CWE

Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

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ID

CWE-790: Improper Filtering of Special Elements

Weakness ID: 790
Abstraction: Class
Status: Incomplete
Presentation Filter:
+ Description

Description Summary

The software receives data from an upstream component, but does not filter or incorrectly filters special elements before sending it to a downstream component.
+ Common Consequences
ScopeEffect
Integrity

Technical Impact: Unexpected state

+ Demonstrative Examples

Example 1

The following code takes untrusted input and uses a regular expression to filter "../" from the input. It then appends this result to the /home/user/ directory and attempts to read the file in the final resulting path.

(Bad Code)
Example Language: Perl 
my $Username = GetUntrustedInput();
$Username =~ s/\.\.\///;
my $filename = "/home/user/" . $Username;
ReadAndSendFile($filename);

Since the regular expression does not have the /g global match modifier, it only removes the first instance of "../" it comes across. So an input value such as:

(Attack)
 
../../../etc/passwd

will have the first "../" stripped, resulting in:

(Result)
 
../../etc/passwd

This value is then concatenated with the /home/user/ directory:

(Result)
 
/home/user/../../etc/passwd

which causes the /etc/passwd file to be retrieved once the operating system has resolved the ../ sequences in the pathname. This leads to relative path traversal (CWE-23).

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness ClassWeakness Class138Improper Neutralization of Special Elements
Development Concepts (primary)699
Research Concepts (primary)1000
ParentOfWeakness BaseWeakness Base791Incomplete Filtering of Special Elements
Development Concepts (primary)699
Research Concepts (primary)1000
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
2009-12-04MITREInternal CWE Team
Modifications
Modification DateModifierOrganizationSource
2010-02-16CWE Content TeamMITREInternal
updated Demonstrative_Examples
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences
2011-06-27CWE Content TeamMITREInternal
updated Common_Consequences
2017-01-19CWE Content TeamMITREInternal
updated Relationships

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Page Last Updated: January 18, 2017