Common Weakness Enumeration

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CWE-184: Incomplete Blacklist

Weakness ID: 184
Abstraction: Base
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
An application uses a "blacklist" of prohibited values, but the blacklist is incomplete.
+ Extended Description
If an incomplete blacklist is used as a security mechanism, then the software may allow unintended values to pass into the application logic.
+ Relationships

The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

+ Relevant to the view "Weaknesses for Simplified Mapping of Published Vulnerabilities" (CWE-1003)
ChildOfClassClass693Protection Mechanism Failure
+ Relevant to the view "Development Concepts" (CWE-699)
MemberOfCategoryCategory171Cleansing, Canonicalization, and Comparison Errors
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the software life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

Architecture and Design
+ Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.


Class: Language-Independent (Undetermined Prevalence)

+ Common Consequences

The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

Access Control

Technical Impact: Bypass Protection Mechanism

+ Demonstrative Examples

Example 1

The following code attempts to stop XSS attacks by removing all occurences of "script" in an input string.

(bad code)
Example Language: Java 
public String removeScriptTags(String input, String mask) {
return input.replaceAll("script", mask);


Because the code only checks for the lower-case "script" string, it can be easily defeated with upper-case script tags.

+ Observed Examples
PHP remote file inclusion in web application that filters "http" and "https" URLs, but not "ftp".
Programming language does not filter certain shell metacharacters in Windows environment.
XSS filter doesn't filter null characters before looking for dangerous tags, which are ignored by web browsers. MIE and validate-before-cleanse.
Web-based mail product doesn't restrict dangerous extensions such as ASPX on a web server, even though others are prohibited.
Resultant XSS from incomplete blacklist (only <script> and <style> are checked).
Privileged program does not clear sensitive environment variables that are used by bash. Overlaps multiple interpretation error.
SQL injection protection scheme does not quote the "\" special character.
Incomplete blacklist prevents user from automatically executing .EXE files, but allows .LNK, allowing resultant Windows symbolic link.
product doesn't protect one dangerous variable against external modification
Chain: only removes SCRIPT tags, enabling XSS
Chain: only checks "javascript:" tag
Chain: incomplete blacklist for OS command injection
"\" not in blacklist for web server, allowing path traversal attacks when the server is run in Windows and other OSes.
+ Potential Mitigations

Phase: Implementation

Strategy: Input Validation

Combine use of blacklist with appropriate use of whitelists.

Phase: Implementation

Strategy: Input Validation

Do not rely exclusively on blacklist validation to detect malicious input or to encode output. There are too many variants to encode a character; you're likely to miss some variants.
+ Weakness Ordinalities
(where the weakness exists independent of other weaknesses)
+ Detection Methods

Black Box

Exploitation of incomplete blacklist weaknesses using the obvious manipulations might fail, but minor variations might succeed.
+ Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
MemberOfCategoryCategory990SFP Secondary Cluster: Tainted Input to Command
+ Notes


An incomplete blacklist frequently produces resultant weaknesses.

Some incomplete blacklist issues might arise from multiple interpretation errors, e.g. a blacklist for dangerous shell metacharacters might not include a metacharacter that only has meaning in one particular shell, not all of them; or a blacklist for XSS manipulations might ignore an unusual construct that's supported by one web browser, but not others.

+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERIncomplete Blacklist
+ References
[REF-140] Greg Hoglund and Gary McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. 2004-02-27. <>.
[REF-141] Steve Christey. "Blacklist defenses as a breeding ground for vulnerability variants". 2006-02-03. <>.
[REF-62] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 8, "Eliminating Metacharacters", Page 435. 1st Edition. Addison Wesley. 2006.
+ Content History
Submission DateSubmitterOrganization
Modification DateModifierOrganization
2008-07-01Sean EidemillerCigital
added/updated demonstrative examples
2008-07-01Eric DalciCigital
updated Potential_Mitigations, Time_of_Introduction
2008-09-08CWE Content TeamMITRE
updated Detection_Factors, Relationships, Other_Notes, Relationship_Notes, Taxonomy_Mappings, Weakness_Ordinalities
2008-11-24CWE Content TeamMITRE
updated Observed_Examples
2009-05-27CWE Content TeamMITRE
updated Description, Other_Notes, Relationship_Notes, Time_of_Introduction
2010-02-16CWE Content TeamMITRE
updated Relationships
2010-04-05CWE Content TeamMITRE
updated Related_Attack_Patterns
2010-06-21CWE Content TeamMITRE
updated Demonstrative_Examples
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated References, Related_Attack_Patterns, Relationships
2013-02-21CWE Content TeamMITRE
updated Potential_Mitigations
2014-07-30CWE Content TeamMITRE
updated Demonstrative_Examples, Relationships
2015-12-07CWE Content TeamMITRE
updated Relationships
2017-05-03CWE Content TeamMITRE
updated Potential_Mitigations, Related_Attack_Patterns
2017-11-08CWE Content TeamMITRE
updated Applicable_Platforms, References

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Page Last Updated: January 18, 2018