CWE-20: Improper Input Validation

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. When software does not validate input properly, an attacker is able to craft the input in a form that is not expected by the rest of the application. This will lead to parts of the system receiving unintended input, which may result in altered control flow, arbitrary control of a resource, or arbitrary code execution.

Input validation is not the only technique for processing input, however. Other techniques 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. Other techniques exist as well (see CWE-138 for more examples.)

Input validation can be applied to:

• 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.

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, and 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.

Related Weaknesses

Modes of Introduction

Applicable Platforms

Languages

Class: Not Language-Specific

Technologies

Likelihood Of Exploit

High

Common Consequences

Detection Methods

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 previous 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

When custom input validation is required, such as when enforcing business rules, manual analysis is necessary to ensure that the validation is properly implemented.

Fuzzing

Fuzzing techniques can be useful for detecting input validation errors. When unexpected inputs are provided to the software, the software should not crash or otherwise become unstable, and it should generate application-controlled error messages. If exceptions or interpreter-generated error messages occur, this indicates that the input was not detected and handled within the application logic itself.

Automated Static Analysis - Binary or Bytecode

According to SOAR, the following detection techniques may be useful:

Cost effective for partial coverage:

• Bytecode Weakness Analysis - including disassembler + source code weakness analysis
• Binary Weakness Analysis - including disassembler + source code weakness analysis

Effectiveness: SOAR Partial

Manual Static Analysis - Binary or Bytecode

According to SOAR, the following detection techniques may be useful:

Cost effective for partial coverage:

• Binary / Bytecode disassembler - then use manual analysis for vulnerabilities & anomalies

Effectiveness: SOAR Partial

Dynamic Analysis with Automated Results Interpretation

According to SOAR, the following detection techniques may be useful:

Highly cost effective:

• Web Application Scanner
• Web Services Scanner
• Database Scanners

Effectiveness: High

Dynamic Analysis with Manual Results Interpretation

According to SOAR, the following detection techniques may be useful:

Highly cost effective:

• Fuzz Tester
• Framework-based Fuzzer

Cost effective for partial coverage:

• Host Application Interface Scanner
• Monitored Virtual Environment - run potentially malicious code in sandbox / wrapper / virtual machine, see if it does anything suspicious

Effectiveness: High

Manual Static Analysis - Source Code

According to SOAR, the following detection techniques may be useful:

Highly cost effective:

• Focused Manual Spotcheck - Focused manual analysis of source
• Manual Source Code Review (not inspections)

Effectiveness: High

Automated Static Analysis - Source Code

According to SOAR, the following detection techniques may be useful:

Highly cost effective:

• Source code Weakness Analyzer
• Context-configured Source Code Weakness Analyzer

Effectiveness: High

Architecture or Design Review

According to SOAR, the following detection techniques may be useful:

Highly cost effective:

• Inspection (IEEE 1028 standard) (can apply to requirements, design, source code, etc.)
• Formal Methods / Correct-By-Construction

Cost effective for partial coverage:

• Attack Modeling

Effectiveness: High

Potential Mitigations

Architecture and Design

Strategy: Attack Surface Reduction

Consider using language-theoretic security (LangSec) techniques that characterize inputs using a formal language and build “recognizers” for that language. This effectively requires parsing to be a distinct layer that effectively enforces a boundary between raw input and internal data representations, instead of allowing parser code to be scattered throughout the program, where it could be subject to errors or inconsistencies that create weaknesses. [REF-1109] [REF-1110] [REF-1111]

Architecture and Design

Strategy: Libraries or Frameworks

Use an input validation framework such as Struts or the OWASP ESAPI Validation API. Note that using a framework does not automatically address all input validation problems; be mindful of weaknesses that could arise from misusing the framework itself (CWE-1173).

Architecture and Design

Strategy: Attack Surface Reduction

Understand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide data to the application. Remember that such inputs may be obtained indirectly through API calls.

Implementation

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 about 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.

Implementation

When your application combines data from multiple sources, perform the validation after the sources have been combined. The individual data elements may pass the validation step but violate the intended restrictions after they have been combined.

Implementation

Be especially careful to validate all input when invoking code that crosses language boundaries, such as from an interpreted language to native code. This could create an unexpected interaction between the language boundaries. Ensure that you are not violating any of the expectations of the language with which you are interfacing. For example, even though Java may not be susceptible to buffer overflows, providing a large argument in a call to native code might trigger an overflow.

