CriticalCVE-2021-34448

Scripting Engine Memory Corruption Vulnerability (CVE-2021-34448)

Published 16.07.2021Last modified 10.08.2026In CISA KEV since 03.11.2021Sources: SingCERT, CVE.org

Actively exploited

What is affected, what to do

Scripting Engine Memory Corruption Vulnerability

The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Windows 10 Version 1507 to version 10.0.10240.19003, Microsoft Windows 10 Version 1607 to version 10.0.14393.4530, Microsoft Windows 10 Version 1809 to version 10.0.17763.2061. Immediate actions, mitigation and detection rules are in the runbook below.

Affected: Microsoft Windows 10 Version 1507 lt 10.0.10240.0..10.0.10240.19003
Microsoft Windows 10 Version 1607 lt 10.0.14393.0..10.0.14393.4530
Microsoft Windows 10 Version 1809 lt 10.0.17763.0..10.0.17763.2061
Microsoft Windows 10 Version 1909 lt 10.0.0..10.0.18363.1679
Microsoft Windows 10 Version 2004 lt 10.0.0..10.0.19041.1110
Microsoft Windows 10 Version 20H2 lt 10.0.0..10.0.19042.1110
Microsoft Windows 10 Version 21H1 lt 10.0.0..10.0.19043.1110
Microsoft Windows 7 lt 6.1.0..6.1.7601.25661
Microsoft Windows 7 Service Pack 1 lt 6.1.0..6.1.7601.25661
Microsoft Windows 8.1 lt 6.3.0..6.3.9600.20069
Microsoft Windows Server 2008 R2 Service Pack 1 lt 6.1.7601.0..6.1.7601.25661
Microsoft Windows Server 2012 lt 6.2.9200.0..6.2.9200.23409
Microsoft Windows Server 2012 R2 lt 6.3.9600.0..6.3.9600.20069
Microsoft Windows Server 2016 lt 10.0.14393.0..10.0.14393.4530
Microsoft Windows Server 2019 lt 10.0.17763.0..10.0.17763.2061
Fixed in: Microsoft Windows 10 Version 1507 10.0.10240.19003
Microsoft Windows 10 Version 1607 10.0.14393.4530
Microsoft Windows 10 Version 1809 10.0.17763.2061
Microsoft Windows 10 Version 1909 10.0.18363.1679
Microsoft Windows 10 Version 2004 10.0.19041.1110
Microsoft Windows 10 Version 20H2 10.0.19042.1110
Microsoft Windows 10 Version 21H1 10.0.19043.1110
Microsoft Windows 7 6.1.7601.25661
Microsoft Windows 7 Service Pack 1 6.1.7601.25661
Microsoft Windows 8.1 6.3.9600.20069
Microsoft Windows Server 2008 R2 Service Pack 1 6.1.7601.25661
Microsoft Windows Server 2012 6.2.9200.23409
Microsoft Windows Server 2012 R2 6.3.9600.20069
Microsoft Windows Server 2016 10.0.14393.4530
Microsoft Windows Server 2019 10.0.17763.2061

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Metrics

6.8

Priority

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

Source: NVD, CVSS v3

6.8

CVSS v3

Severity Medium on the scale from 0 to 10.

Source: SingCERT, CVE.org

40.1 %

EPSS score

Elevated — model estimates 10-50% exploitation likelihood.

Source: FIRST EPSS, percentile 98.6

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: 211.82 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: 6.8 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: 6.8
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: 40.1 %; Elevated — model estimates 10-50% exploitation likelihood.
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: 98.6; 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: 16.07.2021
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: 03.11.2021
CISA assessment (SSVC): Exploitation: active · automatable · Technical impact: total; Source: CISA Vulnrichment, as of 4 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 flaw sits in the scripting engine used by Internet Explorer and MSHTML-based components. It was exploited as a zero-day in targeted campaigns, typically via crafted web pages or link-delivered content that yields code execution in the user's context. Exploitation requires user interaction and is more complex to pull off, but it is network-reachable without authentication. The likelihood of exploitation is still rated very high, and CISA does not flag ransomware use. For fully patched estates with IE disabled, residual risk is low. Unpatched clients, non-ESU Windows 7 systems and internet-facing terminal servers should be treated as high priority and recorded as critical deviations in NIS2-scoped environments.

