Improper Handling of Invalid Use of Special Elements

Description

This vulnerability occurs when an application fails to properly validate or neutralize special characters and control elements within user-supplied input. Without this filtering, attackers can inject these elements to manipulate the application's logic, leading to security breaches or system instability.

Extended Description

At its core, this weakness is about trust. Applications often treat special elements—like command delimiters, escape sequences, or formatting tags—as instructions. When user input containing these elements isn't sanitized, the system mistakenly executes them as code. This can allow attackers to alter data flows, bypass security checks, or directly inject malicious commands, compromising the application from within. To prevent this, developers must implement strict input validation and context-aware output encoding. Treat all user input as untrusted by default. Use allow-list validation to permit only expected, safe characters, and always encode data based on its final output context (like HTML, SQL, or OS commands). This defense-in-depth approach ensures special elements are treated as inert data, not executable instructions, closing a common attack vector.

Common Consequences 1

Scope: Integrity

Impact: Unexpected State

Potential Mitigations 4

Developers should anticipate that special elements will be injected/removed/manipulated in the input vectors of their software system. Use an appropriate combination of denylists and allowlists to ensure only valid, expected and appropriate input is processed by the system.

Phase: 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.

Phase: Implementation

Strategy: Output Encoding

While it is risky to use dynamically-generated query strings, code, or commands that mix control and data together, sometimes it may be unavoidable. Properly quote arguments and escape any special characters within those arguments. The most conservative approach is to escape or filter all characters that do not pass an extremely strict allowlist (such as everything that is not alphanumeric or white space). If some special characters are still needed, such as white space, wrap each argument in quotes after the escaping/filtering step. Be careful of argument injection (Improper Neutralization of Argument Delimiters in a Command ('Argument Injection')).

Phase: Implementation

Strategy: Input Validation

Inputs should be decoded and canonicalized to the application's current internal representation before being validated (Incorrect Behavior Order: Validate Before Canonicalize). Make sure that the application does not decode the same input twice (Double Decoding of the Same Data). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.

Observed Examples 2

CVE-2002-1362Crash via message type without separator character

CVE-2000-0116Extra "<" in front of SCRIPT tag bypasses XSS prevention.

Applicable Platforms

Languages:

Not Language-Specific : Undetermined

Modes of Introduction

Implementation

Related Weaknesses

ChildOf:

Improper Neutralization of Special Elements (CWE-138)

Taxonomy Mapping

PLOVER
Software Fault Patterns

Notes

MaintenanceThe list of children for this entry is far from complete. However, the types of special elements might be too precise for use within CWE.

TerminologyPrecise terminology for the underlying weaknesses does not exist. Therefore, these weaknesses use the terminology associated with the manipulation.

Research GapCustomized languages and grammars, even those that are specific to a particular product, are potential sources of weaknesses that are related to special elements. However, most researchers concentrate on the most commonly used representations for data transmission, such as HTML and SQL. Any representation that is commonly used is likely to be a rich source of weaknesses; researchers are encouraged to investigate previously unexplored representations.