The product filters data in a way that causes it to be reduced or "collapsed" into an unsafe value that violates an expected security property.

Primary

Implementation

Access Control
Bypass Protection Mechanism

Automated Static Analysis
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
High

Architecture and Design
Input Validation
Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.

Implementation
Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list 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. This 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, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

Implementation
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 allowlist validation schemes by introducing dangerous inputs after they have been checked.

Canonicalize the name to match that of the file system's representation of the name. This can sometimes be achieved with an available API (e.g. in Win32 the GetFullPathName function).

CVE-2004-0815
"/.////" in pathname collapses to absolute path.
https://www.cve.org/CVERecord?id=CVE-2004-0815

CVE-2005-3123
"/.//..//////././" is collapsed into "/.././" after ".." and "//" sequences are removed.
https://www.cve.org/CVERecord?id=CVE-2005-3123

CVE-2002-0325
".../...//" collapsed to "..." due to removal of "./" in web server.
https://www.cve.org/CVERecord?id=CVE-2002-0325

CVE-2002-0784
chain: HTTP server protects against ".." but allows "." variants such as "////./../.../". If the server removes "/.." sequences, the result would collapse into an unsafe value "////../" (CWE-182).
https://www.cve.org/CVERecord?id=CVE-2002-0784

CVE-2005-2169
MFV. Regular expression intended to protect against directory traversal reduces ".../...//" to "../".
https://www.cve.org/CVERecord?id=CVE-2005-2169

CVE-2001-1157
XSS protection mechanism strips a <script> sequence that is nested in another <script> sequence.
https://www.cve.org/CVERecord?id=CVE-2001-1157

Collapse of Data into Unsafe Value

IDS11-J
Eliminate noncharacter code points before validation

Allowed
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.
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.

Overlaps regular expressions, although an implementation might not necessarily use regexp's.

PLOVER
2006-07-19
Draft 3
2006-07-19

Eric Dalci
Cigital
2008-07-01
updated Potential_Mitigations, Time_of_Introduction

CWE Content Team
MITRE
2008-09-08
updated Description, Relationships, Relationship_Notes, Relevant_Properties, Taxonomy_Mappings

CWE Content Team
MITRE
2008-11-24
updated Observed_Examples

CWE Content Team
MITRE
2009-03-10
updated Relationships

CWE Content Team
MITRE
2009-07-27
updated Potential_Mitigations

CWE Content Team
MITRE
2010-06-21
updated Description, Observed_Examples

CWE Content Team
MITRE
2010-12-13
updated Relationships

CWE Content Team
MITRE
2011-03-29
updated Potential_Mitigations

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences, Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2012-05-11
updated References, Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2012-10-30
updated Potential_Mitigations

CWE Content Team
MITRE
2014-07-30
updated Relationships

CWE Content Team
MITRE
2017-11-08
updated Applicable_Platforms, Relevant_Properties

CWE Content Team
MITRE
2019-01-03
updated Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2020-02-24
updated Potential_Mitigations, Relationships

CWE Content Team
MITRE
2020-06-25
updated Potential_Mitigations

CWE Content Team
MITRE
2023-01-31
updated Description

CWE Content Team
MITRE
2023-04-27
updated Detection_Factors, Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2025-04-03
4.17
2025-04-03
updated Relationships

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Applicable_Platforms, Weakness_Ordinalities