Improper Neutralization of Substitution Characters

Description

This vulnerability occurs when an application accepts user input and fails to properly sanitize special characters that can trigger command or variable substitution before passing that data to another system component. This allows attackers to inject malicious substitutions that alter the system's intended behavior.

Extended Description

At its core, this flaw is about a broken data pipeline. An upstream component (like a user form, API, or file upload) sends raw data containing characters like backticks (`), dollar signs ($), or percent signs (%), which many systems interpret as instructions to execute a command or expand a variable. The vulnerable component acts as a passive conduit, forwarding this tainted data without cleaning it, effectively letting user input dictate actions in the downstream component. For developers, the fix involves implementing strict input validation and context-aware output encoding. Treat all incoming data as untrusted and neutralize substitution characters based on exactly how the downstream component will use the data. For example, if passing data to a shell, escape shell metacharacters; if passing to a script engine, escape its specific substitution syntax. Never rely on blacklists; instead, use allowlists of expected safe characters or robust encoding libraries designed for the target interpreter.

Common Consequences 1

Scope: Integrity

Impact: Unexpected State

Potential Mitigations 4

Developers should anticipate that substitution characters 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.

Observed Examples 1

CVE-2002-0770Server trusts client to expand macros, allows macro characters to be expanded to trigger resultant information exposure.

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

Research GapUnder-studied.