Collapse of Data into Unsafe Value

Description

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.

Common Consequences 1

Scope: Access Control

Impact: Bypass Protection Mechanism

Detection Methods 1

Automated Static AnalysisHigh

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.)

Potential Mitigations 4

Phase: Architecture and Design

Strategy: Input Validation

Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.

Phase: Implementation

Strategy: 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.

Phase: Implementation

Strategy: Input Validation

Inputs should be decoded and canonicalized to the application's current internal representation before being validated (Incorrect Behavior Order: Validate Before Canonicalize). Make sure that the application does not decode the same input twice (Double Decoding of the Same Data). 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).

Observed Examples 6

CVE-2004-0815"/.////" in pathname collapses to absolute path.

CVE-2005-3123"/.//..//////././" is collapsed into "/.././" after ".." and "//" sequences are removed.

CVE-2002-0325".../...//" collapsed to "..." due to removal of "./" in web server.

CVE-2002-0784chain: HTTP server protects against ".." but allows "." variants such as "////./../.../". If the server removes "/.." sequences, the result would collapse into an unsafe value "////../" (Collapse of Data into Unsafe Value).

CVE-2005-2169MFV. Regular expression intended to protect against directory traversal reduces ".../...//" to "../".

CVE-2001-1157XSS protection mechanism strips a <script> sequence that is nested in another <script> sequence.

References 1

The Art of Software Security Assessment

Mark Dowd, John McDonald, and Justin Schuh

Addison Wesley

2006

ID: REF-62

Applicable Platforms

Languages:

Not Language-Specific : Undetermined

Modes of Introduction

Implementation

Related Weaknesses

ChildOf:

Improper Neutralization (CWE-707)

CanPrecede:

Path Traversal: '....' (Multiple Dot) (CWE-33)

CanPrecede:

Path Traversal: '....//' (CWE-34)

CanPrecede:

Path Traversal: '.../...//' (CWE-35)

Taxonomy Mapping

PLOVER
The CERT Oracle Secure Coding Standard for Java (2011)

Notes

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