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



+ Observed Examples
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

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

Phase: Architecture and Design

Assume all input is malicious. Use a standard input validation mechanism to validate all input for length, type, syntax, and business rules before accepting the data to be displayed or stored. Use an "accept known good" validation strategy.

Use and specify a strong output encoding (such as ISO 8859-1 or UTF 8).

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.

Inputs should be decoded and canonicalized to the application's current internal representation before being validated. Make sure that your application does not decode the same input twice. Such errors could be used to bypass whitelist 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
+ Research Gaps

Probably under-studied.

+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERDouble Encoding
+ Content History
Submission DateSubmitterOrganizationSource
PLOVERExternally Mined
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
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Double Encoding