CWE - CWE-795: Only Filtering Special Elements at a Specified Location (4.16)

Weakness ID: 795

Vulnerability Mapping: ALLOWED This CWE ID may be used to map to real-world vulnerabilities
Abstraction: Base Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.

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Description

The product receives data from an upstream component, but only accounts for special elements at a specified location, thereby missing remaining special elements that may exist before sending it to a downstream component.

Extended Description

A filter might only account for instances of special elements when they occur:

relative to a marker (e.g. "at the beginning/end of string; the second argument"), or
at an absolute position (e.g. "byte number 10").

This may leave special elements in the data that did not match the filter position, but still may be dangerous.

Common Consequences

This table 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

Relationships

This table 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	Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.	791	Incomplete Filtering of Special Elements
ParentOf	Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.	796	Only Filtering Special Elements Relative to a Marker
ParentOf	Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.	797	Only Filtering Special Elements at an Absolute Position

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

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 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.

Demonstrative Examples

Example 1

The following code takes untrusted input and uses a regular expression to filter a "../" element located at the beginning of the input string. 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 is only looking for an instance of "../" at the beginning of the string, it only removes the first "../" element. 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-22).

Example 2

The following code takes untrusted input and uses a substring function to filter a 3-character "../" element located at the 0-index position of the input string. 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 $Username = GetUntrustedInput();
if (substr($Username, 0, 3) eq '../') {

$Username = substr($Username, 3);

}
my $filename = "/home/user/" . $Username;
ReadAndSendFile($filename);

Since the if function is only looking for a substring of "../" between the 0 and 2 position, it only removes that specific "../" element. So an input value such as:

(attack code)

../../../etc/passwd

will have the first "../" filtered, 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-22).

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.

Nature	Type	ID	Name
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1407	Comprehensive Categorization: Improper Neutralization

Vulnerability Mapping Notes

Usage: ALLOWED

(this CWE ID may be used to map to real-world vulnerabilities)

Reason: Acceptable-Use

Rationale:

This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.

Comments:

Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

Content History

Submissions
Submission Date	Submitter	Organization
2009-12-04 (CWE 1.7, 2009-12-28)	CWE Content Team	MITRE
2009-12-04 (CWE 1.7, 2009-12-28)
Modifications
Modification Date	Modifier	Organization
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
2020-02-24	CWE Content Team	MITRE
2020-02-24	updated Relationships
2021-03-15	CWE Content Team	MITRE
2021-03-15	updated Demonstrative_Examples
2023-01-31	CWE Content Team	MITRE
2023-01-31	updated Description
2023-04-27	CWE Content Team	MITRE
2023-04-27	updated Relationships
2023-06-29	CWE Content Team	MITRE
2023-06-29	updated Mapping_Notes


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Common Weakness Enumeration

CWE-795: Only Filtering Special Elements at a Specified Location

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