Microsoft DirectWrite TrueType Font Parsing Remote Code Execution Vulnerability (CVE-2015-1671, MS15-044)
Executive & SecOps Summary
SECOPS ASSESSMENTMicrosoft .NET Framework, Office 2007/2010, Lync 2010/2013, and Silverlight 5 utilizing the DirectWrite library
Remote attackers can execute arbitrary code (RCE) with the privileges of the active user by tricking the system into rendering a specially crafted TrueType font
Immediately apply the official Microsoft security updates (MS15-044) and restrict the loading of untrusted fonts via system policies
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
This vulnerability is addressed in the MS15-044 security bulletin. Administrators should deploy the latest cumulative updates. Use the following PowerShell commands to verify patch status and force updates:
# Check if the required MS15-044 hotfixes (e.g., KB3045171, KB3051468) are installed
Get-HotFix | Where-Object { $_.HotFixID -match 'KB3045171|KB3051468' }
# Force Windows Update to retrieve and install missing security patches
Install-Module PSWindowsUpdate -Force -SkipPublisherCheck
Get-WindowsUpdate -AcceptAll -Install -AutoReboot2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible, mitigate the risk by enabling the 'Untrusted Font Blocking' feature on Windows or disabling font downloads in Internet zones.
# Enable Untrusted Font Blocking feature via Registry
Set-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Control\Session Manager\Kernel' -Name 'MitigationOptions' -Value ([byte](0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x01,0x00,0x00,0x00,0x00,0x00,0x00,0x00))
# Disable Font Downloads in the Internet Zone (Zone 3)
New-ItemProperty -Path 'HKCU:\Software\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\3' -Name '1604' -PropertyType DWord -Value 3 -Force3. Log Audit & IoC Verification Commands
Run the following script to audit the system for vulnerable Silverlight installations (versions prior to 5.1.40416.00):
# Audit Silverlight version to identify vulnerability exposure
$silverlightPath = '${env:ProgramFiles}\Microsoft Silverlight\sliverlight.configuration.exe'
if (Test-Path $silverlightPath) {
$version = [System.Diagnostics.FileVersionInfo]::GetVersionInfo($silverlightPath).FileVersion
Write-Host 'Current Silverlight Version:' $version
if ([Version]$version -lt [Version]'5.1.40416.00') {
Write-Warning 'VULNERABLE: Silverlight version is below 5.1.40416.00. Apply MS15-044 immediately.'
} else {
Write-Host 'SECURE: Silverlight is patched.' -ForegroundColor Green
}
} else {
Write-Host 'Silverlight is not present on this system.' -ForegroundColor Green
}Block Kernel Privilege Escalation & Ransomware Payloads Before You Can Reboot
Protect unpatched Windows, Linux, and macOS workloads against in-memory exploitation and lateral movement with autonomous EDR.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2015-1671) exists in the Microsoft DirectWrite library, which is responsible for high-quality text rendering across various Windows applications. The root cause lies in the improper validation of input data when parsing TrueType font (.ttf) structures. When processing malformed font tables, the DirectWrite engine fails to perform adequate bounds checking, leading to a memory corruption condition that can be leveraged to hijack execution flow.
2. Exploitation Mechanics & Attack Scenario
An attacker exploits this vulnerability by embedding a corrupted TrueType font with malformed table offsets or sizes into a delivery vehicle. Common attack vectors include:
- Web-based Vector: Hosting a malicious website containing Silverlight content or web fonts that trigger the vulnerability when rendered by a visitor's browser.
- Document-based Vector: Sending a spear-phishing email with an attached Office document or email body that references the malicious font.
Upon rendering the font, the DirectWrite parser triggers memory corruption, allowing the attacker's payload to execute within the security context of the logged-in user.
3. Infrastructure & Business Impact
As a vulnerability listed in the CISA KEV catalog, CVE-2015-1671 represents a proven threat vector that has been actively exploited in targeted campaigns. Successful exploitation grants initial access to endpoint systems, which can serve as a pivot point for lateral movement, credential harvesting, and full domain compromise, posing severe risks to enterprise confidentiality and operational continuity.
Official Vendor Advisories & References
The Windows DirectWrite library, as used in Microsoft .NET Framework 3.0 SP2, 3.5, 3.5.1, 4, 4.5, 4.5.1, and 4.5.2; Office 2007 SP3 and 2010 SP2; Live Meeting 2007 Console; Lync 2010; Lync 2010 Attendee; Lync 2013 SP1; Lync Basic 2013 SP1; Silverlight 5 before 5.1.40416.00; and Silverlight 5 Developer Runtime before 5.1.40416.00, allows remote attackers to execute arbitrary code via a crafted TrueType font, aka "TrueType Font Parsing Vulnerability."
Frequently Asked SecOps Questions (CVE-2015-1671)
FAQQ1. How critical is CVE-2015-1671 and is it actively exploited in the wild?
CVE-2015-1671 is rated CVSS 7.8 (HIGH) with a 30-day EPSS exploitation probability of 48.8%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2015-1671?
Primary impacted vendors include microsoft across net_framework, office, live_meeting, lync, silverlight. Scope summary: Microsoft .NET Framework 3.0 SP2 to 4.5.2, Office 2007 SP3 / 2010 SP2, Live Meeting 2007 Console, Lync 2010 / 2013 SP1, and Silverlight 5 (prior to version 5.1.40416.00)
Q3. What temporary mitigations or workarounds exist for CVE-2015-1671 before patching?
[Mitigation] Immediately apply the official Microsoft security updates (MS15-044) and restrict the loading of untrusted fonts via system policies Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
Generate an Internal Engineering Notice or CISO Report for CVE-2015-1671
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