Path Traversal: 'dir\..\..\filename'

Description

This vulnerability occurs when an application builds file paths using user input but fails to properly block sequences like 'dir\..\..\filename'. Attackers can use these multiple backslash-dot-dot sequences to escape the intended directory and access files or folders anywhere on the file system.

Extended Description

This specific pattern, using multiple internal '\..\' sequences, is often a technique to bypass weak security checks. Some defenses might only remove a single instance of '..\' or only check for it at the start of a path. By embedding several levels deep (like 'dir\..\..\'), an attacker can trick the system into moving up more directory levels than intended, effectively neutralizing the protection. For developers, the core issue is relying on simple string filtering or blacklisting. The secure solution is to use canonicalization functions provided by the operating system to resolve the full, absolute path, and then explicitly verify that this final path remains within the allowed directory boundary. Never trust that manually stripping a few '..\' sequences is sufficient.

Common Consequences 1

Scope: ConfidentialityIntegrity

Impact: Read Files or DirectoriesModify Files or Directories

Potential Mitigations 2

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. When validating filenames, use stringent allowlists that limit the character set to be used. If feasible, only allow a single "." character in the filename to avoid weaknesses such as Relative Path Traversal, and exclude directory separators such as "/" to avoid Absolute Path Traversal. Use a list of allowable file extensions, which will help to avoid Unrestricted Upload of File with Dangerous Type. Do not rely exclusively on a filtering mechanism that removes potentially dangerous characters. This is equivalent to a denylist, which may be incomplete (Incomplete List of Disallowed Inputs). For example, filtering "/" is insufficient protection if the filesystem also supports the use of "\" as a directory separator. Another possible error could occur when the filtering is applied in a way that still produces dangerous data (Collapse of Data into Unsafe Value). For example, if "../" sequences are removed from the ".../...//" string in a sequential fashion, two instances of "../" would be removed from the original string, but the remaining characters would still form the "../" string.

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.

Observed Examples 1

CVE-2002-0160The administration function in Access Control Server allows remote attackers to read HTML, Java class, and image files outside the web root via a "..\.." sequence in the URL to port 2002.

References 1

24 Deadly Sins of Software Security

Michael Howard, David LeBlanc, and John Viega

McGraw-Hill

2010

ID: REF-44

Applicable Platforms

Languages:

Not Language-Specific : Undetermined

Modes of Introduction

Implementation

Functional Areas

File Processing

Affected Resources

File or Directory

Related Weaknesses

ChildOf:

Relative Path Traversal (CWE-23)

Taxonomy Mapping

PLOVER
Software Fault Patterns