Incorrect Permission Assignment for Critical Resource

Description

When a resource is given a permission setting that provides access to a wider range of actors than required, it could lead to the exposure of sensitive information, or the modification of that resource by unintended parties. This is especially dangerous when the resource is related to program configuration, execution, or sensitive user data. For example, consider a misconfigured storage account for the cloud that can be read or written by a public or anonymous user.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following code sets the umask of the process to 0 before creating a file and writing "Hello world" into the file.

#define OUTFILE "hello.out"

umask(0);
FILE *out;
/* Ignore link following (CWE-59) for brevity */

out = fopen(OUTFILE, "w");
if (out) {
  fprintf(out, "hello world!\n");
  fclose(out);
}

After running this program on a UNIX system, running the "ls -l" command might return the following output:

-rw-rw-rw- 1 username 13 Nov 24 17:58 hello.out

The "rw-rw-rw-" string indicates that the owner, group, and world (all users) can read the file and write to it.

Example Two

This code creates a home directory for a new user, and makes that user the owner of the directory. If the new directory cannot be owned by the user, the directory is deleted.

function createUserDir($username){
  $path = '/home/'.$username;
  if(!mkdir($path)){
    return false;
  }
  if(!chown($path,$username)){
    rmdir($path);
    return false;
  }
  return true;
}

Because the optional "mode" argument is omitted from the call to mkdir(), the directory is created with the default permissions 0777. Simply setting the new user as the owner of the directory does not explicitly change the permissions of the directory, leaving it with the default. This default allows any user to read and write to the directory, allowing an attack on the user's files. The code also fails to change the owner group of the directory, which may result in access by unexpected groups.

This code may also be vulnerable to Path Traversal (CWE-22) attacks if an attacker supplies a non alphanumeric username.

Example Three

The following code snippet might be used as a monitor to periodically record whether a web site is alive. To ensure that the file can always be modified, the code uses chmod() to make the file world-writable.

$fileName = "secretFile.out";

if (-e $fileName) {
  chmod 0777, $fileName;
}

my $outFH;
if (! open($outFH, ">>$fileName")) {
  ExitError("Couldn't append to $fileName: $!");
}
my $dateString = FormatCurrentTime();
my $status = IsHostAlive("cwe.mitre.org");
print $outFH "$dateString cwe status: $status!\n";
close($outFH);

The first time the program runs, it might create a new file that inherits the permissions from its environment. A file listing might look like:

-rw-r--r-- 1 username 13 Nov 24 17:58 secretFile.out

This listing might occur when the user has a default umask of 022, which is a common setting. Depending on the nature of the file, the user might not have intended to make it readable by everyone on the system.

The next time the program runs, however - and all subsequent executions - the chmod will set the file's permissions so that the owner, group, and world (all users) can read the file and write to it:

-rw-rw-rw- 1 username 13 Nov 24 17:58 secretFile.out

Perhaps the programmer tried to do this because a different process uses different permissions that might prevent the file from being updated.

Example Four

This program creates and reads from an admin file to determine privilege information.

If the admin file doesn't exist, the program will create one. In order to create the file, the program must have write privileges to write to the file. After the file is created, the permissions need to be changed to read only.

const adminFile = "/etc/admin-users"
func createAdminFileIfNotExists() error {
  file, err := os.Create(adminFile)
  if err != nil {

    return err
  }
  return nil
}


func changeModeOfAdminFile() error {
  fileMode := os.FileMode(0440)
  if err := os.Chmod(adminFile, fileMode); err != nil {

    return err
  }
  return nil
}

os.Create will create a file with 0666 permissions before umask if the specified file does not exist. A typical umask of 0022 would result in the file having 0644 permissions. That is, the file would have world-writable and world-readable permissions.

In this scenario, it is advised to use the more customizable method of os.OpenFile with the os.O_WRONLY and os.O_CREATE flags specifying 0640 permissions to create the admin file.

This is because on a typical system where the umask is 0022, the perm 0640 applied in os.OpenFile will result in a file of 0620 where only the owner and group can write.

Example Five

The following command recursively sets world-readable permissions for a directory and all of its children:

chmod -R ugo+r DIRNAME

If this command is run from a program, the person calling the program might not expect that all the files under the directory will be world-readable. If the directory is expected to contain private data, this could become a security problem.

Example Six

The following Azure command updates the settings for a storage account:

az storage account update --name <storage-account> --resource-group <resource-group> --allow-blob-public-access true

However, "Allow Blob Public Access" is set to true, meaning that anonymous/public users can access blobs.

The command could be modified to disable "Allow Blob Public Access" by setting it to false.

az storage account update --name <storage-account> --resource-group <resource-group> --allow-blob-public-access false

Example Seven

The following Google Cloud Storage command gets the settings for a storage account named 'BUCKET_NAME':

gsutil iam get gs://BUCKET_NAME

Suppose the command returns the following result:

{

  "bindings":[{

    "members":[

      "projectEditor: PROJECT-ID",
      "projectOwner: PROJECT-ID"
    ],
    "role":"roles/storage.legacyBucketOwner"

  },
  {

    "members":[
      "allUsers",
      "projectViewer: PROJECT-ID"
      ],
      "role":"roles/storage.legacyBucketReader"

    }

  ]

}

This result includes the "allUsers" or IAM role added as members, causing this policy configuration to allow public access to cloud storage resources. There would be a similar concern if "allAuthenticatedUsers" was present.

The command could be modified to remove "allUsers" and/or "allAuthenticatedUsers" as follows:

gsutil iam ch -d allUsers gs://BUCKET_NAME
gsutil iam ch -d allAuthenticatedUsers gs://BUCKET_NAME

See Also