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]] .
Non-Compliant Code Example
Compliant Code Example (POSIX)
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]].
On Linux, it is possible to lock certain applications by attempting to open devices rather than files, for example:
/dev/mouse /dev/console /dev/tty0 /dev/zero etc.
A web browser that failed to check for these devices would allow an attacker to create a website with image tags such as <IMG SRC=file:///dev/mouse that would lock the user's mouse.
>
Programmers can use the POSIX stat() function to obtain information about a named file, and the S_ISREG() macro to determine if the file is a regular file.
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]] Section 5.6, "Device Files"
[[Howard 02]] Chapter 11, "Canonical Representation Issues"
[[ISO/IEC 9899-1999]] Section
FIO45-C. Do not reopen a file stream 09. Input Output (FIO) FIO45-C. Do not reopen a file stream