Filenames on various operating systems including Windows and UNIX may be used to access "special" files which are actually devices. Sample reserved MS-DOS device names include AUX, CON, PRN, COM1, and LPT1. Device files on UNIX systems are used to apply access rights and to direct operations on the files to the appropriate device drivers.

Performing operations on device files which are intended for ordinary character or binary files can result in crashes and denial-of-service attacks.  For example, when Windows attempts to interpret the device name as a file resource, it performs an illegal resource access that usually results in a crash \[[Howard 02|AA. C References#Howard 02]\] .

Non-Compliant Code Example




Compliant Code Example (UNIX)



Device files in UNIX can be a major security hazard when an attacker is able to access them in an unauthorized way. For instance, if attackers can read or write to the {{/dev/kmem}} device, they may be able to alter their priority, UID, or other attributes of their process or simply crash the system. Similarly, access to disk devices, tape devices, network devices, and terminals being used by others all can lead to problems \[[Garfinkel 96|AA. C References#Garfinkel 96]\].



struct stat s;

if (stat(filename, &s) == 0) {
  if (S_ISREG (s.st_mode)) {
    /* operate on file */
  }
}






Compliant Solution (Windows)



In the compliant solution, the log file is only opened once upon program startup, and is closed upon program termination. The log_message() function only writes the message to the already opened file.


HANDLE hFile = CreateFile(pFullPathName,
  0, 0, NULL, OPEN_EXISTING, 0, NULL
);
if (hFile == INVALID_HANDLE_VALUE) {
  /* handle error */
}
if (GetFileType(hFile) != FILE_TYPE_DISK) {
  /* handle error */
}




Risk Assessment







 Rule 


 Severity 


 Likelihood 


 Remediation Cost 


 Priority 


 Level 






 FIO46-C 


 2 (medium) 


 1 (unlikely) 


 2 (medium) 


 P4 


 L3 








Related Vulnerabilities



Search for vulnerabilities resulting from the violation of this rule on the CERT website.



References



\[[Garfinkel 96|AA. C References#Garfinkel 96]\] Section 5.6, "Device Files"
\[[Howard 02|AA. C References#Howard 02]\] Chapter 11, "Canonical Representation Issues"
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section