Improper Neutralization of Macro Symbols

Description

This vulnerability occurs when an application accepts user input containing macro symbols (like those used in templates or configuration files) and passes it along without properly sanitizing it. This allows attackers to inject malicious macro commands that get executed by downstream systems, potentially leading to data manipulation, system access, or other unintended behaviors.

Extended Description

Think of this like handing someone a note written in a special code language they understand, but you didn't check what the note actually says. If an attacker can sneak their own commands into that 'code' (the macro symbols), the downstream component will faithfully execute those commands as if they were legitimate instructions. This is common in systems that process templates, configuration files, or documents that support embedded scripting or command macros. To prevent this, developers must treat all user-supplied input that will be interpreted as code or commands as untrusted. Implement strict validation and sanitization routines that either escape or remove dangerous macro symbols before the data is sent for processing. Always design systems with a clear separation between data (which can be user-provided) and the actual commands or code that operate on that data.

Common Consequences 1

Scope: Integrity

Impact: Unexpected State

Potential Mitigations 4

Phase: Implementation

Strategy: Input Validation

Developers should anticipate that macro symbols 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

Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component.

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

CVE-2008-2018Attacker can obtain sensitive information from a database by using a comment containing a macro, which inserts the data during expansion.

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.