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Perform file File operations should be performed in a secure directory. In most cases, a secure directory is a directory in which no one other than the user, or possibly the administrator, has the ability to write, create, moverename, delete files , or otherwise manipulate files. (Other users may read or search the directory , but generally may not modify the directory's contents in any way.) Also, other users must not be able to delete or rename files in the parent of the secure directory and all higher directories, although creating new files or deleting or renaming files they own is permissible.

Performing file operations in a secure directory eliminates the possibility that an attacker might tamper with the files or file system to exploit a file system vulnerability in a program. These vulnerabilities often exist because there is a loose binding between the file name and the actual file. (see See FIO01-AC. Be careful using functions that use file names for identification.) . In some cases, file operations can be performed securely (and should be)anywhere. In other cases, the only way to ensure secure file operations is to perform the operation within a secure directory.

Ensuring that file systems are configured in a safe manner is typically a system administration function. However, programs can often check that a file system is securely configured before performing file operations that may potentially lead to security vulnerabilities if the system is misconfigured. There is a slight possibility that file systems will be reconfigured in an insecure manner while a process is running and after the check has been made. As a result, it is always advisable to implement your code in a secure manner (that is, consistent with the other rules and recommendations in this section) even when running in a secure directory.

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Noncompliant Code Example

In this non-compliant noncompliant code example, the file identified by file_name is opened, processed, closed, and removed. :

char *file_name;
FILE *fp;

/* initializeInitialize file_name */

fp = fopen(file_name, "w");
if (fp == NULL) {
  /* Handle error */
}

/* ... Process file ... */

if (fclose(fp) != 0) {
  /* Handle error */
}

if (remove(file_name) != 0) {
  /* Handle error */
}

An attacker can replace the file object identified by file_name with a link to an arbitrary file before the call to fopen(). It is also possible that the file object identified by file_name in the call to remove() is not the same file object identified by file_name in the call to fopen(). If the file is not in a secure directory, for example, /tmp/app/tmpdir/passwd, then an attacker can manipulate the location of the file as follows:

% cd /tmp/app/ 
% rm -rf tmpdir
% ln -s /etc tmpdir

There is not Not much that can be done programmatically done to ensure the file removed is the same file that was opened, processed, and closed, except to make sure that the file is opened in a secure directory with privileges that would prevent the file from being manipulated by an untrusted user.

Compliant Solution (POSIX)

This example sample implementation of a the function secure_dir() function will ensure that path ensures that the directory fullpath and all directories above it are owned by either by the user or the superuser , and may not be modified by any other usersand that other users do not have write access to the directories. When checking directories, it is important to traverse from the root to the top in order leaf to avoid a dangerous race condition where in which an attacker who has privileges to at least one of the directories can delete rename and recreate re-create a directory after the privilege verification.

This function uses the realpath() function to canonicalize the input path...fullpath need not be canonicalized (see FIO02-AC. Canonicalize path names originating from untrusted sources for more information on realpath(). It then checks every directory in the canonical path, ensuring that every directory is owned by the current user or by root. Furthermore, every directory must disallow write access to everyone but the owner, either by turning off group write access and world write access, or by turning on the sticky bittainted sources). If the path contains a symbolic link, this routine recursively invokes itself on the linked-to directory and ensures it is also secure. A symbolically linked directory may be secure if both its source and linked-to directory are secure.

Note that this function is effective only on file systems that are fully compatible with UNIX permissions, and it may not behave normally for file systems with other permission mechanisms, such as AFS (Andrew File System).

#include <stdlib.h>
#include <unistd<limits.h>
#include <limits<string.h>
#include <libgen.h>
#include <unistd.h>
#include <sys/stat.h>
#include <string<sys/types.h>

enum { MAX_SYMLINKS = 5 };

/* Returns nonzero if directory is secure, zero otherwise */
int secure_dir(const char * pathfullpath) {
  charstatic *fullpathunsigned = NULL;
  char *path_copyint num_symlinks = NULL0;
  char *dirnamepath_rescopy = NULL;
  char ** dirs = NULL;
  int num_of_dirs = 01;
  int secure = 1;
  int i, r;
  struct stat buf;
  uid_t my_uid = geteuid();
  size_t linksize;
  /char* link;
   *
 Canonicalize pathif and store the result in fullpath
(!fullpath || fullpath[0] != '/') {
    /* Handle see FIO02-A. Canonicalize path names originating from
   * untrusted sources
   */
error */
  }
   
  if (num_symlinks > MAX_SYMLINKS) {  /* Could be a symlink loop */
    /* Handle error */
  }
  
  if (!(path_copy = strdup(fullpath))) {
    /* Handle error */
  }

  dirname_res = path_copy;
  /* Figure out how far it is to the root */
  while (1) {char* path_parent = path_copy;
  for  dirname_res = dirname(dirname_res);

