CWE

Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

CWE/SANS Top 25 Most Dangerous Software Errors
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ID

CWE-180: Incorrect Behavior Order: Validate Before Canonicalize

Weakness ID: 180
Abstraction: Base
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
The software validates input before it is canonicalized, which prevents the software from detecting data that becomes invalid after the canonicalization step.
+ Extended Description
This can be used by an attacker to bypass the validation and launch attacks that expose weaknesses that would otherwise be prevented, such as injection.
+ 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)
NatureTypeIDName
ChildOfBaseBase179Incorrect Behavior Order: Early Validation
+ Relevant to the view "Development Concepts" (CWE-699)
+ 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.

PhaseNote
Implementation
+ 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.

Languages

(Language-Independent classes): (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.

ScopeImpactLikelihood
Access Control

Technical Impact: Bypass Protection Mechanism

+ Demonstrative Examples

Example 1

The following code attempts to validate a given input path by checking it against a whitelist and then return the canonical path. In this specific case, the path is considered valid if it starts with the string "/safe_dir/".

(bad)
Example Language: Java 
String path = getInputPath();
if (path.startsWith("/safe_dir/"))
{
File f = new File(path);
return f.getCanonicalPath();

}

The problem with the above code is that the validation step occurs before canonicalization occurs. An attacker could provide an input path of "/safe_dir/../" that would pass the validation step. However, the canonicalization process sees the double dot as a traversal to the parent directory and hence when canonicized the path would become just "/".

To avoid this problem, validation should occur after canonicalization takes place. In this case canonicalization occurs during the initialization of the File object. The code below fixes the issue.

(good)
Example Language: Java 
String path = getInputPath();
File f = new File(path);
if (f.getCanonicalPath().startsWith("/safe_dir/"))
{
return f.getCanonicalPath();

}

+ Observed Examples
ReferenceDescription
Product allows remote attackers to view restricted files via an HTTP request containing a "*" (wildcard or asterisk) character.
Product modifies the first two letters of a filename extension after performing a security check, which allows remote attackers to bypass authentication via a filename with a .ats extension instead of a .hts extension.
Database consumes an extra character when processing a character that cannot be converted, which could remove an escape character from the query and make the application subject to SQL injection attacks.
Overlaps "fakechild/../realchild"
Product checks URI for "<" and other literal characters, but does it before hex decoding the URI, so "%3E" and other sequences are allowed.
+ Potential Mitigations

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.
+ Notes

Relationship

This overlaps other categories.
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERValidate-Before-Canonicalize
OWASP Top Ten 2004A1CWE More SpecificUnvalidated Input
CERT Java Secure CodingIDS01-JNormalize strings before validating them
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
PLOVER
Modifications
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigital
updated Potential_Mitigations, Time_of_Introduction
2008-08-15Veracode
Suggested OWASP Top Ten 2004 mapping
2008-09-08CWE Content TeamMITRE
updated Relationships, Other_Notes, Taxonomy_Mappings, Type
2008-10-14CWE Content TeamMITRE
updated Description
2009-05-27CWE Content TeamMITRE
updated Other_Notes, Relationship_Notes
2010-02-16CWE Content TeamMITRE
updated Demonstrative_Examples
2011-03-29CWE Content TeamMITRE
updated Potential_Mitigations
2011-06-01CWE Content TeamMITRE
updated Common_Consequences, Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITRE
updated Demonstrative_Examples, Observed_Examples, Related_Attack_Patterns, Relationships, Taxonomy_Mappings
2014-07-30CWE Content TeamMITRE
updated Relationships
2017-01-19CWE Content TeamMITRE
updated Relationships
2017-11-08CWE Content TeamMITRE
updated Applicable_Platforms, Functional_Areas
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Validate-Before-Canonicalize

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Page Last Updated: November 14, 2017