CriticalCVE-2013-3896

    Microsoft Silverlight Information Disclosure Vulnerability (CVE-2013-3896)

    Published In CISA KEV since Sources: CVE.org

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    What is affected, what to do

    Microsoft Silverlight 5 before 5.1.20913.0 does not properly validate pointers during access to Silverlight elements, which allows remote attackers to obtain sensitive information via a crafted Silverlight application, aka "Silverlight Vulnerability."

    The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Silverlight to version 5.1.20913.0. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Silverlight lt 5.0..5.1.20913.0
    Fixed in
    • Microsoft Silverlight 5.1.20913.0

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

    Metrics

    5.5

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    5.5

    CVSS v3

    Severity Medium on the scale from 0 to 10.

    Source: CVE.org

    68.0 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.32

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    206.42 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploited
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    5.5 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    5.5
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    68.0 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    99.32
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CISA assessment (SSVC)
    Exploitation: active · not automatable · Technical impact: partial
    Source: CISA Vulnrichment, as of 10 February 2025

    Response and mitigation

    Why act now?

    Prioritisation rationale

    On its own this flaw doesn't execute code — it discloses memory contents, and in the wild it was chained by exploit kits with separate memory-corruption bugs to bypass ASLR and then achieve drive-by code execution, which explains its KEV listing and very high exploitation likelihood despite a moderate base severity. Exploitation requires a user to visit a malicious page on a host that still has Silverlight and a browser able to load it; CISA reports no known ransomware use. For most organisations real exposure is low once IE11 is retired, but any Silverlight finding is unsupported software and therefore a compliance gap under NIS2 and ISO 27001 A.8.8 — treat it as high priority and close it by removal, not patching. Pay particular attention to legacy systems in OT-adjacent control rooms and internet-connected terminal server farms.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Build an inventory: Sweep all Windows endpoints and terminal servers for Microsoft Silverlight, e.g. Get-ItemProperty HKLM:\SOFTWARE\Microsoft\Silverlight -Name Version, or query the Uninstall registry keys for DisplayName -like "*Silverlight*". Reconcile against your CMDB/software inventory.
      • Uninstall as the primary fix: Silverlight reached end of support in October 2021 and no longer receives security updates. Remove it fleet-wide via your deployment tooling (SCCM/Intune, Get-Package -Name "Microsoft Silverlight" | Uninstall-Package) rather than merely upgrading.
      • If removal is not immediately possible: Upgrade to at least version 5.1.20913.0, which fixes CVE-2013-3896. Obtain the installer via the Microsoft advisory — never from third-party download sites.
      • Identify dependent business apps: Catalogue internal web applications still serving .xap content (often legacy control-room, video or ERP front ends), notify the application owners, and log them as technical debt in the risk register.
      • Close the browser path: Silverlight only loads in Internet Explorer 11 or Microsoft Edge IE mode. Review the IE mode site list for entries that require Silverlight and remove any that aren't strictly needed.

    2. Runbook step 2

      Mitigation layers

      • Endpoint hardening / GPO: Use the Internet Explorer "Add-on Management" policy to disable the Silverlight add-on and enable "Deny all add-ons unless specifically allowed", preventing users from re-enabling it. Back this up with AppLocker/WDAC execution control.
      • Web proxy / secure web gateway: Block internet downloads with MIME type application/x-silverlight-app or the .xap extension; allow only explicitly approved internal hosts.
      • IPS / network: Ensure exploit-kit landing-page and Silverlight exploit signatures are enabled in your IPS (Suricata/Snort or commercial), and switch the relevant categories from alert to block.
      • Exploit mitigation: Because this bug leaks memory addresses to defeat ASLR, enforce Windows Exploit Protection (DEP, mandatory ASLR, CFG) for iexplore.exe and msedge.exe to raise the bar for the follow-on code-execution stage.
      • Least privilege: Run users without local admin rights so that any payload delivered via a successful chain cannot trivially persist or escalate.

    3. Runbook step 3

      Detection rules

      • Proxy/web logs: Requests for .xap files or responses with Content-Type: application/x-silverlight-app from external, non-allowlisted domains. SPL: index=proxy (uri_path="*.xap" OR content_type="application/x-silverlight-app") NOT dest_domain IN (allowlist) | stats count by src_ip, dest_domain.
      • Sysmon EID 7 (Image Loaded): npctrl.dll or agcore.dll loaded into iexplore.exe or msedge.exe on hosts where inventory says Silverlight should be gone. KQL (Defender): DeviceImageLoadEvents | where FileName in~ ("npctrl.dll","agcore.dll") | summarize by DeviceName, InitiatingProcessFileName.
      • EDR process ancestry (Sysmon EID 1 / Defender): Suspicious browser child processes shortly after Silverlight content is fetched, e.g. iexplore.exe → cmd.exe/powershell.exe/rundll32.exe/regsvr32.exe. Sigma shape: ParentImage|endswith: '\iexplore.exe' with Image|endswith matching a shell/LOLBin list.
      • Network telemetry (Zeek): files.log entries with mime_type == "application/x-silverlight-app" followed within minutes by an executable download (application/x-dosexec) to the same host — the classic exploit-kit sequence.
      • Inventory drift: Reappearance of HKLM\SOFTWARE\Microsoft\Silverlight (Sysmon EID 12/13) after removal points to shadow IT or deliberate reinstallation.

    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 5.1.20913.0 does not properly validate pointers during access to Silverlight elements, which allows remote attackers to obtain sensitive information via a crafted Silverlight application, aka "Silverlight Vulnerability."

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

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    microsoftsilverlight
    • 5.0 – < 5.1.20913.0
    • 5.1.20913.0

    Vendors and products

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited

    References & sources

    • docs.microsoft.comMS13-087Vendor advisory, x_refsource_MS, Patch, Vendor advisory
    • oval.cisecurity.orgoval:org.mitre.oval:def:19003Vulnerability database, signature, x_refsource_OVAL, Broken link
    • us-cert.govTA13-288AThird-party advisory, x_refsource_CERT, Third-party advisory, US government resource
    • oval.cisecurity.orgoval:org.mitre.oval:def:19055Vulnerability database, signature, x_refsource_OVAL, Broken link
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2013-3896government-resource, US government resource
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