CriticalCVE-2018-8373

    Microsoft Scripting Engine Memory Corruption Vulnerability (CVE-2018-8373)

    Published In CISA KEV since Sources: CVE.org

    What is affected, what to do

    A remote code execution vulnerability exists in the way that the scripting engine handles objects in memory in Internet Explorer, aka "Scripting Engine Memory Corruption Vulnerability." This affects Internet Explorer 9, Internet Explorer 11, Internet Explorer 10. This CVE ID is unique from CVE-2018-8353, CVE-2018-8355, CVE-2018-8359, CVE-2018-8371, CVE-2018-8372, CVE-2018-8385, CVE-2018-8389, CVE-2018-8390.

    The vulnerability is being actively exploited; treat it as a priority. The available sources name no fixed version. Check the vendor guidance in the references below. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Internet Explorer 9 eq Windows Server 2008 for 32-bit Systems Service Pack 2..Windows Server 2008 for 32-bit Systems Service Pack 2
    • Microsoft Internet Explorer 9 eq Windows Server 2008 for x64-based Systems Service Pack 2..Windows Server 2008 for x64-based Systems Service Pack 2
    • Microsoft Internet Explorer 11 eq Windows 10 for 32-bit Systems..Windows 10 for 32-bit Systems
    • Microsoft Internet Explorer 11 eq Windows 10 for x64-based Systems..Windows 10 for x64-based Systems
    • Microsoft Internet Explorer 11 eq Windows 10 Version 1607 for 32-bit Systems..Windows 10 Version 1607 for 32-bit Systems
    • Microsoft Internet Explorer 11 eq Windows 10 Version 1607 for x64-based Systems..Windows 10 Version 1607 for x64-based Systems
    • Microsoft Internet Explorer 11 eq Windows 10 Version 1703 for 32-bit Systems..Windows 10 Version 1703 for 32-bit Systems
    • Microsoft Internet Explorer 11 eq Windows 10 Version 1703 for x64-based Systems..Windows 10 Version 1703 for x64-based Systems
    • Microsoft Internet Explorer 11 eq Windows 10 Version 1709 for 32-bit Systems..Windows 10 Version 1709 for 32-bit Systems
    • Microsoft Internet Explorer 11 eq Windows 10 Version 1709 for x64-based Systems..Windows 10 Version 1709 for x64-based Systems
    • Microsoft Internet Explorer 11 eq Windows 10 Version 1803 for 32-bit Systems..Windows 10 Version 1803 for 32-bit Systems
    • Microsoft Internet Explorer 11 eq Windows 10 Version 1803 for x64-based Systems..Windows 10 Version 1803 for x64-based Systems
    • Microsoft Internet Explorer 11 eq Windows 7 for 32-bit Systems Service Pack 1..Windows 7 for 32-bit Systems Service Pack 1
    • Microsoft Internet Explorer 11 eq Windows 7 for x64-based Systems Service Pack 1..Windows 7 for x64-based Systems Service Pack 1
    • Microsoft Internet Explorer 11 eq Windows 8.1 for 32-bit systems..Windows 8.1 for 32-bit systems
    • Microsoft Internet Explorer 11 eq Windows 8.1 for x64-based systems..Windows 8.1 for x64-based systems
    • Microsoft Internet Explorer 11 eq Windows RT 8.1..Windows RT 8.1
    • Microsoft Internet Explorer 11 eq Windows Server 2008 R2 for x64-based Systems Service Pack 1..Windows Server 2008 R2 for x64-based Systems Service Pack 1
    • Microsoft Internet Explorer 11 eq Windows Server 2012 R2..Windows Server 2012 R2
    • Microsoft Internet Explorer 11 eq Windows Server 2016..Windows Server 2016
    • Microsoft Internet Explorer 10 eq Windows Server 2012..Windows Server 2012

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

    Metrics

    7.5

    Priority

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

    Source: NVD, CVSS v3

    7.5

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: CVE.org

    61.5 %

    EPSS score

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

    Source: FIRST EPSS, percentile 99.16

    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
    216.16 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
    7.5 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
    7.5
    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
    61.5 %
    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
    99.16
    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: total
    Source: CISA Vulnrichment, as of 7 February 2025
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-787

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This is a memory-corruption bug in the VBScript engine, triggered through crafted web pages or Office documents embedding IE content. It is network-reachable without authentication but requires user interaction and precise exploitation. Successful exploitation fully compromises confidentiality, integrity and availability within the user's context. It has been used in targeted spear-phishing and watering-hole operations and still carries a very high exploitation likelihood, though CISA has no record of ransomware use. For organisations on current Windows builds with IE disabled, the residual risk is low and a compliance report closes it out. It becomes critical wherever legacy estate persists — OT HMIs, line-of-business apps tied to IE, or unpatched Server 2012 hosts — and those systems should be treated as high priority and isolated.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Verify and enforce patch level: Microsoft fixed this in the August 2018 security updates. Any later cumulative Internet Explorer or Windows update also covers it. Look up the KB number per OS build in the Microsoft Security Update Guide entry for CVE-2018-8373. Pull a WSUS, Intune or SCCM compliance report and update any host lacking a cumulative update since 08/2018.
      • Flag unsupported builds: Windows 10 RTM, 1607, 1703 and 1709, plus Windows Server 2012 without ESU, no longer receive fixes. Inventory them, cut their internet access, or fast-track migration to a supported build.
      • Disable VBScript in IE via GPO: Use Computer Configuration > Administrative Templates > Windows Components > Internet Explorer > Internet Control Panel > Security Page > Internet Zone > Allow VBScript to run in Internet Explorer = Disabled. Apply the same setting to the Restricted Sites and Intranet zones. Registry equivalent: ...\Internet Settings\Zones\3\140C = 3.
      • Retire IE as a browser: Enforce the "Disable Internet Explorer 11 as a standalone browser" policy and redirect users to Microsoft Edge. Allow IE mode only for an explicit site list.
      • Hunt on exposed clients: On unpatched hosts with IE usage, sweep the last 30 days of EDR telemetry for suspicious iexplore.exe child processes (see Detection rules).

