Microsoft Silverlight Runtime Remote Code Execution Vulnerability (CVE-2016-0034)

What is affected, what to do

This vulnerability allows an attacker to execute arbitrary code or perform a denial-of-service attack remotely via a crafted website due to mishandling of negative offsets during decoding in Microsoft Silverlight versions before 5.1.41212.0. It is actively exploited in ransomware campaigns and requires no authentication for exploitation. Affected organizations should urgently upgrade to version 5.1.41212.0 to mitigate the risk.

The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update Microsoft Silverlight to version 5.1.41212.0. Immediate actions, mitigation and detection rules are in the runbook below.

Affected

• Microsoft Silverlight lt 5.0..5.1.41212.0

Fixed in

• Microsoft Silverlight 5.1.41212.0

Not sure whether and how you are affected? Ask a question about this vulnerability

Metrics

Response and mitigation

1. 01

   Runbook step 1

   Immediate response (0-24 h)

   • Inventory: Sweep all Windows endpoints and terminal servers for Microsoft Silverlight. Check the registry value HKLM\SOFTWARE\Microsoft\Silverlight\Version or the folder C:\Program Files\Microsoft Silverlight\, using SCCM/Intune inventory, EDR software lists or Get-ItemProperty.
   • Uninstall rather than patch: Silverlight is end-of-support. Where no business dependency exists, remove it entirely via software distribution or msiexec /x with the product code from the local Uninstall key.
   • If removal is not immediately possible: Upgrade Silverlight 5 to at least 5.1.41212.0, since every earlier 5.x build is vulnerable. The patch is available from the vendor; look up the bulletin and KB number in the vendor advisory. Confirm the deployed version through inventory once the rollout completes.
   • Prioritise exposed hosts: Start with systems running Internet Explorer or Edge IE mode with unrestricted web access. Terminal servers, jump hosts and admin workstations come first, because compromise there has the widest blast radius.
   • Retro-hunt: Wherever outdated versions turn up, search proxy logs for the past weeks for .xap downloads from unknown domains. Flag matching hosts for forensic preservation.
2. 02

   Runbook step 2

   Mitigation layers

   • Network/proxy: Block MIME type application/x-silverlight-app and the .xap extension at the web proxy or SWG. Allow exceptions only for documented internal applications.
   • Browser hardening (GPO): Disable the Silverlight ActiveX control in Internet Explorer through Group Policy, either via the add-on list or by disabling ActiveX for the Internet and Restricted zones. Restrict Edge IE mode to an enterprise site list.
   • Endpoint hardening: Use WDAC/AppLocker to stop npctrl.dll and agcore.dll from loading outside approved hosts. Enable ASR rules that restrict child-process creation from browser and Office contexts.
   • IPS/exploit-kit defence: Turn on vendor IPS signatures for Silverlight exploitation and common exploit-kit landing pages. Add DNS filtering for newly registered and known-malicious domains, because delivery is typically drive-by via crafted websites.
   • Least privilege: Run users without local admin rights. This limits the impact of code execution in the browser context and makes persistence harder.
   • Legacy dependencies: Move any line-of-business app that genuinely needs Silverlight into isolated VDI or remote browser isolation with no internet egress, and schedule its replacement.
3. 03

   Runbook step 3

   Detection rules

   • EDR process ancestry: Watch for iexplore.exe with npctrl.dll/agcore.dll loaded that then spawns cmd.exe, powershell.exe, wscript.exe, mshta.exe, rundll32.exe or regsvr32.exe. KQL example: DeviceProcessEvents | where InitiatingProcessFileName =~ "iexplore.exe" and FileName in~ ("cmd.exe","powershell.exe","wscript.exe","mshta.exe","rundll32.exe")
   • Sysmon EID 7 + EID 1: Look for an image load of npctrl.dll or agcore.dll followed within seconds by process creation (EID 1) whose parent is iexplore.exe. Implement this as a Sigma correlation rule.
   • Proxy/web logs: Flag responses with Content-Type: application/x-silverlight-app or URIs ending in .xap from external or unknown hosts, especially after redirect chains (several 302s in a row). SPL example: index=proxy (uri="*.xap" OR content_type="application/x-silverlight-app") | stats count by src, dest_host
   • Zeek/Suricata: In http.log or files.log, look for Silverlight-MIME downloads followed by a PE download to the same client. That pattern indicates a second-stage payload.
   • Crash telemetry: Clusters of Windows Event ID 1000 (Application Error) for iexplore.exe with faulting module agcore.dll may indicate failed exploitation attempts (object-header corruption).

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2

NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

Read in glossary

Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).

CRA

The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

Read in glossary

Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.

ISO/IEC 27001

ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

Read in glossary

Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

Microsoft Silverlight 5 before 5.1.41212.0 mishandles negative offsets during decoding, which allows remote attackers to execute arbitrary code or cause a denial of service (object-header corruption) via a crafted web site, aka "Silverlight Runtime Remote Code Execution Vulnerability."

Source: CISA KEV Catalog, cvelistv5, NVD (NIST), CVE.org (MITRE)

Vendors and products

• Microsoft Microsoft Silverlight

References & sources

• securitytracker.com1034655Vulnerability database, x_refsource_SECTRACK
• docs.microsoft.comMS16-006Vendor advisory, x_refsource_MS
• cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2016-0034government-resource