01
Runbook step 1
Immediate response (0-24 h)
- Deploy the July 2021 cumulative security update (or any later cumulative update) to all affected Windows clients. Minimum target builds are 10.0.10240.19003 (1507), 10.0.14393.4530 (1607), 10.0.17763.2061 (1809), 10.0.18363.1679 (1909), and 10.0.19041.1110 / 10.0.19042.1110 / 10.0.19043.1110 (2004/20H2/21H1). Look up the per-build KB number in the MSRC advisory for CVE-2021-34448.
- Build a patch-gap inventory with Intune, SCCM or Defender TVM and list every endpoint below the target build. For Windows Server hosts used for interactive browsing (RDS/Citrix), confirm whether they are affected in the vendor advisory.
- Windows 7 (target 6.1.7601.25661) only receives this fix under ESU. Treat non-ESU machines as unpatchable: remove their internet browsing, and isolate or decommission them.
- Treat Internet Explorer 11 and legacy MSHTML hosts as the primary attack path. Disable standalone IE via GPO ("Disable Internet Explorer 11 as a standalone browser") and redirect users to Chromium Edge. Restrict IE mode to sites listed in the Enterprise Mode Site List.
- Fast-track patch verification for high-exposure user groups (executives, finance, IT admins). They are the usual targets of crafted links and documents.
02
Runbook step 2
Mitigation layers
- Configuration/endpoint hardening: Restrict legacy JScript execution in IE internet zones using the option Microsoft documents in its advisory. Use the jscript.dll ACL workaround (takeown/icacls) only after functional testing, because it breaks applications that depend on that library.
- Attack Surface Reduction (Defender for Endpoint): Enforce the following rules in block mode: "Block JavaScript or VBScript from launching downloaded executable content" (d3e037e1-3eb8-44c8-a917-57927947596d), "Block all Office applications from creating child processes" (d4f940ab-401b-4efc-aadc-ad5f3c50688a), and "Block Office applications from injecting code into other processes".
- Network/proxy: Force all web traffic through an authenticated proxy with URL categorisation and TLS inspection. Block newly registered and uncategorised domains for MSIE/Trident/7.0 user agents. Deny direct egress on 80/443 from client VLANs at the perimeter firewall.
- Mail gateway: Detonate attachments carrying embedded web content in a sandbox, including HTML, MHT, and Office documents with external OLE or MSHTML references. Enable click-time URL rewriting.
- Least privilege: Remove local admin rights from standard users (LAPS or Windows LAPS) so that successful code execution stays confined to the user context. Use WDAC or AppLocker to block dropped binaries in %TEMP% and %APPDATA%.
03
Runbook step 3
Detection rules
- Sysmon EID 1 / EDR process ancestry: Watch for iexplore.exe, winword.exe, excel.exe or mshta.exe spawning cmd.exe, powershell.exe, rundll32.exe, regsvr32.exe or wscript.exe. Example KQL: DeviceProcessEvents | where InitiatingProcessFileName in~ ("iexplore.exe","winword.exe","excel.exe","mshta.exe") and FileName in~ ("cmd.exe","powershell.exe","rundll32.exe","regsvr32.exe").
- Crash telemetry (Application log EID 1000/1001, WER): IE or Office crashes with faulting module jscript.dll or jscript9.dll often indicate failed exploitation attempts. SPL: index=wineventlog EventCode=1000 (Faulting_module="jscript*.dll").
- Sysmon EID 7 (image load): Flag jscript.dll or jscript9.dll loading into processes that normally do not use a script engine, such as winword.exe or excel.exe. Sigma shape: ImageLoaded|endswith: '\jscript9.dll' with a filter for expected host processes.
- Sysmon EID 11 / file events: Look for executables or scripts (.exe, .dll, .js, .hta) written to INetCache, %TEMP% or %APPDATA% shortly after IE or Office starts.
- Proxy/Zeek HTTP logs: Flag Trident/7.0 user agents on networks where IE should be disabled. Also watch for large obfuscated JavaScript responses from freshly registered domains, followed by new outbound connections from the same parent process.

