CVE-2025-39967: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-39967 was disclosed on October 15, 2025, affecting the Linux kernel's framebuffer console (fbcon) subsystem. The vulnerability involves integer overflow issues in the fbcon_do_set_font() function when handling user-controlled font parameters (NVD).

The vulnerability stems from multiple integer overflow conditions in the fbcon_do_set_font() function. Specifically, there are two key overflow points: 1) The CALC_FONTSZ(h, pitch, charcount) calculation where multiplication of height, pitch, and character count with user-controlled values can overflow, and 2) The FONT_EXTRA_WORDS * sizeof(int) + size addition operation which can also overflow. These overflows result in smaller memory allocations than intended, potentially leading to buffer overflows during font data copying operations (NVD).

When successfully exploited, this vulnerability can lead to buffer overflows during font data copying operations due to insufficient memory allocation. This could potentially result in memory corruption and system instability (NVD).

The vulnerability requires the ability to manipulate font parameters in the Linux kernel's framebuffer console subsystem. The exploitation involves providing specially crafted values for height, pitch, and character count parameters to trigger the integer overflow conditions (NVD).

The vulnerability has been patched by implementing explicit overflow checking using check_mul_overflow() and check_add_overflow() kernel helper functions to validate all size calculations before memory allocation (NVD). Ubuntu has released fixes for version 6.17.0-6.6 in the 25.10 (questing) release (Ubuntu).