CWE-20: Improper Input Validation

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Description

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.

Extended Description

Input validation is a frequently-used technique for checking potentially dangerous inputs in order to ensure that the inputs are safe for processing within the code, or when communicating with other components. Input can consist of: raw data - strings, numbers, parameters, file contents, etc. metadata - information about the raw data, such as headers or size Data can be simple or structured. Structured data can be composed of many nested layers, composed of combinations of metadata and raw data, with other simple or structured data. Many properties of raw data or metadata may need to be validated upon entry into the code, such as: specified quantities such as size, length, frequency, price, rate, number of operations, time, etc. implied or derived quantities, such as the actual size of a file instead of a specified size indexes, offsets, or positions into more complex data structures symbolic keys or other elements into hash tables, associative arrays, etc. well-formedness, i.e. syntactic correctness - compliance with expected syntax lexical token correctness - compliance with rules for what is treated as a token specified or derived type - the actual type of the input (or what the input appears to be) consistency - between individual data elements, between raw data and metadata, between references, etc. conformance to domain-specific rules, e.g. business logic equivalence - ensuring that equivalent inputs are treated the same authenticity, ownership, or other attestations about the input, e.g. a cryptographic signature to prove the source of the data Implied or derived properties of data must often be calculated or inferred by the code itself. Errors in deriving properties may be considered a contributing factor to improper input validation.

ThreatScore

Threat Mapped score: 1.8

Industry: Finiancial

Threat priority: P4 - Informational (Low)

Observed Examples (CVEs)

Related Attack Patterns (CAPEC)

Attack TTPs

Malware

APTs (Intrusion Sets)

Modes of Introduction

Phase Note
Architecture and Design N/A
Implementation REALIZATION: This weakness is caused during implementation of an architectural security tactic. If a programmer believes that an attacker cannot modify certain inputs, then the programmer might not perform any input validation at all. For example, in web applications, many programmers believe that cookies and hidden form fields can not be modified from a web browser (CWE-472), although they can be altered using a proxy or a custom program. In a client-server architecture, the programmer might assume that client-side security checks cannot be bypassed, even when a custom client could be written that skips those checks (CWE-602).

Common Consequences

Potential Mitigations

Applicable Platforms

Demonstrative Examples

Intro: This example demonstrates a shopping interaction in which the user is free to specify the quantity of items to be purchased and a total is calculated.

Body: The user has no control over the price variable, however the code does not prevent a negative value from being specified for quantity. If an attacker were to provide a negative value, then the user would have their account credited instead of debited.

... public static final double price = 20.00; int quantity = currentUser.getAttribute("quantity"); double total = price * quantity; chargeUser(total); ...

Intro: This example asks the user for a height and width of an m X n game board with a maximum dimension of 100 squares.

Body: While this code checks to make sure the user cannot specify large, positive integers and consume too much memory, it does not check for negative values supplied by the user. As a result, an attacker can perform a resource consumption (CWE-400) attack against this program by specifying two, large negative values that will not overflow, resulting in a very large memory allocation (CWE-789) and possibly a system crash. Alternatively, an attacker can provide very large negative values which will cause an integer overflow (CWE-190) and unexpected behavior will follow depending on how the values are treated in the remainder of the program.

... #define MAX_DIM 100 ... /* board dimensions */ int m,n, error; board_square_t *board; printf("Please specify the board height: \n"); error = scanf("%d", &m); if ( EOF == error ){ die("No integer passed: Die evil hacker!\n"); } printf("Please specify the board width: \n"); error = scanf("%d", &n); if ( EOF == error ){ die("No integer passed: Die evil hacker!\n"); } if ( m > MAX_DIM || n > MAX_DIM ) { die("Value too large: Die evil hacker!\n"); } board = (board_square_t*) malloc( m * n * sizeof(board_square_t)); ...

Intro: The following example shows a PHP application in which the programmer attempts to display a user's birthday and homepage.

Body: The programmer intended for $birthday to be in a date format and $homepage to be a valid URL. However, since the values are derived from an HTTP request, if an attacker can trick a victim into clicking a crafted URL with <script> tags providing the values for birthday and / or homepage, then the script will run on the client's browser when the web server echoes the content. Notice that even if the programmer were to defend the $birthday variable by restricting input to integers and dashes, it would still be possible for an attacker to provide a string of the form:

$birthday = $_GET['birthday']; $homepage = $_GET['homepage']; echo "Birthday: $birthday<br>Homepage: <a href=$homepage>click here</a>"

Intro: The following example takes a user-supplied value to allocate an array of objects and then operates on the array.

Body: This example attempts to build a list from a user-specified value, and even checks to ensure a non-negative value is supplied. If, however, a 0 value is provided, the code will build an array of size 0 and then try to store a new Widget in the first location, causing an exception to be thrown.

private void buildList ( int untrustedListSize ){ if ( 0 > untrustedListSize ){ die("Negative value supplied for list size, die evil hacker!"); } Widget[] list = new Widget [ untrustedListSize ]; list[0] = new Widget(); }

Intro: This Android application has registered to handle a URL when sent an intent:

Body: The application assumes the URL will always be included in the intent. When the URL is not present, the call to getStringExtra() will return null, thus causing a null pointer exception when length() is called.

... IntentFilter filter = new IntentFilter("com.example.URLHandler.openURL"); MyReceiver receiver = new MyReceiver(); registerReceiver(receiver, filter); ... public class UrlHandlerReceiver extends BroadcastReceiver { @Override public void onReceive(Context context, Intent intent) { if("com.example.URLHandler.openURL".equals(intent.getAction())) { String URL = intent.getStringExtra("URLToOpen"); int length = URL.length(); ... } } }

Notes

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