External Control of Critical State Data

The software stores security-critical state information about its users, or the software itself, in a location that is accessible to unauthorized actors.


Description

If an attacker can modify the state information without detection, then it could be used to perform unauthorized actions or access unexpected resources, since the application programmer does not expect that the state can be changed.

State information can be stored in various locations such as a cookie, in a hidden web form field, input parameter or argument, an environment variable, a database record, within a settings file, etc. All of these locations have the potential to be modified by an attacker. When this state information is used to control security or determine resource usage, then it may create a vulnerability. For example, an application may perform authentication, then save the state in an "authenticated=true" cookie. An attacker may simply create this cookie in order to bypass the authentication.

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

In the following example, an authentication flag is read from a browser cookie, thus allowing for external control of user state data.

Cookie[] cookies = request.getCookies();
for (int i =0; i< cookies.length; i++) {
  Cookie c = cookies[i];
  if (c.getName().equals("authenticated") && Boolean.TRUE.equals(c.getValue())) {
    authenticated = true;
  }
}

Example Two

The following code uses input from an HTTP request to create a file name. The programmer has not considered the possibility that an attacker could provide a file name such as "../../tomcat/conf/server.xml", which causes the application to delete one of its own configuration files (CWE-22).

String rName = request.getParameter("reportName");
File rFile = new File("/usr/local/apfr/reports/" + rName);
...
rFile.delete();

Example Three

The following code uses input from a configuration file to determine which file to open and echo back to the user. If the program runs with privileges and malicious users can change the configuration file, they can use the program to read any file on the system that ends with the extension .txt.

fis = new FileInputStream(cfg.getProperty("sub")+".txt");
amt = fis.read(arr);
out.println(arr);

Example Four

This program is intended to execute a command that lists the contents of a restricted directory, then performs other actions. Assume that it runs with setuid privileges in order to bypass the permissions check by the operating system.

#define DIR "/restricted/directory"

char cmd[500];
sprintf(cmd, "ls -l %480s", DIR);
/* Raise privileges to those needed for accessing DIR. */

RaisePrivileges(...);
system(cmd);
DropPrivileges(...);
...

This code may look harmless at first, since both the directory and the command are set to fixed values that the attacker can't control. The attacker can only see the contents for DIR, which is the intended program behavior. Finally, the programmer is also careful to limit the code that executes with raised privileges.

However, because the program does not modify the PATH environment variable, the following attack would work:

The user sets the PATH to reference a directory under that user's control, such as "/my/dir/".

The user creates a malicious program called "ls", and puts that program in /my/dir

The user executes the program.

When system() is executed, the shell consults the PATH to find the ls program

The program finds the malicious program, "/my/dir/ls". It doesn't find "/bin/ls" because PATH does not contain "/bin/".

The program executes the malicious program with the raised privileges.

Example Five

This code prints all of the running processes belonging to the current user.

//assume getCurrentUser() returns a username that is guaranteed to be alphanumeric (CWE-78)
$userName = getCurrentUser();
$command = 'ps aux | grep ' . $userName;
system($command);

This program is also vulnerable to a PATH based attack (CWE-426), as an attacker may be able to create malicious versions of the ps or grep commands. While the program does not explicitly raise privileges to run the system commands, the PHP interpreter may by default be running with higher privileges than users.

Example Six

The following code segment implements a basic server that uses the "ls" program to perform a directory listing of the directory that is listed in the "HOMEDIR" environment variable. The code intends to allow the user to specify an alternate "LANG" environment variable. This causes "ls" to customize its output based on a given language, which is an important capability when supporting internationalization.

$ENV{"HOMEDIR"} = "/home/mydir/public/";
my $stream = AcceptUntrustedInputStream();
while (<$stream>) {

  chomp;
  if (/^ENV ([\w\_]+) (.*)/) {
    $ENV{$1} = $2;
  }
  elsif (/^QUIT/) { ... }
  elsif (/^LIST/) {
    open($fh, "/bin/ls -l $ENV{HOMEDIR}|");
    while (<$fh>) {
      SendOutput($stream, "FILEINFO: $_");
    }
    close($fh);
  }

}

The programmer takes care to call a specific "ls" program and sets the HOMEDIR to a fixed value. However, an attacker can use a command such as "ENV HOMEDIR /secret/directory" to specify an alternate directory, enabling a path traversal attack (CWE-22). At the same time, other attacks are enabled as well, such as OS command injection (CWE-78) by setting HOMEDIR to a value such as "/tmp; rm -rf /". In this case, the programmer never intends for HOMEDIR to be modified, so input validation for HOMEDIR is not the solution. A partial solution would be an allowlist that only allows the LANG variable to be specified in the ENV command. Alternately, assuming this is an authenticated user, the language could be stored in a local file so that no ENV command at all would be needed.

While this example may not appear realistic, this type of problem shows up in code fairly frequently. See CVE-1999-0073 in the observed examples for a real-world example with similar behaviors.

See Also

Authorize Actors

Weaknesses in this category are related to the design and architecture of a system's authorization components. Frequently these deal with enforcing that agents have th...

SFP Secondary Cluster: Exposed Data

This category identifies Software Fault Patterns (SFPs) within the Exposed Data cluster (SFP23).

Comprehensive CWE Dictionary

This view (slice) covers all the elements in CWE.

CWE Cross-section

This view contains a selection of weaknesses that represent the variety of weaknesses that are captured in CWE, at a level of abstraction that is likely to be useful t...

Weaknesses Introduced During Implementation

This view (slice) lists weaknesses that can be introduced during implementation.


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