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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 FIO01-C. Be careful using functions that use file names for identification.) In some cases, file operations can be performed securely anywhere. In other cases, the only way to ensure secure file operations is to perform the operation within a secure directory.

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Code Block

bgColor	#FFCCCC
lang	c

char *file_name;
FILE *fp;

/* Initialize file_name */

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

/* ... processProcess file ... */

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

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

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Code Block
% 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 sample implementation of the function secure_dir() ensures that the directory fullpath and all directories above it are owned by either the user or the superuser and that other users do not have write access to the directories. When checking directories, it is important to traverse from the root to the leaf to avoid a dangerous race condition in which an attacker who has privileges to at least one of the directories can rename and re-create a directory after the privilege verification.

fullpath need not be canonicalized (See see FIO02-C. Canonicalize path names originating from untrusted tainted 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.

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Code Block

bgColor	#ccccff
lang	c

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

enum { MAX_SYMLINKS = 5 };

/* Returns nonzero if directory is secure, zero otherwise */
int secure_dir(const char *fullpath) {
  static unsigned int num_symlinks = 0;
  char *path_copy = NULL;
  char **dirs = NULL;
  int num_of_dirs = 1;
  int secure = 1;
  int i, r;
  struct stat buf;
  uid_t my_uid = geteuid();
  size_t linksize;
  char* link;
   
  if (!fullpath || fullpath[0] != '/') {
    /* Handle error */
  }
   
  if (num_symlinks > MAX_SYMLINKS) {  //* Could be a symlink loop */
    /* Handle error */
  }
  
  if (!(path_copy = strdup(fullpath))) {
    /* Handle error */
  }
  
  //* Figure out how far it is to the root */
  char* path_parent = path_copy;
  for (; ((strcmp(path_copyparent, "/") != 0) &&
          (strcmp(path_copyparent, "//") != 0)); &&
       path_copy   (strcmp(path_parent, ".") != 0));
       path_parent = dirname(path_copyparent)) {
    num_of_dirs++;
  } //* nowNow num_of_dirs indicates # of dirs we must check */
  free(path_copy);
  path_copy = NULL;
  
  if (!(dirs = (char **)malloc(num_of_dirs * sizeof(char *)))) {
    /* Handle error */
  }
   
  if (!(dirs[num_of_dirs - 1] = strdup(fullpath))) {
    /* Handle error */
  }
  
  if (!(path_copy = strdup(fullpath))) {
    /* Handle error */
  }
  
  //* Now fill the dirs array array */
  path_parent = path_copy;
  for (i = num_of_dirs - 2; i >= 0; i--) {
    path_copyparent = dirname(path_copyparent);
    if (!(dirs[i] = strdup(path_copyparent))) {
      /* Handle error */
    }
  }
  free(path_copy);
  path_copy = NULL;
  
  //*
   * Traverse from the root to the fullpath,
  // * checking permissions along the way.
   */
 for (i = 0; i < num_of_dirs; i++) {
    if (lstat(dirs[i], &buf) != 0) {
      /* Handle error */
    }
     
    if (S_ISLNK(buf.st_mode)) { //* symlinkSymlink, 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)) { //* notNot a directory */
      secure = 0;
      break;
    }
     
    if ((buf.st_uid != my_uid) && (buf.st_uid != 0)) {
      //* Directory is owned by someone besides user or rootroot */
      secure = 0;
      break;
    }
     
    if (buf.st_mode & (S_IWGRP | S_IWOTH)) { //* dir is writable by others */
      secure = 0;
      break;
    }
  }
   
  for (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 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.

Code Block

bgColor	#ccccff
lang	c

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

/* Initialize dir_name */

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

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

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

/* ... processProcess file ... */

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

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

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Failing to perform file I/O operations in a secure directory that cannot otherwise be securely performed can result in a broad range of file system vulnerabilities.

Recommendation	Severity	Likelihood	Detectable

Remediation Cost

Repairable	Priority	Level
FIO15-C	Medium

medium

Probable

probable

No

high

No

P4

L3

Related Vulnerabilities

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

Related Guidelines

SEI CERT C++

Secure

Coding Standard	VOID FIO15-CPP. Ensure that file operations are performed in a secure directory
MITRE CWE	CWE-379, Creation of temporary file in directory with insecure permissions CWE-552, Files or directories accessible to external parties

Bibliography

[

Open Group 2004

IEEE Std 1003.1:2013] XSH, System Interfaces, dirname

()

XSH, System Interfaces, realpath

()


[Viega 2003]	Section 2.4, "Determining Whether a Directory Is Secure"

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Compliant Solution (POSIX)

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