Variant · Low-Medium

CWE-156: Improper Neutralization of Whitespace

The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could be interpreted as whitespace when they are sent to a downstream...

CWE-156 · Variant Level ·3 CVEs ·4 Mitigations

Description

The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could be interpreted as whitespace when they are sent to a downstream component.

This can include space, tab, etc.

Potential Impact

Integrity

Unexpected State

Mitigations & Prevention

General

Developers should anticipate that whitespace 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.

Implementation

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 relat

Implementation

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 qu

Implementation

Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.

Real-World CVE Examples

CVE IDDescription
CVE-2002-0637MIE. virus protection bypass with RFC violations involving extra whitespace, or missing whitespace.
CVE-2004-0942CPU consumption with MIME headers containing lines with many space characters, probably due to algorithmic complexity (RESOURCE.AMP.ALG).
CVE-2003-1015MIE. whitespace interpreted differently by mail clients.

Taxonomy Mappings

  • PLOVER: SPEC.WHITESPACE — Whitespace
  • Software Fault Patterns: SFP24 — Tainted input to command

Frequently Asked Questions

What is CWE-156?

CWE-156 (Improper Neutralization of Whitespace) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Variant-level weakness. The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could be interpreted as whitespace when they are sent to a downstream...

How can CWE-156 be exploited?

Attackers can exploit CWE-156 (Improper Neutralization of Whitespace) to unexpected state. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-156?

Key mitigations include: Developers should anticipate that whitespace will be injected/removed/manipulated in the input vectors of their product. Use an appropriate combination of denylists and allowlists to ensure only valid

What is the severity of CWE-156?

CWE-156 is classified as a Variant-level weakness (Low-Medium abstraction). It has been observed in 3 real-world CVEs.