fontconfig before 2.17.1 has an off-by-one error in allocation during sfnt capability handling, leading to a one-byte out-of-bounds write, and potentially a crash or code execution. This is in FcFontCapabilities in fcfreetype.c.
fontconfig before 2.12.1 does not validate offsets, which allows local users to trigger arbitrary free calls and consequently conduct double free attacks and execute arbitrary code via a crafted cache file.