CWE-20: Improper Input Validation

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:

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:

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.

DoS: Crash, Exit, or Restart; DoS: Resource Consumption (CPU); DoS: Resource Consumption (Memory)

Read Memory; Read Files or Directories

Modify Memory; Execute Unauthorized Code or Commands

Architecture and Design

Strategy: Attack Surface Reduction

Architecture and Design

Strategy: Libraries or Frameworks

Architecture and Design; Implementation

Strategy: Attack Surface Reduction

Strategy: Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.

When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue."

Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

Architecture and Design

For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602 . Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.

Even though client-side checks provide minimal benefits with respect to server-side security, they are still useful. First, they can support intrusion detection. If the server receives input that should have been rejected by the client, then it may be an indication of an attack. Second, client-side error-checking can provide helpful feedback to the user the expectations for valid input. Third, there may be a reduction in server-side processing time for accidental input errors, although this is typically a small savings.

Inputs should be decoded and canonicalized to the application's current internal representation before being validated ( CWE-180 , CWE-181 ). Make sure that your application does not inadvertently decode the same input twice ( CWE-174 ). Such errors could be used to bypass allowlist schemes by introducing dangerous inputs after they have been checked. Use libraries such as the OWASP ESAPI Canonicalization control.

Consider performing repeated canonicalization until your input does not change any more. This will avoid double-decoding and similar scenarios, but it might inadvertently modify inputs that are allowed to contain properly-encoded dangerous content.

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 ).

Class: Not Language-Specific (Often Prevalent)

AI/ML (Often Prevalent)

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.

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.

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

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.

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

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 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:

If this data were used in a SQL statement, it would treat the remainder of the statement as a . The could disable other security-related logic in the statement. In this case, encoding combined with input validation would be a more useful protection mechanism.

Furthermore, an XSS ( CWE-79 ) attack or SQL injection ( CWE-89 ) are just a few of the potential consequences when input validation is not used. Depending on the context of the code, CRLF Injection ( CWE-93 ), Argument Injection ( CWE-88 ), or Command Injection ( CWE-77 ) may also be possible.

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

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.

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

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.

Note: this is a curated list of examples for users to understand the variety of ways in which this weakness can be introduced. It is not a complete list of all CVEs that are related to this CWE entry.

Automated Static Analysis

Some instances of improper input validation can be detected using automated static analysis.

A static analysis tool might allow the user to specify which application-specific methods or functions perform input validation; the tool might also have built-in knowledge of validation frameworks such as Struts. The tool may then suppress or de-prioritize any associated warnings. This allows the analyst to focus on areas of the software in which input validation does not appear to be present.

Except in the cases described in the paragraph, automated static analysis might not be able to recognize when proper input validation is being performed, leading to false positives - i.e., warnings that do not have any security consequences or require any code changes.

Manual Static Analysis

Automated Static Analysis - Binary or Bytecode

According to SOAR [ REF-1479 ], the following detection techniques may be useful:

Effectiveness: SOAR Partial

Manual Static Analysis - Binary or Bytecode

According to SOAR [ REF-1479 ], the following detection techniques may be useful:

Effectiveness: SOAR Partial

Dynamic Analysis with Automated Results Interpretation

According to SOAR [ REF-1479 ], the following detection techniques may be useful:

Dynamic Analysis with Manual Results Interpretation

According to SOAR [ REF-1479 ], the following detection techniques may be useful:

Manual Static Analysis - Source Code

According to SOAR [ REF-1479 ], the following detection techniques may be useful:

Automated Static Analysis - Source Code

According to SOAR [ REF-1479 ], the following detection techniques may be useful:

Architecture or Design Review

According to SOAR [ REF-1479 ], the following detection techniques may be useful:

Within CWE, the "input validation" term focuses on the act of checking whether an input is already safe, which is different from other techniques that ensure safe processing of input. Carefully perform root-cause analysis to be sure that the issue is not due to techniques that attempt to transform potentially-dangerous input into something safe, such as filtering ( CWE-790 ) - which attempts to remove dangerous inputs - or encoding/escaping ( CWE-116 ), which attempts to ensure that the input is not misinterpreted when it is included in output to another component.

If the issue is truly due to imroper input validation, consider using lower-level children.

CWE-116 and CWE-20 have a close association because, depending on the nature of the structured message, proper input validation can indirectly prevent special characters from changing the meaning of a structured message. For example, by validating that a numeric ID field should only contain the 0-9 characters, the programmer effectively prevents injection attacks.

Multiple techniques exist to transform potentially dangerous input into something safe, which is different than "validation," which is a technique to check if an input is already safe. CWE users need to be cautious during root cause analysis to ensure that an issue is truly an input-validation problem.

The "input validation" term is extremely common, but it is used in many different ways. In some cases its usage can obscure the real underlying weakness or otherwise hide chaining and composite relationships.

Some people use "input validation" as a general term that covers many different neutralization techniques for ensuring that input is appropriate, such as filtering, i.e., attempting to remove dangerous inputs (related to CWE-790 ); encoding/escaping, i.e., attempting to ensure that the input is not misinterpreted when it is included in output to another component (related to CWE-116 ); or canonicalization, which often indirectly removes otherwise-dangerous inputs. Others use the term in a narrower context to simply mean "checking if an input conforms to expectations without changing it." CWE uses this narrow interpretation.

Note that "input validation" has very different meanings to different people, or within different classification schemes. Caution must be used when referencing this CWE entry or mapping to it. For example, some weaknesses might involve inadvertently giving control to an attacker over an input when they should not be able to provide an input at all, but sometimes this is referred to as input validation.

Finally, it is important to emphasize that the distinctions between input validation and output escaping are often blurred. Developers must be careful to understand the difference, including how input validation is not always sufficient to prevent vulnerabilities, especially when less stringent data types must be supported, such as free-form text. Consider a SQL injection scenario in which a person's last name is inserted into a query. The name "O'Reilly" would likely pass the validation step since it is a common last name in the English language. However, this valid name cannot be directly inserted into the database because it contains the "'" apostrophe character, which would need to be escaped or otherwise transformed. In this case, removing the apostrophe might reduce the risk of SQL injection, but it would produce incorrect behavior because the wrong name would be recorded.