    2. Runbook step 2

      Mitigation layers

      • Endpoint hardening: Enable the ASR rules "Block Office applications from creating child processes" and "Block all Office applications from injecting code". The engine is also reachable through MSHTML/IE content embedded in Office documents. Use WDAC or AppLocker to restrict script-host execution from user-writable paths.
      • Network / web proxy: On the secure web gateway, block responses containing text/vbscript or <script language="vbscript"> from uncategorised or newly registered domains. Log the Trident/7.0 and MSIE user agents, and deny them for hosts not on the exception list.
      • IPS signatures: Enable vendor signatures for scripting-engine memory corruption (VBScript use-after-free, CVE-2018-8373 class) on Suricata/Snort or your NGFW, in blocking mode. Check ET Open/Pro rulesets for matching CVE references.
      • Exploit protection: Enforce Windows Defender Exploit Protection (DEP, ASLR, CFG) for iexplore.exe. Where possible, enable Enhanced Protected Mode and 64-bit IE processes.
      • Least privilege: Users should browse with standard accounts only and have no local admin rights. This confines successful code execution to the user context.
      • Server separation: Prohibit interactive browsing on servers, especially Windows Server 2012. Force server VLAN egress HTTP/HTTPS through an allowlisting proxy.

    3. Runbook step 3

      Detection rules

      • Process ancestry (Sysmon EID 1 / Security 4688): Alert when ParentImage ends with \iexplore.exe and Image is one of cmd.exe, powershell.exe, wscript.exe, mshta.exe, rundll32.exe, regsvr32.exe or certutil.exe. KQL: DeviceProcessEvents | where InitiatingProcessFileName =~ "iexplore.exe" and FileName in~ ("cmd.exe","powershell.exe","mshta.exe","rundll32.exe","regsvr32.exe")
      • vbscript.dll loads (Sysmon EID 7): Watch for ImageLoaded ending with \vbscript.dll where Image is \iexplore.exe, \winword.exe, \excel.exe or \powerpnt.exe. Once the GPO is in place, any hit signals a bypass or a misconfiguration.
      • File drops (Sysmon EID 11): Look for new PE or script files written by iexplore.exe into %TEMP%, %APPDATA% or INetCache and executed shortly afterwards.
      • Crashes as failed exploitation: Application log EID 1000/1001 with iexplore.exe and faulting module vbscript.dll points to failed heap grooming. Correlate repeated crashes across hosts.
      • Network (Zeek http.log / proxy): Watch for MSIE/Trident user agents hitting first-seen domains, followed within minutes by outbound connections from that host to unknown destinations. Sigma shape: proxy | ua contains "Trident/7.0" and dst_domain_age < 30d.

    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

    A remote code execution vulnerability exists in the way that the scripting engine handles objects in memory in Internet Explorer, aka "Scripting Engine Memory Corruption Vulnerability." This affects Internet Explorer 9, Internet Explorer 11, Internet Explorer 10. This CVE ID is unique from CVE-2018-8353, CVE-2018-8355, CVE-2018-8359, CVE-2018-8371, CVE-2018-8372, CVE-2018-8385, CVE-2018-8389, CVE-2018-8390.

    Source: NVD (NIST), cvelistv5, CISA KEV Catalog

    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
    MicrosoftInternet Explorer 10
    • Windows Server 2012
    • —
    MicrosoftInternet Explorer 11
    • Windows 7 for x64-based Systems Service Pack 1
    • Windows RT 8.1
    • Windows Server 2008 R2 for x64-based Systems Service Pack 1
    • Windows Server 2012 R2
    • Windows Server 2016
    • Windows 10 Version 1703 for x64-based Systems
    • Windows 10 Version 1607 for 32-bit Systems
    • Windows 10 Version 1703 for 32-bit Systems
    • Windows 7 for 32-bit Systems Service Pack 1
    • Windows 8.1 for x64-based systems
    • Windows 10 Version 1709 for 32-bit Systems
    • Windows 10 Version 1803 for x64-based Systems
    • Windows 10 for 32-bit Systems
    • Windows 10 Version 1709 for x64-based Systems
    • Windows 8.1 for 32-bit systems
    • Windows 10 for x64-based Systems
    • Windows 10 Version 1803 for 32-bit Systems
    • Windows 10 Version 1607 for x64-based Systems
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    MicrosoftInternet Explorer 9
    • Windows Server 2008 for x64-based Systems Service Pack 2
    • Windows Server 2008 for 32-bit Systems Service Pack 2
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    microsoftinternet_explorer
    • 9
    • 10
    • 11
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    Vendors and products

    Weakness classes (CWE)

    • CWE-787Out-of-bounds Write

      Base

      The product writes data past the end, or before the beginning, of the intended buffer.

      Entry at MITRE CWE-787

    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

    • securitytracker.com1041483Vulnerability database, x_refsource_SECTRACK, Broken link, Third-party advisory, Vulnerability database
    • securityfocus.com105037Vulnerability database, x_refsource_BID, Broken link, Third-party advisory, Vulnerability database
    • portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2018-8373x_refsource_CONFIRM, Patch, Vendor advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2018-8373government-resource, US government resource
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