What’s happening with the Apple font bug?
A serious vulnerability in Apple’s font parsing system has been disclosed, potentially causing devices to crash or suffer memory corruption when a malicious font is processed. The flaw, identified as CVE-2025-43400, can be triggered simply by opening a crafted font-bearing document or launching an app that loads a malicious font. Security authorities, including BürgerCERT of Germany’s Federal Office for Information Security, urge users to apply the latest security updates promptly to mitigate the risk of Denial-of-Service-like crashes and other memory-related exploits.
Affected systems and versions
The issue affects several Apple operating systems, with the following versions specifically impacted:
- iOS before 18.7.1
- iOS before 26.0.1
- iPadOS before 18.7.1
- iPadOS before 26.0.1
- macOS Sequoia before 15.7.1
- macOS Sonoma before 14.8.1
- macOS Tahoe before 15.7.1
Apple notes that the root cause lies in the Font Parser, the component responsible for rendering fonts for display. When exploited, it may write to memory areas it should not access, potentially opening doors to further attacks.
How the bug works
Fonts are frequently embedded in web pages, documents, or software, and are often loaded dynamically. An attacker could craft a malicious font or document so that, upon being processed by the font parser, it triggers a memory write event. In practice, this can lead to a device crash or memory corruption, which could enable additional, unlisted attacks. The risk persists even if the user does not directly install a rogue font—the font could be delivered via a compromised webpage, document, or app.
Why this matters: risk and impact
While the immediate symptom for users might be an unexpected crash, the underlying memory damage could pave the way for more sophisticated exploits. A Denial-of-Service effect (repeated crashes) is explicitly highlighted by security authorities as a potential outcome, and memory corruption can complicate device stability and security posture. Those using iPhones, iPads, or Mac computers should treat this patch with priority to reduce exposure.
How to protect yourself: install the security update
The recommended protection is to install the latest security update for your device. Here’s how to proceed:
On iPhone or iPad
- Open the Settings app
- Select General
- Tap Software Update
- Download and install the available update (the version that fixes the font parser bug)
On Mac
- Open System Settings (or System Preferences on older macOS)
- Go to General and choose Software Update
- Install any pending updates that address the font parsing vulnerability
For best protection, enable automatic updates so new security patches install as soon as they’re released. Additionally, avoid opening documents or fonts from untrusted sources and be cautious of websites or apps that load fonts from unknown origins.
What to expect after patching
The fix closes the vulnerability in the Font Parser, preventing unauthorized writes to memory paths that could be exploited by attackers. After applying the update, devices should be resilient against attempts to crash the parser or corrupt process memory through crafted fonts, reducing the risk of follow-on compromises.
Best practices beyond the patch
Keeping your devices up to date is the first line of defense. Consider these steps as part of a broader security routine:
- Keep all apps and the OS current with the latest security updates
- Be cautious with documents or fonts from untrusted sources
- Use reputable apps and browsers that handle fonts securely
- Regularly back up data so you can recover quickly if an issue occurs
In short, act promptly on the available security update to mitigate the Apple font bug CVE-2025-43400, and maintain a security-first posture by applying future patches without delay.
