CVE-2015-1671: Microsoft Windows Remote Code Execution Vulnerability

CVE-2015-1671 is a remote code execution vulnerability in Microsoft Windows and related components, including .NET Framework, Office, Lync, and Silverlight. It arises when these components fail to properly handle TrueType fonts. Successful abuse can let an attacker run code in the context of the affected process or user, which matters because font handling is common in everyday document and web workflows and can turn a single malicious file or page into a foothold on the host.

Defenders should treat this as a high-priority patch item for any environment still running the affected Microsoft stack. Confirm exact product scope, fixed builds, and deployment guidance against the vendor advisory; do not rely on secondary summaries alone.

How it works

The weakness is categorized under CWE-19 (data processing errors). In plain terms, the vulnerable components do not correctly process certain TrueType font data. An attacker who can supply a crafted font—typically embedded in a document, web content, or other file that the target application will parse—can trigger the flawed handling path and achieve remote code execution.

Abuse generally depends on the victim opening or rendering the malicious content with a vulnerable component (for example, a document viewer, browser-hosted control, or collaboration client that exercises the font parser). No further exploit mechanics, payload details, or reliability claims are stated in the available record; treat any public proof-of-concept material with caution and validate behavior only in isolated lab conditions if needed for detection engineering.

Am I affected? How to find it in your systems

The vulnerability affects Microsoft Windows and the listed related products that process TrueType fonts: .NET Framework, Office, Lync, and Silverlight. These commonly appear on end-user workstations, terminal servers, and some application servers that render documents or host legacy Silverlight/.NET content.

• Inventory Windows hosts and installed Microsoft desktop and productivity software (Office suites, Lync/Skype for Business clients, Silverlight runtimes, and .NET Framework versions in use).
• Map which systems still open untrusted or semi-trusted documents, email attachments, or web content that can embed fonts.
• Compare installed builds and update levels to the fixed versions named in the Microsoft advisory for CVE-2015-1671; if the advisory lists multiple bulletins or packages, apply the full set that covers your SKUs.
• For exploitation signs, review endpoint and application logs for crashes or faults in font-related or document-rendering processes, unexpected child processes spawned from Office/Lync/browser hosts, and alerts from existing EDR or application-control tools around font parsing. Public detail on specific telemetry indicators is limited, so tune detections to your baseline and the vendor’s guidance.

How to remediate

Patch first. Apply the updates Microsoft released for this vulnerability exactly as directed in the vendor advisory and CISA’s required action: follow vendor instructions for the applicable Windows, .NET Framework, Office, Lync, and Silverlight packages.

• Use your standard enterprise channel (WSUS, ConfigMgr/Intune, Microsoft Update Catalog, or approved third-party patch management) to deploy and verify installation across all affected SKUs.
• After patching, validate that the expected security updates are present and that dependent services restart cleanly.
• Harden the broader class: keep Office and collaboration clients current, prefer protected-view or sandboxed document opening for untrusted files, and remove or disable legacy components (such as unused Silverlight) that are no longer required.
• Reduce the attack surface by limiting which users and machines need full document-rendering privileges and by enforcing least privilege so that code execution in a user process does not immediately yield admin rights.

If you can't patch immediately

Until updates are installed, reduce exposure with compensating controls appropriate to font- and document-parsing flaws.

• Segment and restrict: keep high-risk workstations off sensitive networks where possible; limit outbound connectivity from document-heavy hosts.
• Virtual patching / filtering: where a WAF, email gateway, or secure web gateway can block or quarantine attachments and web content known to carry suspicious fonts or exploit patterns for this class, enable those rules and monitor for false positives.
• Disable or constrain the affected feature path: remove unused Silverlight and legacy .NET browser plugins; use application control or Office hardening (Protected View, disabling automatic download of external content) so untrusted fonts are less likely to be parsed with full privileges.
• Increase monitoring: alert on anomalous process trees from Office, Lync, or browser processes, sudden crashes in font subsystems, and new persistence after document open events. Retain logs long enough to support incident review.

These steps lower risk but do not replace the vendor update. Schedule patching as soon as operationally feasible.

If your data may have been exposed

Actively exploited remote code execution flaws are a common path into broader compromise and data theft, even when ransomware use is not documented for this specific CVE. If you have reason to believe systems were exposed before patching—suspicious documents opened, unexplained process activity, or confirmed intrusion—follow your incident-response plan: isolate hosts, preserve evidence, credential-reset as needed, and assess what data those hosts could reach.

As a quick external check, you can run a free exposure scan of your email addresses against known breach datasets to see whether credentials or personal data associated with your accounts have already appeared in public breach collections, then prioritize password changes and monitoring accordingly.