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

Scripting Engine Memory Corruption Vulnerability

Source: CSA/SingCERT, CISA KEV Catalog, NVD (NIST), cvelistv5, 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 versions	Fixed in
Microsoft	Windows 10 Version 1507	`10.0.10240.0 – < 10.0.10240.19003`	`10.0.10240.19003`
Microsoft	Windows 10 Version 1607	`10.0.14393.0 – < 10.0.14393.4530`	`10.0.14393.4530`
Microsoft	Windows 10 Version 1809	`10.0.17763.0 – < 10.0.17763.2061`	`10.0.17763.2061`
Microsoft	Windows 10 Version 1909	`10.0.0 – < 10.0.18363.1679`	`10.0.18363.1679`
Microsoft	Windows 10 Version 2004	`10.0.0 – < 10.0.19041.1110`	`10.0.19041.1110`
Microsoft	Windows 10 Version 20H2	`10.0.0 – < 10.0.19042.1110`	`10.0.19042.1110`
Microsoft	Windows 10 Version 21H1	`10.0.0 – < 10.0.19043.1110`	`10.0.19043.1110`
Microsoft	Windows 7	`6.1.0 – < 6.1.7601.25661`	`6.1.7601.25661`
Microsoft	Windows 7 Service Pack 1	`6.1.0 – < 6.1.7601.25661`	`6.1.7601.25661`
Microsoft	Windows 8.1	`6.3.0 – < 6.3.9600.20069`	`6.3.9600.20069`
Microsoft	Windows Server 2008 R2 Service Pack 1	`6.1.7601.0 – < 6.1.7601.25661`	`6.1.7601.25661`
Microsoft	Windows Server 2012	`6.2.9200.0 – < 6.2.9200.23409`	`6.2.9200.23409`
Microsoft	Windows Server 2012 R2	`6.3.9600.0 – < 6.3.9600.20069`	`6.3.9600.20069`
Microsoft	Windows Server 2016	`10.0.14393.0 – < 10.0.14393.4530`	`10.0.14393.4530`
Microsoft	Windows Server 2019	`10.0.17763.0 – < 10.0.17763.2061`	`10.0.17763.2061`

Vendors and products

Affected operating systems

Affected operating systems

Windows	microsoft	windows_10_1507	`10.0.10240.19003`
Windows	microsoft	windows_10_1507	—
Windows	microsoft	windows_10_1607	`10.0.14393.4530`
Windows	microsoft	windows_10_1607	—
Windows	microsoft	windows_10_1809	`10.0.17763.2061`
Windows	microsoft	windows_10_1809	`10.0.18363.1679`
Windows	microsoft	windows_10_1809	`10.0.19041.1110`
Windows	microsoft	windows_10_1809	—
Windows	microsoft	windows_10_1909	`10.0.18363.1679`
Windows	microsoft	windows_10_1909	—
Windows	microsoft	windows_10_2004	—
Windows	microsoft	windows_10_20H2	`10.0.19042.1110`
Windows	microsoft	windows_10_20h2	—
Windows	microsoft	windows_10_21H1	`10.0.19043.1110`
Windows	microsoft	windows_10_21h1	—
Windows	microsoft	windows_7	`1.001`
Windows	microsoft	windows_7	`6.1.7601.25661`
Windows	microsoft	windows_7	—
Windows	microsoft	windows_8.1	`1.001`
Windows	microsoft	windows_8.1	`6.3.9600.20069`
Windows	microsoft	windows_8.1	—
Windows	microsoft	windows_rt_8.1	`6.3.9600.20069`
Windows	microsoft	windows_rt_8.1	—
Windows	microsoft	windows_server_2008	`r2`

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 Catalog	CISA Known Exploited Vulnerability (opens in a new tab)cisa.gov	actively exploited	03.11.2021

References & sources

portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2021-34448x_refsource_MISC, x_transferred
Microsoft Security Update GuideScripting Engine Memory Corruption VulnerabilityVendor advisory, Patch
cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-34448government-resource

Changes to this CVE

Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

10.08.2026NVD analysis modifiedNVD

New references:

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-34448

Show the original change

Individual changes of this event as published by NVD
Type	Action	Before	After
Reference type	added	—	Microsoft Corporation: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-34448 Types: Patch, Vendor Advisory

Source identifier: nvd@nist.gov

10.08.2026Record modifiedMicrosoft

New references:

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-34448

Show 4 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Reference	added	—	https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-34448
Reference	removed	https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-34448	—
Reference type	removed	https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-34448 Types: Patch, Vendor Advisory	—
Affected versions (publisher)	changed	Windows 10 Version 1809, Windows Server 2019, Windows 10 Version 1909 (+12)	Windows 10 Version 1507, Windows 10 Version 1607, Windows 10 Version 1809 (+13)

Source identifier: secure@microsoft.com

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<a href="https://intel.neosec.eu/en/cve/CVE-2021-34448"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-34448.svg?lang=en" alt="CVE-2021-34448 – NEOSEC Intel" height="20"></a>

What is affected, what to do

Metrics

Response and mitigation

Prioritisation rationale

Immediate response (0-24 h)

Mitigation layers

Detection rules

Detection rules

Classification under NIS2 and CRA

Description

Affected products

Vendors and products

Affected operating systems

Weakness classes (CWE)

Public exploit references

References & sources

Changes to this CVE