Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

CWE/SANS Top 25 Most Dangerous Software Errors
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CWE-177: Improper Handling of URL Encoding (Hex Encoding)

Weakness ID: 177
Abstraction: Variant
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
The software does not properly handle when all or part of an input has been URL encoded.
+ 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 "Research Concepts" (CWE-1000)
ChildOfClassClass172Encoding Error
+ Relevant to the view "Development Concepts" (CWE-699)
ChildOfClassClass172Encoding Error
+ 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.

+ 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.


Technical Impact: Unexpected State

+ Observed Examples
Hex-encoded path traversal variants - "%2e%2e", "%2e%2e%2f", "%5c%2e%2e"
Hex-encoded path traversal variants - "%2e%2e", "%2e%2e%2f", "%5c%2e%2e"
Hex-encoded path traversal variants - "%2e%2e", "%2e%2e%2f", "%5c%2e%2e"
"%20" (encoded space)
"%20" (encoded space)
"%20" (encoded space)
"%20" (encoded space)
Crash via hex-encoded space "%20".
"%00" (encoded null)
"%00" (encoded null)
"%00" (encoded null)
"%00" (encoded null)
"%00" (encoded null)
"%00" (encoded null)
"%00" (encoded null)
"%2f" (encoded slash)
"%5c" (encoded backslash) and "%2e" (encoded dot) sequences
"%5c" (encoded backslash)
"%0a" (overlaps CRLF)
+ Potential Mitigations

Phase: Architecture and Design

Strategy: Input Validation

Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.

Phase: Implementation

Strategy: Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a whitelist 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 (i.e., do not rely on a blacklist). A blacklist 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, blacklists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

Phase: Implementation

Strategy: Input Validation

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 whitelist validation schemes by introducing dangerous inputs after they have been checked.
+ 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.
MemberOfCategoryCategory992SFP Secondary Cluster: Faulty Input Transformation
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERURL Encoding (Hex Encoding)
+ Content History
Submission DateSubmitterOrganization
Modification DateModifierOrganization
2008-07-01Eric DalciCigital
updated Potential_Mitigations, Time_of_Introduction
Suggested OWASP Top Ten 2004 mapping
2008-09-08CWE Content TeamMITRE
updated Relationships, Observed_Example, Taxonomy_Mappings
2009-07-27CWE Content TeamMITRE
updated Potential_Mitigations
2010-12-13CWE Content TeamMITRE
updated Name
2011-03-29CWE Content TeamMITRE
updated Potential_Mitigations
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2011-06-27CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated Related_Attack_Patterns, Relationships
2012-10-30CWE Content TeamMITRE
updated Potential_Mitigations
2014-07-30CWE Content TeamMITRE
updated Relationships
2017-11-08CWE Content TeamMITRE
updated Applicable_Platforms
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11URL Encoding (Hex Encoding)
2010-12-13Failure to Handle URL Encoding (Hex Encoding)

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Page Last Updated: January 18, 2018