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CWE-174: Double Decoding of the Same Data

 
Double Decoding of the Same Data
Weakness ID: 174 (Weakness Variant)Status: Draft
+ Description

Description Summary

The software decodes the same input twice, which can limit the effectiveness of any protection mechanism that occurs in between the decoding operations.
+ Time of Introduction
  • Implementation
+ Applicable Platforms

Languages

All

+ Common Consequences
ScopeEffect
Access Control
Confidentiality
Availability
Integrity
Other

Technical Impact: Bypass protection mechanism; Execute unauthorized code or commands; Varies by context

+ Observed Examples
ReferenceDescription
CVE-2004-1315Forum software improperly URL decodes the highlight parameter when extracting text to highlight, which allows remote attackers to execute arbitrary PHP code by double-encoding the highlight value so that special characters are inserted into the result.
CVE-2004-1939XSS protection mechanism attempts to remove "/" that could be used to close tags, but it can be bypassed using double encoded slashes (%252F)
CVE-2001-0333Directory traversal using double encoding.
CVE-2004-1938"%2527" (double-encoded single quote) used in SQL injection.
CVE-2005-1945Double hex-encoded data.
CVE-2005-0054Browser executes HTML at higher privileges via URL with hostnames that are double hex encoded, which are decoded twice to generate a malicious hostname.
+ 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: Output Encoding

Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component.

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.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness ClassWeakness Class172Encoding Error
Development Concepts (primary)699
Research Concepts1000
ChildOfWeakness ClassWeakness Class675Duplicate Operations on Resource
Research Concepts (primary)1000
ChildOfCategoryCategory896SFP Cluster: Tainted Input
Software Fault Pattern (SFP) Clusters (primary)888
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
+ Research Gaps

Probably under-studied.

+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERDouble Encoding
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
PLOVERExternally Mined
Modifications
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Potential_Mitigations
2008-09-08CWE Content TeamMITREInternal
updated Relationships, Observed_Example, Taxonomy_Mappings
2008-11-24CWE Content TeamMITREInternal
updated Observed_Examples
2009-07-27CWE Content TeamMITREInternal
updated Potential_Mitigations
2011-03-29CWE Content TeamMITREInternal
updated Potential_Mitigations
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences
2012-05-11CWE Content TeamMITREInternal
updated Common_Consequences, Observed_Examples, Relationships
2012-10-30CWE Content TeamMITREInternal
updated Potential_Mitigations
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Double Encoding
Page Last Updated: February 18, 2014