Summary: A Critical Apple Font Bug Requires Immediate Patch
Security researchers and national CERT bodies have flagged a critical vulnerability in Apple’s font parsing system. The flaw, tracked as CVE-2025-43400, can be exploited by a malicious font embedded in web pages, documents, or software to crash a device or corrupt its memory. German authorities through BürgerCERT are urging users to install the security update as soon as possible to prevent denial-of-service-like crashes and other potential attacks. The patch applies to iPhones, iPads, and Macs via the appropriate operating-system updates.
What is the Apple font bug?
The defect lies in the Font Parser, the routine responsible for loading and rendering fonts so they display correctly in apps and on the web. When a specially crafted font is processed, the parser can be tricked into writing to memory areas it should not touch. This can cause the hosting process to crash or, in the worst case, enable further exploits. Because fonts are embedded in webpages, documents, and applications, successful exploitation can occur without user awareness—simply by opening a malicious document or visiting a compromised site.
Affected devices and affected versions
Apple has identified several operating-system versions that are susceptible until the security update is installed. These include:
- iOS on devices before 18.7.1 and before 26.0.1
- iPadOS on devices before 18.7.1 and before 26.0.1
- macOS Sequoia before 15.7.1
- macOS Sonoma before 14.8.1
- macOS Tahoe before 15.7.1
The fix is embedded in the latest security updates released by Apple. Users should update to the latest available version to close the vulnerability.
Why this matters: risk and impact
The vulnerability is not limited to a single app or file type. Since fonts can be loaded from websites, documents, and software, a single crafted font could trigger a crash or memory corruption across various scenarios. In practice, this means potential disruption of device use, and in some cases, the patch could prevent attackers from executing additional, non-specified attacks once the memory is compromised. BürgerCERT emphasizes the potential for repeated crashes or Denial-of-Service-like conditions if unpatched systems remain exposed.
What users should do now
Timely installation of the update is essential. Here are practical steps to protect devices:
- On iPhone and iPad: Open Settings, General, Software Update. If an update is available, download and install it. Prefer using a stable Wi‑Fi connection and ensure the device is plugged in or has sufficient battery life.
- On Mac: Open System Settings (or System Preferences on older macOS), then Software Update. Install all available security updates, restarting if prompted.
- Consider enabling automatic updates so future security fixes install without delay.
- Back up important data before applying updates, as a precaution against any unexpected issues during installation.
- Avoid opening documents or links from untrusted sources until devices are fully patched.
About CVE-2025-43400 and the CERT role
CVE numbers uniquely identify publicly disclosed security vulnerabilities. The CVE-2025-43400 entry confirms a documented weakness in the font parsing code. National CERTs, including BürgerCERT in Germany, monitor such findings and issue advisories to help users apply fixes quickly. The coordinated response—dissemination of guidance and push for updates—helps minimize broad, real-world impact and reduces risk exposure for individuals and organizations alike.
Longer-term security considerations
Beyond applying the patch, users and organizations should adopt general best practices: keep software up to date, rely on reputable sources for documents and media, and employ layered security measures such as web-content filtering and device management policies. Regular audits of installed apps and extensions can also reduce attack surfaces where malicious fonts could otherwise hide.
Bottom line
The Apple font bug CVE-2025-43400 is a clear reminder that trusted software components—like the font parser—deserve timely updates. An urgent security patch is available for iOS, iPadOS, and macOS, and BürgerCERT is urging users to install it promptly to prevent crashes and memory-related attacks. By updating now, users protect personal data, maintain system stability, and reduce the chance of disruption from attackers leveraging this vulnerability.
