Improper Neutralization of Comment Delimiters

Description

This vulnerability occurs when an application accepts user input and fails to properly sanitize characters that can be interpreted as comment markers (like /*, */, //, #, or <!--) before passing that data to another system component. This allows an attacker to inject malicious comments that can break data processing, alter logic, or expose sensitive information.

Extended Description

Think of comment delimiters as hidden instructions that tell a parser or interpreter to ignore everything that follows on a line or within a block. When an application doesn't filter these characters from untrusted input, an attacker can inject them to "comment out" critical parts of a command, query, or configuration file. This manipulation can disrupt data flows, bypass security checks, or corrupt files in downstream systems like databases, log processors, or configuration parsers. The core issue is a failure in input validation and output encoding specific to the context of the receiving component. For example, injecting `/*` into a SQL query might break its structure, while injecting `<!--` into an XML document could hide malicious payloads from an XML parser. To prevent this, developers must always treat user input as untrusted and sanitize or encode it according to the specific syntax rules of the target interpreter before any processing occurs.

Common Consequences 1

Scope: Integrity

Impact: Unexpected State

Potential Mitigations 4

Developers should anticipate that comments 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 5

CVE-2002-0001Mail client command execution due to improperly terminated comment in address list.

CVE-2004-0162MIE. RFC822 comment fields may be processed as other fields by clients.

CVE-2004-1686Well-placed comment bypasses security warning.

CVE-2005-1909Information hiding using a manipulation involving injection of comment code into product. Note: these vulnerabilities are likely vulnerable to more general XSS problems, although a regexp might allow ">!--" while denying most other tags.

CVE-2005-1969Information hiding using a manipulation involving injection of comment code into product. Note: these vulnerabilities are likely vulnerable to more general XSS problems, although a regexp might allow "<!--" while denying most other tags.

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