CWE

Collapse of Data into Unsafe Value

ID: 182		Date: (C)2012-05-14   (M)2022-10-10
Type: weakness		Status: DRAFT
Abstraction Type: Base

Description

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

Applicable Platforms
Language Class: All

Time Of Introduction

• Implementation

Common Consequences

Scope	Technical Impact	Notes
Access_Control	Bypass protection mechanism

Detection Methods
None

Potential Mitigations

Phase	Strategy	Description	Effectiveness	Notes
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 whitelist 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 (i.e., do not rely on a blacklist). A blacklist 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, blacklists 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 whitelist 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).

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

Related CWE	Type	View	Chain
CWE-182 ChildOf CWE-896	Category	CWE-888

Demonstrative Examples
None

Observed Examples

1. CVE-2004-0815 : "/.////" in pathname collapses to absolute path.
2. CVE-2005-3123 : "/.//..//////././" is collapsed into "/.././" after ".." and "//" sequences are removed.
3. CVE-2002-0325 : ".../...//" collapsed to "..." due to removal of "./" in web server.
4. 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).
5. CVE-2005-2169 : MFV. Regular expression intended to protect against directory traversal reduces ".../...//" to "../".
6. CVE-2001-1157 : XSS protection mechanism strips a <script> sequence that is nested in another <script> sequence.

For more examples, refer to CVE relations in the bottom box.

White Box Definitions
None

Black Box Definitions
None

Taxynomy Mappings

Taxynomy	Id	Name	Fit
PLOVER		Collapse of Data into Unsafe Value
CERT Java Secure Coding	IDS11-J	Eliminate noncharacter code points before validation

References:

1. Mark Dowd John McDonald Justin Schuh .The Art of Software Security Assessment 1st Edition. Addison Wesley. Section:'Chapter 8, "Character Stripping Vulnerabilities", Page 437.'. Published on 2006.