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Path Traversal: '..\filedir'

Incomplete Variant
Structure: Simple
Description

The product uses external input to construct a pathname that should be within a restricted directory, but it does not properly neutralize "..\" sequences that can resolve to a location that is outside of that directory.

Extended Description

This allows attackers to traverse the file system to access files or directories that are outside of the restricted directory. The '..\' manipulation is the canonical manipulation for operating systems that use "\" as directory separators, such as Windows. However, it is also useful for bypassing path traversal protection schemes that only assume that the "/" separator is valid.

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
  1. File Processing
Affected Resources
  1. File or Directory
Related Weaknesses
ChildOf: Relative Path Traversal (CWE-23)
Taxonomy Mapping
  • PLOVER
  • Software Fault Patterns