logo

Improper Neutralization of Trailing Special Elements

Incomplete Variant
Structure: Simple
Description

This vulnerability occurs when an application fails to properly sanitize or remove trailing special characters from user-supplied input before passing it to another system component. These leftover characters can trick the downstream parser into executing unintended commands or altering the data flow.

Extended Description

Think of trailing special elements like unexpected punctuation at the end of a sentence—characters such as newlines (\n), carriage returns (\r), semicolons (;), or command delimiters. When an application doesn't strip these from input, they travel into functions that parse or process data, like database queries, shell commands, or log handlers. The downstream component interprets these characters as legitimate instructions, not data, which can lead to injection attacks, data corruption, or system manipulation. To prevent this, developers must implement strict input validation and output encoding specifically for the context where the data will be used. Always sanitize input by escaping or removing control characters and command delimiters at the boundaries between different system components. Treat all input as untrusted and ensure your validation logic accounts for the entire data string, not just the primary content, to neutralize these hidden trailing threats.
Common Consequences 1
Scope: Integrity

Impact: Unexpected State
Potential Mitigations 4
Developers should anticipate that trailing special elements 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 3
CVE-2004-0847web framework for .NET allows remote attackers to bypass authentication for .aspx files in restricted directories via a request containing a (1) "\" (backslash) or (2) "%5C" (encoded backslash)
CVE-2002-1451Trailing space ("+" in query string) leads to source code disclosure.
CVE-2001-0446Application server allows remote attackers to read source code for .jsp files by appending a / to the requested URL.
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