Full Report
Microsoft has confirmed that .NET Framework updates released as part of the August 2026 Patch Tuesday are breaking printing and PDF export in WPF applications. [...]
Analysis Summary
# Vulnerability: August 2026 .NET Framework Security Update Regression (WPF Printing)
## CVE Details
*Note: The provided article discusses a regression caused by the August 2026 Patch Tuesday updates. While the specific CVE IDs for the underlying vulnerabilities addressed by the patch are not explicitly named in the text, these updates typically address critical security flaws in font handling.*
- **CVE ID:** Not explicitly specified (Regression caused by August 2026 Security Updates)
- **CVSS Score:** Unknown/Not listed
- **CWE:** CWE-94 (Improper Control of Generation of Code - implied by the need for "Cmap and Sbit Overflow Protection")
## Affected Systems
- **Products:** Applications built using the Windows Presentation Foundation (WPF) UI framework.
- **Versions:**
- Windows 10 (all supported versions)
- Windows 11 (all supported versions)
- Windows Server 2012 through Windows Server 2025
- **Configurations:** Systems where the August 2026 .NET Framework cumulative update is installed and applications utilize specific fonts (including **Calibri**) for printing or PDF/XPS export.
## Vulnerability Description
The August 2026 security updates for .NET Framework introduced "Cmap and Sbit Overflow Protection" to mitigate potential security exploits related to font parsing. However, this security hardening has caused a functional regression. Affected WPF applications fail when attempting to process certain fonts, resulting in a `System.IO.FileFormatException`. This occurs specifically during the generation of PDF/XPS content or when sending documents to a printer.
## Exploitation
- **Status:** Not exploited (This report concerns a **functional break** caused by a security patch). However, the underlying vulnerabilities fixed by the update are presumed to be exploitable if the patch is bypassed.
- **Complexity:** N/A (Functional failure)
- **Attack Vector:** Local (Triggered by processing malicious font files if protection is disabled).
## Impact
- **Confidentiality:** None (from the regression)
- **Integrity:** None (from the regression)
- **Availability:** **High** (Causes application crashes/failures during core printing and export functions).
## Remediation
### Patches
- Microsoft is currently investigating a permanent fix. Users are advised to keep the August 2026 updates installed to remain protected against the underlying security threats.
### Workarounds
**Warning:** Applying this workaround disables security protections and exposes the system to the vulnerabilities the August update was intended to fix.
1. Locate the application configuration file (`app.config` or `[appname].exe.config`).
2. Add the following AppContext switch to the `<runtime>` section:
xml
<runtime>
<AppContextSwitchOverrides value="Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection=true" />
</runtime>
## Detection
- **Indicators of Compromise:** N/A (Functional issue).
- **Detection Methods:**
- Application event logs showing `System.IO.FileFormatException`.
- Failure of WPF applications to render/print when using Calibri or other standard fonts.
- Verification of August 2026 .NET Cumulative Update installation via Windows Update History or `Get-HotFix` in PowerShell.
## References
- BleepingComputer Article: hxxps[:]//www[.]bleepingcomputer[.]com/news/microsoft/microsoft-august-updates-break-printing-pdf-export-in-wpf-apps/
- Microsoft Windows Release Health Dashboard (Internal/Vendor source referenced in context).