CWE - CWE-794: Incomplete Filtering of Multiple Instances of Special Elements (3.1)

Weakness ID: 794

Abstraction: Variant
Structure: Simple

Status: Incomplete

Presentation Filter:

Description

The software receives data from an upstream component, but does not filter all instances of a special element before sending it to a downstream component.

Extended Description

Incomplete filtering of this nature may be applied to

sequential elements (special elements that appear next to each other) or
non-sequential elements (special elements that appear multiple times in different locations).

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)

Nature	Type	ID	Name
ChildOf	Variant - a weakness that is described at a very low level of detail, typically limited to a specific language or technology. More specific than a Base weakness.	792	Incomplete Filtering of One or More Instances of Special Elements

Relevant to the view "Architectural Concepts" (CWE-1008)

Nature	Type	ID	Name
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1019	Validate Inputs

Relevant to the view "Development Concepts" (CWE-699)

Nature	Type	ID	Name
ChildOf	Variant - a weakness that is described at a very low level of detail, typically limited to a specific language or technology. More specific than a Base weakness.	792	Incomplete Filtering of One or More Instances of Special Elements

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.

Phase	Note
Implementation	REALIZATION: This weakness is caused during implementation of an architectural security tactic.

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.

Scope	Impact	Likelihood
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 code)

../../../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).

Content History

Submissions
Submission Date	Submitter	Organization
2009-12-04	CWE Content Team	MITRE
2009-12-04
Modifications
Modification Date	Modifier	Organization
2010-02-16	CWE Content Team	MITRE
2010-02-16	updated Demonstrative_Examples
2010-06-21	CWE Content Team	MITRE
2010-06-21	updated Description
2011-06-01	CWE Content Team	MITRE
2011-06-01	updated Common_Consequences
2011-06-27	CWE Content Team	MITRE
2011-06-27	updated Common_Consequences
2017-01-19	CWE Content Team	MITRE
2017-01-19	updated Relationships
2017-11-08	CWE Content Team	MITRE
2017-11-08	updated Modes_of_Introduction, Relationships


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Common Weakness Enumeration

CWE-794: Incomplete Filtering of Multiple Instances of Special Elements