Windows 2000Operating system · Microsoft

CVE-2009-0091

HIGH · 9.3 CVSS v2.0 Published 2009-10-14

Mitigation only

No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.

See remediation →

100/100

Remediation priority · Urgent

Remotely reachable No privileges Zero-click

Official description

NVD · unedited

Microsoft .NET Framework 2.0, 2.0 SP1, and 3.5 does not properly enforce a certain type-equality constraint in .NET verifiable code, which allows remote attackers to execute arbitrary code via (1) a crafted XAML browser application (XBAP), (2) a crafted ASP.NET application, or (3) a crafted .NET Framework application, aka "Microsoft .NET Framework Type Verification Vulnerability."

Technical summary

dbcve analysis · high confidence

The vulnerability exists in .NET Framework 2.0, 2.0 SP1, and 3.5 where type-equality constraints in verifiable code are not properly enforced. This allows specially crafted XBAP browser applications, ASP.NET applications, or .NET Framework applications to bypass type safety checks and achieve arbitrary code execution with full user privileges.

MitigationApply Microsoft security patches for the affected .NET Framework versions and conduct compatibility testing with existing applications, particularly those utilizing XBAP or ASP.NET components.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions

NVD · CPE data

Windows 2000Operating system

Affected:all versions

.net FrameworkApplication

Affected:= 1.1= 2.0= 3.5= 1.0

Windows Server 2003Operating system

Affected:all versions

Windows Server 2008Operating system

Affected:all versions

Windows VistaOperating system

Affected:all versions

Windows 7Operating system

Affected:all versions

Windows XpOperating system

Affected:all versions

CVSS breakdown

From the vector

Attack vector: Network
Complexity: M
Authentication: None
Confidentiality: Complete
Integrity: Complete
Availability: Complete

AV:N/AC:M/Au:N/C:C/I:C/A:C

Am I affected?

dbcve checks

Work through these to decide whether this CVE applies to you.

Identify installed .NET Framework version

Open Registry Editor and navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NET Framework Setup\NDP\v2.0.50727 (for .NET 2.0) or v3.5 (for .NET 3.5). Check the 'Version' value in the 'Install' subkey. Alternatively, run 'reg query "HKLM\SOFTWARE\Microsoft\NET Framework Setup\NDP" /s /v Version' from command prompt.

Affected if The installed version is 2.0, 2.0 SP1, or 3.5 without security patches
Verify if XBAP browser application support is enabled

Check Internet Explorer security settings. In IE, go to Internet Options > Security > Custom Level > .NET Framework-related settings. Look for 'Enable XAML Browser Applications' or 'Allow XBAP' option. Also check Windows Features for .NET Framework 3.0 (which includes XBAP support).

Affected if XBAP support is enabled and .NET Framework 2.0/3.5 unpatched version is installed
Confirm ASP.NET is installed and enabled

Check for ASP.NET in IIS (run inetmgr) or look for ASP.NET version in C:\Windows\Microsoft.NET\Framework\{version}\aspnet_isapi.dll. Also verify ASP.NET is registered with IIS using 'aspnet_regiis -lk'.

Affected if ASP.NET is installed and running on an unpatched .NET Framework 2.0/3.5 installation
Check Windows version and .NET Framework installation

Run 'winver' or 'systeminfo' from command prompt to identify the Windows version. Then cross-reference with the .NET Framework version found in Step 1.

Affected if Running Windows 2000, XP, Server 2003, Server 2008, Vista, or Windows 7 with unpatched .NET Framework 1.0, 1.1, 2.0, or 3.5

The environment is affected if .NET Framework version 1.0, 1.1, 2.0, 2.0 SP1, or 3.5 is installed without Microsoft security patches, and XBAP or ASP.NET features are in use.

Generated from the published advisory. Verify against your own configuration.

Remediation

dbcve · scoped

Mitigation available No clean upgrade yet — mitigate in the meantime

Mitigation

Apply Microsoft security patches for the affected .NET Framework versions and conduct compatibility testing with existing applications, particularly those utilizing XBAP or ASP.NET components.

Recommended fix High confidence

Apply Microsoft Security Update MS09-036 (patches for .NET Framework 1.0 SP3, 1.1 SP1, 2.0 SP1/SP2, and 3.5)

1. Identify the exact installed version of .NET Framework by checking the Windows registry or using the Microsoft .NET Framework Setup Verification Tool
2. Determine the applicable Microsoft security bulletin - this vulnerability was addressed in MS09-036
3. For .NET Framework 1.0 SP3 on Windows XP/2000: apply KB967617
4. For .NET Framework 1.1 SP1 on all supported Windows versions: apply KB967618
5. For .NET Framework 2.0 SP1 on all supported Windows versions: apply KB967612
6. For .NET Framework 2.0 SP2 on all supported Windows versions: apply KB974417
7. For .NET Framework 3.5 on all supported Windows versions: apply KB974418 (includes fixes for .NET 2.0 SP1 components)
8. Download the applicable patch from Microsoft Update Catalog or use Windows Update

Caveat Patches for older .NET Framework versions may require service pack updates; ensure compatibility with existing applications before deployment

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Windows 2000 Scoped from the published advisory

Consultation4.0 h
Implementation8.0 h
Testing12.0 h
Review / QA4.0 h

28.0 hours of engineering $4,760

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Scan for this in your stack

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References

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2009-0091 in production — separate from our analysis above.

Notes0 Guidelines

Know something about CVE-2009-0091?

The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.

The version that genuinely resolved it — not the one the vendor claimed
A config change or rule that shut the vector down
A gotcha in the upgrade path that cost you an afternoon

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here

Verified mitigations, workarounds, and config changes
Version or environment caveats, and links to real fixes
No weaponised exploit code, or anything meant to cause harm
No spam, self-promotion, credentials, or personal data

At a glance

A fix existsA vendor remedy is published

Severity 9.3 High

Exploit likelihood EPSS, next 30 days 25.8%

Affects: Windows 2000

Remediation

A high-priority fix

Fix this in Windows 2000 28 hrs · Moderate

Typical engagement from $4,760

Get a fix See the full breakdown ↓

A high-priority vulnerability. Send your environment and a specialist will scope CVE-2009-0091 and return a fixed quote and timeline.

You’ll deal with our team directly and deploy the reviewed fix to staging first — never straight to production.

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

Check your environment

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

Scan for this in your stack

References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Practitioner notes

No notes yet

Official description

Technical summary

Affected products & versions

CVSS breakdown

Am I affected?

Remediation

References