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According to section subclause 5.2.1 of the C Standard [ISO/IEC 9899:2011],

Two sets of characters and their associated collating sequences shall be defined: the set in which source files are written (the source character set), and the set interpreted in the execution environment (the execution character set). Each set is further divided into a basic character set, whose contents are given by this subclause, and a set of zero or more locale-specific members (which are not members of the basic character set) called extended characters. The combined set is also called the extended character set. The values of the members of the execution character set are implementation-defined.

There are several national variants of ASCII. As a result, the original ASCII is often called US-ASCII. ISO/IEC 646-1991 defines a character set, similar to US-ASCII, but with code positions corresponding to US-ASCII characters @[]{|} as national use positions [ISO/IEC 646-1991]. It also gives some liberties with the characters #$^`~. In particular characters (e.g., #$^`~).  In ISO/IEC 646-1991, several national variants of ASCII are defined, assigning different letters and symbols to the national use positions. Consequently, the characters that appear in those positions, including those in US-ASCII, are less portable in international data transfer. Because of the national variants, some characters are less portable than others: they might be transferred or interpreted incorrectly.

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File names containing particular characters can be troublesome and can cause unexpected behavior leading to potential vulnerabilities. If a program allows the user to specify a file name in the creation or renaming of a file, certain checks should be made to disallow the following characters and patterns:

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Noncompliant Code Example (File Name 1)

In the following this noncompliant code example, unsafe characters are used as part of a file name.:

#include <fcntl.h>
#include <sys/stat.h>

int main(void) {
   char *file_name = "&#xBB;&#xA3;???&#xAB;\xe5ngstr\xf6m";
   mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;

   int fd = open(file_name, O_CREAT | O_EXCL | O_WRONLY, mode);
   if (fd == -1) {
      /* Handle error */
   }
}

An implementation is free to define its own mapping of the "nonsafe" characters. For example, when tested run on a Red Hat Enterprise Linux distribution7.5, this noncompliant code example resulted in the following file name being revealed by the ls command:

?ngstr?????m

Compliant Solution (File Name 1)

Use a descriptive file name containing only the subset of ASCII previously described.:

#include <fcntl.h>
#include <sys/stat.h>

int main(void) {
   char *file_name = "name.ext";
   mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;

   int fd = open(file_name, O_CREAT | O_EXCL | O_WRONLY, mode);
   if (fd == -1) {
      /* Handle error */
   }
}

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This noncompliant code example is derived from FIO30-C. Exclude user input from format strings, except that a newline is removed on the assumption that fgets() will include it.:

char myFilename[1000];
const char elimNewLn[] = "\n";

fgets(myFilename, sizeof(myFilename)-1, stdin);
myFilename[sizeof(myFilename)-1] = '\0';
myFilename[strcspn(myFilename, elimNewLn)] = '\0';

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In this compliant solution, the program rejects file names that violate the guidelines for selecting safe characters.:

char myFilename[1000];
const char elimNewln[] = "\n";
const char badChars[] = "-\n\r ,;'\\<\"";
do {
  fgets(myFilename, sizeof(myFilename)-1, stdin);
  myFilename[sizeof(myFilename)-1] ='\0';
  myFilename[strcspn(myFilename, elimNewln)]='\0';
} while ( (strcspn(myFilename, badChars))
           < (strlen(myFilename)));

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Failing to use only the subset of ASCII that is guaranteed to work can result in misinterpreted data.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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