Improper Neutralization of Expression/Command Delimiters

Description

This vulnerability occurs when an application fails to properly sanitize input that contains special characters used as delimiters in expressions or commands. When this unsanitized input is passed to another component for processing, those characters can be misinterpreted, altering the intended logic or structure.

Extended Description

Think of delimiters like semicolons in code or pipe characters in commands—they tell a system where one instruction ends and another begins. If an attacker can inject or manipulate these delimiters in user-controlled input (like a form field or API parameter), they can break out of the intended data context and inject arbitrary expressions or commands. This is similar to SQL injection, but targets other interpreters like shell commands, configuration parsers, or templating engines. The core risk is that the downstream component, expecting clean data, instead receives input it interprets as executable logic. This can lead to data corruption, unauthorized information disclosure, or even full system compromise. To prevent this, always validate and sanitize input by escaping or stripping delimiter characters based on the specific syntax of the receiving component before any processing occurs.

Common Consequences 1

Scope: ConfidentialityIntegrityAvailabilityOther

Impact: Execute Unauthorized Code or CommandsAlter Execution Logic

Potential Mitigations 4

Developers should anticipate that inter-expression and inter-command delimiters will be injected/removed/manipulated in the input vectors of their product. 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.

References 1

The Art of Software Security Assessment

Mark Dowd, John McDonald, and Justin Schuh

Addison Wesley

2006

ID: REF-62

Applicable Platforms

Languages:

Not Language-Specific : Undetermined

Modes of Introduction

Implementation

Related Attack Patterns

Related Weaknesses

ChildOf:

Improper Neutralization of Delimiters (CWE-140)

Taxonomy Mapping

PLOVER
Software Fault Patterns

Notes

RelationshipA shell metacharacter (covered in Improper Neutralization of Escape, Meta, or Control Sequences) is one example of a potential delimiter that may need to be neutralized.