(; ((strcmp(path_parent, "/") != 0) &&
     num_of_dirs++;

    if ((strcmp(dirnamepath_resparent, ".//") !== 0) ||
	&&
          (strcmp(dirnamepath_resparent, "/.") !== 0));
 {
      break;path_parent = dirname(path_parent)) {
    }num_of_dirs++;
  } /* Now num_of_dirs indicates # of dirs we must check */
  free(path_copy);
  path_copy = NULL;

  /* Now allocate and fill the dirs array */
  if (!(dirs = (char **)malloc(num_of_dirs * sizeof(char *dirs)))) {
    /* Handle error */
  }
   
  if (!(dirs[num_of_dirs - 1] = strdup(fullpath))) {
    /* Handle error */
  }
  
  if (!(path_copy = strdup(fullpath))) {
    /* Handle error */
  }
  
  dirname_res/* Now fill the dirs array */
  path_parent = path_copy;
  for (i = 1 num_of_dirs - 2; i < num_of_dirs>= 0; i++--) {
    dirnamepath_resparent = dirname(dirnamepath_resparent);

    if (!(dirs[num_of_dirs - i - 1] = strdup(dirnamepath_res);
parent))) {
      /* Handle error */
    }
  }
  free(path_copy);
  path_copy = NULL;
  
  /*
   * Traverse from the root to the topfullpath, checking
   * checking permissions along the way.
   */
  for (i = 0; i < num_of_dirs; i++) {
    if (statlstat(dirs[i], &buf) != 0) {
      /* Handle error */
    }
     
    if (S_ISLNK(buf.st_mode)) { /* Symlink, test linked-to file */
      linksize = buf.st_size + 1;
      if (!(link = (char *)malloc(linksize))) {
        /* Handle error */
      }
       
      r = readlink(dirs[i], link, linksize);
      if (r == -1) {
        /* Handle error */
      } else if (r >= linksize) {
        /* Handle truncation error */
      }
      link[r] = '\0';
 
      num_symlinks++;
      r = secure_dir(link);
      num_symlinks--;
       
      if (!r) {
        secure = 0;
 
        free(link);
        link = NULL;
        break;
      }
       
      free(link);
      link = NULL;
       
      continue;
    }
     
    if (!S_ISDIR(buf.st_mode)) { /* Not a directory */
      secure = 0;
      break;
    }
     
    if ((buf.st_uid != my_uid) && (buf.st_uid != 0)) {
      /* Directory is owned by someone besides user or root */
      secure = 0;
    } else if (!(buf.st_mode & S_ISVTX) &&
	  break;
    }
     
    if (buf.st_mode & (S_IWGRP | S_IWOTH))) { /* dir is writable by others */
      secure = 0;
  /* Others have permission tobreak;
 rename or remove files}
 here */}
   
  for secure(i = 0;
 i <  }num_of_dirs; i++) {
    free(dirs[i]);
    dirs[i] = NULL;
  }
  
  free(dirs);
  dirs = NULL;
  
  return secure;
}

This compliant solution uses the secure_dir() function above to ensure that an attacker may not tamper with the file to be opened and subsequently removed. Note that once the path name of a directory has been checked using secure_dir(), all further file operations on that directory must be performed using the same path.

char *dir_name;
const char *file_name = "passwd"; /* File name within the secure directory */
FILE *fp;

/* initializeInitialize filedir_name */

if (!secure_dir(filedir_name)) {
  /* Handle error */
}

if (chdir(dir_name) == -1) {
  /* Handle error */
}

fp = fopen(file_name, "w");
if (fp == NULL) {
  /* Handle error */
}

/* ... Process file ... */

if (fclose(fp) != 0) {
  /* Handle error */
}

if (remove(file_name) != 0) {
  /* Handle error */
}

Risk Assessment

Failing to ensure proper permissions perform file I/O operations in a directory may lead to sensitive data getting saved to (or critical configuration or other input files being read from) public directories to which an attacker has accesssecure directory that cannot otherwise be securely performed can result in a broad range of file system vulnerabilities.

Related Vulnerabilities

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

References

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]
\[[Open Group 04|AA. C References#Open Group 04]\] [{{dirname()}}|http://www.opengroup.org/onlinepubs/009695399/functions/dirname.html], [{{realpath()}}|http://www.opengroup.org/onlinepubs/009695399/functions/realpath.html]
\[[Viega 03|AA. C References#Viega 03]\] Section 2.4, "Determining Whether a Directory Is Secure"

Related Guidelines

Bibliography

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Image Added Image Added Image AddedFIO14-A. Understand the difference between text mode and binary mode with file streams      09. Input Output (FIO)       FIO16-A. Limit access to files by creating a jail