Implementation

Directly convert your input type into the expected data type, such as using a conversion function that translates a string into a number. After converting to the expected data type, ensure that the input’s values fall within the expected range of allowable values and that multi-field consistencies are maintained.

Implementation

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.

Implementation

When exchanging data between components, ensure that both components are using the same character encoding. Ensure that the proper encoding is applied at each interface. Explicitly set the encoding you are using whenever the protocol allows you to do so.

Observed Examples

• CVE-2021-30860: Chain: improper input validation (CWE-20) leads to integer overflow (CWE-190) in mobile OS, as exploited in the wild per CISA KEV.
• CVE-2021-30663: Chain: improper input validation (CWE-20) leads to integer overflow (CWE-190) in mobile OS, as exploited in the wild per CISA KEV.
• CVE-2021-22205: Chain: backslash followed by a newline can bypass a validation step (CWE-20), leading to eval injection (CWE-95), as exploited in the wild per CISA KEV.
• CVE-2021-21220: Chain: insufficient input validation (CWE-20) in browser allows heap corruption (CWE-787), as exploited in the wild per CISA KEV.
• CVE-2020-9054: Chain: improper input validation (CWE-20) in username parameter, leading to OS command injection (CWE-78), as exploited in the wild per CISA KEV.
• CVE-2020-3452: Chain: security product has improper input validation (CWE-20) leading to directory traversal (CWE-22), as exploited in the wild per CISA KEV.
• CVE-2020-3161: Improper input validation of HTTP requests in IP phone, as exploited in the wild per CISA KEV.
• CVE-2020-3580: Chain: improper input validation (CWE-20) in firewall product leads to XSS (CWE-79), as exploited in the wild per CISA KEV.
• CVE-2021-37147: Chain: caching proxy server has improper input validation (CWE-20) of headers, allowing HTTP response smuggling (CWE-444) using an “LF line ending”
• CVE-2008-5305: Eval injection in Perl program using an ID that should only contain hyphens and numbers.
• CVE-2008-2223: SQL injection through an ID that was supposed to be numeric.
• CVE-2008-3477: lack of input validation in spreadsheet program leads to buffer overflows, integer overflows, array index errors, and memory corruption.
• CVE-2008-3843: insufficient validation enables XSS
• CVE-2008-3174: driver in security product allows code execution due to insufficient validation
• CVE-2007-3409: infinite loop from DNS packet with a label that points to itself
• CVE-2006-6870: infinite loop from DNS packet with a label that points to itself
• CVE-2008-1303: missing parameter leads to crash
• CVE-2007-5893: HTTP request with missing protocol version number leads to crash
• CVE-2006-6658: request with missing parameters leads to information exposure
• CVE-2008-4114: system crash with offset value that is inconsistent with packet size
• CVE-2006-3790: size field that is inconsistent with packet size leads to buffer over-read
• CVE-2008-2309: product uses a denylist to identify potentially dangerous content, allowing attacker to bypass a warning
• CVE-2008-3494: security bypass via an extra header
• CVE-2008-3571: empty packet triggers reboot
• CVE-2006-5525: incomplete denylist allows SQL injection
• CVE-2008-1284: NUL byte in theme name causes directory traversal impact to be worse
• CVE-2008-0600: kernel does not validate an incoming pointer before dereferencing it
• CVE-2008-1738: anti-virus product has insufficient input validation of hooked SSDT functions, allowing code execution
• CVE-2008-1737: anti-virus product allows DoS via zero-length field
• CVE-2008-3464: driver does not validate input from userland to the kernel
• CVE-2008-2252: kernel does not validate parameters sent in from userland, allowing code execution
• CVE-2008-2374: lack of validation of string length fields allows memory consumption or buffer over-read
• CVE-2008-1440: lack of validation of length field leads to infinite loop
• CVE-2008-1625: lack of validation of input to an IOCTL allows code execution
• CVE-2008-3177: zero-length attachment causes crash
• CVE-2007-2442: zero-length input causes free of uninitialized pointer
• CVE-2008-5563: crash via a malformed frame structure
• CVE-2008-5285: infinite loop from a long SMTP request
• CVE-2008-3812: router crashes with a malformed packet
• CVE-2008-3680: packet with invalid version number leads to NULL pointer dereference
• CVE-2008-3660: crash via multiple “.” characters in file extension