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Improper Neutralization of Input Terminators

Draft Variant
Structure: Simple
Description

This vulnerability occurs when an application accepts external input but fails to properly handle special characters that downstream systems interpret as commands to stop processing data.

Extended Description

Many protocols and parsers use specific control characters or sequences to mark the end of a data stream. When an application passes user input containing these terminators without sanitizing them, it can trick the downstream component into prematurely stopping its processing, potentially truncating data or creating unexpected system behavior. For instance, in SMTP, a single period on a line (.) signals the end of an email message body. Similarly, a null character (\0) often denotes the end of a string in C-style languages and many file formats. If an attacker can inject these terminators, they can manipulate data flow, corrupt files, or bypass validation logic that expects more complete input.
Common Consequences 1
Scope: Integrity

Impact: Unexpected State
Potential Mitigations 4
Developers should anticipate that terminators 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 4
CVE-2000-0319MFV. mail server does not properly identify terminator string to signify end of message, causing corruption, possibly in conjunction with off-by-one error.
CVE-2000-0320MFV. mail server does not properly identify terminator string to signify end of message, causing corruption, possibly in conjunction with off-by-one error.
CVE-2001-0996Mail server does not quote end-of-input terminator if it appears in the middle of a message.
CVE-2002-0001Improperly terminated comment or phrase allows commands.
Applicable Platforms
Languages:
Not Language-Specific : Undetermined
Modes of Introduction
Implementation
Related Attack Patterns
Related Weaknesses
ChildOf: Improper Neutralization of Special Elements (CWE-138)
Taxonomy Mapping
  • PLOVER
  • Software Fault Patterns