A format string exploit occurs when the data of an input string is evaluated as a command by the program. This class of attacks is very similar to buffer overflows since an attacker could execute code, read the stack or cause new behaviors that compromise security. Learn more about format string attacks on OWASP attack index.
If format strings can be influenced by an attacker, they can be exploited.
The highlighted line of code below is the trigger point of this particular Fedora 24 format weakness.
- Daemon startup and status notification - Detection of systemd boots You may compile this with -DDISABLE_SYSTEMD to disable systemd support. This makes all those calls NOPs that are directly related to systemd (i.e. only sd_is_xxx() will stay useful). Since this is drop-in code we don't want any of our symbols to be exported in any case. Hence we declare hidden visibility for all of them. You may find an up-to-date version of these source files online: http://cgit.freedesktop.org/systemd/plain/src/sd-daemon.h http://cgit.freedesktop.org/systemd/plain/src/sd-daemon.c This should compile on non-Linux systems, too, but with the exception of the sd_is_xxx() calls all functions will become NOPs. See sd-daemon(7) for more information. */ #ifndef _sd_printf_attr_ #if __GNUC__ >= 4 #define _sd_printf_attr_(a,b) __attribute__ ((format (printf, a, b))) #else #define _sd_printf_attr_(a,b) #endif #endif #ifndef _sd_hidden_ #if (__GNUC__ >= 4) && !defined(SD_EXPORT_SYMBOLS) #define _sd_hidden_ __attribute__ ((visibility("hidden"))) #else #define _sd_hidden_ #endif #endif /* Log levels for usage on stderr: fprintf(stderr, SD_NOTICE "Hello World!\n"); This is similar to printk() usage in the kernel. */ #define SD_EMERG "<0>" /* system is unusable */ #define SD_ALERT "<1>" /* action must be taken immediately */ #define SD_CRIT "<2>" /* critical conditions */ #define SD_ERR "<3>" /* error conditions */ #define SD_WARNING "<4>" /* warning conditions */