09. Input Output (FIO)

Input/Output is a broad topic and includes all the functions defined in C99 [[ISO/IEC 9899-1999]] Section 7.19, "Input/output <stdio.h>" and related functions.

The security of I/O operations is dependent on the versions of the C library, the operating system, and the file system. Older libraries are generally more susceptible to security flaws than newer library versions. Different operating systems have different capabilities and mechanisms for managing file privileges. There are numerous different file systems, including: File Allocation Table (FAT), FAT32, New Technology File System (NTFS), NetWare File System (NWFS), and the Unix File System (UFS). There are also many distributed file systems including: Andrew File System (AFS), Distributed File System (DFS), Microsoft DFS, and Network File System (NFS). These file systems vary in their capabilities and privilege mechanisms.

As a starting point, the I/O topic area describes the use of C99 standard functions. However, because these functions have been generalized to support multiple disparate operating and file systems, they cannot generally be used in a secure fashion. As a result, most of the rules and recommendations in this topic area recommend approaches that are specific to the operating system and file systems in use. Because of the inherent complexity, there may not exist compliant solutions for all operating system and file system combinations. Consequently, you must consider the target operating and file systems when evaluating the applicability of each compliant solution to your environment.

Recommendations

FIO00-A. Take care when creating format strings

FIO01-A. Be careful using functions that use file names for identification

FIO02-A. Canonicalize path names originating from untrusted sources

FIO03-A. Do not make assumptions about fopen() and file creation

FIO04-A. Detect and handle input and output errors

FIO05-A. Identify files using multiple file attributes

FIO06-A. Create files with appropriate access permissions

FIO07-A. Prefer fseek() to rewind()

FIO08-A. Take care when calling remove() on an open file

FIO09-A. Be careful with binary data when transferring data across systems

FIO10-A. Take care when using the rename() function

FIO11-A. Take care when specifying the mode parameter of fopen()

FIO12-A. Prefer setvbuf() to setbuf()

FIO13-A. Take care when using ungetc()

FIO14-A. Understand the difference between text mode and binary mode with file streams

FIO15-A. Do not create temporary files in shared directories

Rules

FIO30-C. Exclude user input from format strings

FIO31-C. Do not simultaneously open the same file multiple times

FIO32-C. Do not perform operations on devices that are only appropriate for files

FIO33-C. Detect and handle input output errors resulting in undefined behavior

FIO34-C. Use int to capture the return value of character IO functions

FIO35-C. Use feof() and ferror() to detect end-of-file and file errors when sizeof(int) == sizeof(char)

FIO36-C. Do not assume a newline character is read when using fgets()

FIO37-C. Don't assume character data has been read

FIO38-C. Do not use a copy of a FILE object for input and output

FIO39-C. Do not read in from a stream directly following output to that stream

FIO40-C. Reset strings on fgets() failure

FIO41-C. Do not call getc() or putc() with stream arguments that have side effects

FIO42-C. Ensure files are properly closed when they are no longer needed

FIO43-C. Do not copy data from an unbounded source to a fixed-length array

FIO44-C. Only use values for fsetpos() that are returned from fgetpos()

FIO45-C. Do not reopen a file stream

FIO46-C. Temporary files must be dealt with in a secure manner

Risk Assessment Summary

Related Rules and Recommendations

      08. Memory Management (MEM)       FIO00-A. Take care when creating format strings