Path Traversal: '\absolute\pathname\here'

Description

This vulnerability occurs when an application accepts user-supplied input containing Windows-style absolute paths (like '\absolute\pathname\here') without proper validation. Attackers can exploit this to navigate outside the intended directory and access or manipulate sensitive files anywhere on the server's file system.

Extended Description

Path traversal vulnerabilities using backslash sequences are a specific threat on Windows systems, where the backslash (\) is the standard directory separator. Unlike relative paths (e.g., '..\'), an absolute path starting with a backslash or drive letter (e.g., '\Windows\System32\' or 'C:\temp\') provides a direct roadmap to a specific location, bypassing the application's intended working directory entirely. If the application fails to sanitize or block these inputs, a simple request could expose critical system files, configuration data, or application source code. To prevent this, developers must implement strict input validation that rejects any user input containing absolute path patterns or directory separators. A stronger defense involves using a whitelist of permitted files or canonicalizing the input path and then verifying it resides within a designated safe directory (like the application's web root). Relying solely on blacklists or simple string replacement for '..\' is insufficient, as attackers can craft numerous bypasses using absolute paths, encoded characters, or trailing dots.

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.

Effectiveness: High

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 3

CVE-1999-1263Mail client allows remote attackers to overwrite arbitrary files via an e-mail message containing a uuencoded attachment that specifies the full pathname for the file to be modified.

CVE-2003-0753Remote attackers can read arbitrary files via a full pathname to the target file in config parameter.

CVE-2002-1525Remote attackers can read arbitrary files via an absolute pathname.

Applicable Platforms

Languages:

Not Language-Specific : Undetermined

Modes of Introduction

Implementation

Functional Areas

File Processing

Affected Resources

File or Directory

Related Weaknesses

ChildOf:

Absolute Path Traversal (CWE-36)

Taxonomy Mapping

PLOVER
CERT C Secure Coding
Software Fault Patterns