")
| project Timestamp, DeviceName, RemoteUrl, InitiatingProcessFileName
Don't overlook that some malware strains actively block Defender updates to prevent the BlueHammer fix from landing. Alongside Quinn's AMSI checks, verify the engine version actually incremented post-patch to ensure the remediation took. You can quickly audit a fleet's engine status with this:
Get-MpComputerStatus | Select-Object AntispywareSignatureVersion, AntivirusSignatureVersion, EngineVersion
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