CVE-2021-33742Exploited in the wildNVD status: Analyzed
Microsoft Windows MSHTML Platform Remote Code Execution
CVE-2021-33742 is a high-severity out-of-bounds write vulnerability in Windows 10 Version 1809 from Microsoft, with a CVSS 3.1 base score of 8.8 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, but only if a user takes part, for example by opening a link or file. The record lists version 10.0.17763.1999 as the first without the flaw, so updating Windows 10 Version 1809 to that release or later removes the exposure. CISA added it to the Known Exploited Vulnerabilities catalogue on November 3, 2021, which means attacks have been observed in the wild.
Patch now
Attackers are already exploiting this vulnerability, according to CISA.
What to do
- Update Windows 10 Version 1809 to 10.0.17763.1999 or a later version.
- Update Windows Server 2019 to 10.0.17763.1999 or a later version.
- Update Windows 10 Version 1909 to 10.0.18363.1621 or a later version.
- CISA's required action: Apply updates per vendor instructions.
- Read the advisory on portal.msrc.microsoft.com for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
- Because the attack needs a user to take part, warn users about unexpected links and files related to this product until it is patched.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Microsoft | Windows 10 Version 1809 | 10.0.0 up to, but not including, 10.0.17763.1999 | 10.0.17763.1999 |
| Microsoft | Windows Server 2019 | 10.0.0 up to, but not including, 10.0.17763.1999 | 10.0.17763.1999 |
| Microsoft | Windows 10 Version 1909 | 10.0.0 up to, but not including, 10.0.18363.1621 | 10.0.18363.1621 |
| Microsoft | Windows 10 Version 21H1 | 10.0.0 up to, but not including, 10.0.19043.1052 | 10.0.19043.1052 |
| Microsoft | Windows 10 Version 2004 | 10.0.0 up to, but not including, 10.0.19041.1052 | 10.0.19041.1052 |
| Microsoft | Windows 10 Version 20H2 | 10.0.0 up to, but not including, 10.0.19042.1052 | 10.0.19042.1052 |
| Microsoft | Windows 10 Version 1507 | 10.0.0 up to, but not including, 10.0.10240.18967 | 10.0.10240.18967 |
| Microsoft | Windows 10 Version 1607 | 10.0.0 up to, but not including, 10.0.14393.4467 | 10.0.14393.4467 |
| Microsoft | Windows Server 2016 | 10.0.0 up to, but not including, 10.0.14393.4467 | 10.0.14393.4467 |
| Microsoft | Windows 7 | 6.1.0 up to, but not including, 6.1.7601.25632; 6.1.0 up to, but not including, publication | 6.1.7601.25632, publication |
| Microsoft | Windows 7 Service Pack 1 | 6.1.0 up to, but not including, 6.1.7601.25632; 6.1.0 up to, but not including, publication | 6.1.7601.25632, publication |
| Microsoft | Windows 8.1 | 6.3.0 up to, but not including, 6.3.9600.20045; 6.3.0 up to, but not including, 1.0.0.0 | 6.3.9600.20045, 1.0.0.0 |
| Microsoft | Windows Server 2008 Service Pack 2 | 6.0.0 up to, but not including, 6.0.6003.21137; 6.0.0 up to, but not including, publication | 6.0.6003.21137, publication |
| Microsoft | Windows Server 2008 R2 Service Pack 1 | 6.1.0 up to, but not including, 6.1.7601.25632; 6.1.0 up to, but not including, publication | 6.1.7601.25632, publication |
| Microsoft | Windows Server 2012 | 6.2.0 up to, but not including, 6.2.9200.23372; 6.2.0 up to, but not including, 1.0.0.0 | 6.2.9200.23372, 1.0.0.0 |
| Microsoft | Windows Server 2012 R2 | 6.3.0 up to, but not including, 6.3.9600.20045; 6.3.0 up to, but not including, 1.0.0.0 | 6.3.9600.20045, 1.0.0.0 |
| Microsoft | Windows 10 1507 | before 10.0.10240.18967 | 10.0.10240.18967 |
| Microsoft | Windows 10 1607 | before 10.0.14393.4467 | 10.0.14393.4467 |
| Microsoft | Windows 10 1809 | before 10.0.17763.1999 | 10.0.17763.1999 |
| Microsoft | Windows 10 1909 | before 10.0.18363.1621 | 10.0.18363.1621 |
| Microsoft | Windows 10 2004 | before 10.0.19041.1052 | 10.0.19041.1052 |
| Microsoft | Windows 10 20h2 | before 10.0.19042.1052 | 10.0.19042.1052 |
| Microsoft | Windows 10 21h1 | before 10.0.19043.1052 | 10.0.19043.1052 |
| Microsoft | Windows Rt 8.1 | Not specified in the record | Not stated |
| Microsoft | Windows Server 2008 | r2; sp2 | Not stated |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- RequiredA user must take an action such as opening a file or link.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 7.5 | High | CNA (microsoft.com) |
| CVSS 3.1 | 8.8 | High | NVD |
| CVSS 2.0 | 6.8 | Medium | NVD |
Official description
Windows MSHTML Platform Remote Code Execution Vulnerability
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Out-of-bounds Write
The product writes data past the end, or before the beginning, of the intended buffer.
How developers prevent this class of flaw
- Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer. Be wary that a language's interface to native code may still be subject to overflows, even if the language itself is theoretically safe.
- Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. Examples include the Safe C String Library (SafeStr) by Messier and Viega, and the Strsafe.h library from Microsoft. These libraries provide safer versions of overflow-prone string-handling functions.
- Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking. D3-SFCV (Stack Frame Canary Validation) from D3FEND discusses canary-based detection in detail.
Timeline
- CVE record published.
- Added to CISA's Known Exploited Vulnerabilities catalogue.
- Deadline for US federal agencies to remediate, set by CISA.
- CISA published its SSVC assessment (exploitation: active, automatable: no, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Third-party and government advisories
- cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33742US Government Resource
Recent Out-of-bounds Write vulnerabilities
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Arm Trusted Firmware Out-of-Bounds Write Exploited
- 8.8 High severity
- 9.8 Critical severity
Tenda AC11 Router Stack Buffer Overflow Exploited
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Microsoft Win32k Privilege Escalation Exploited
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