Path Traversal: '..\filedir'

Description

This vulnerability occurs when an application builds a file path using user input but fails to block or properly handle '..\' sequences. This oversight allows an attacker to break out of the intended directory and navigate to unauthorized locations in the file system.

Extended Description

Attackers exploit this flaw by injecting '..\' (dot-dot-backslash) sequences into file path parameters. Since each '..\' moves up one directory level, they can craft a path like '..\..\windows\system32\config' to access sensitive files well outside the application's allowed directory, potentially leading to data theft, system information disclosure, or server compromise. While this sequence is specific to Windows systems that use the backslash (\) as a separator, it's a critical bypass technique. Many defensive filters only check for the forward slash (/) used in Unix-style paths, making the '..\' variant a common method for evading simple validation and achieving the same traversal effect on Windows servers.

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 5

CVE-2002-0661"\" not in denylist for web server, allowing path traversal attacks when the server is run in Windows and other OSes.

CVE-2002-0946Arbitrary files may be read files via ..\ (dot dot) sequences in an HTTP request.

CVE-2002-1042Directory traversal vulnerability in search engine for web server allows remote attackers to read arbitrary files via "..\" sequences in queries.

CVE-2002-1209Directory traversal vulnerability in FTP server allows remote attackers to read arbitrary files via "..\" sequences in a GET request.

CVE-2002-1178Directory traversal vulnerability in servlet allows remote attackers to execute arbitrary commands via "..\" sequences in an HTTP request.

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