Absolute Path Traversal

Description

This vulnerability occurs when an application builds file paths using user input without properly blocking absolute paths like '/etc/passwd' or 'C:\Windows\system32'. Attackers can exploit this to escape the intended directory and access sensitive files anywhere on the server.

Extended Description

Absolute path traversal happens because the application trusts user-supplied input when constructing filesystem paths, failing to validate or sanitize sequences that point to root directories. Unlike relative path traversals (using '..'), this attack uses full paths starting from the filesystem root, allowing direct access to critical system files, configuration files, or application source code outside the webroot. Preventing this requires validating all user input used in file operations, rejecting any path containing a leading slash or drive letter, and using canonicalization functions to resolve paths before checking if they remain within the allowed directory. While SAST tools can detect the vulnerable pattern, Plexicus uses AI to analyze the context and suggest the precise code fix—such as implementing an allowlist or path normalization—saving developers hours of manual security review and remediation.

Common Consequences 4

Scope: IntegrityConfidentialityAvailability

Impact: Execute Unauthorized Code or Commands

The attacker may be able to create or overwrite critical files that are used to execute code, such as programs or libraries.

Scope: Integrity

Impact: Modify Files or Directories

The attacker may be able to overwrite or create critical files, such as programs, libraries, or important data. If the targeted file is used for a security mechanism, then the attacker may be able to bypass that mechanism. For example, appending a new account at the end of a password file may allow an attacker to bypass authentication.

Scope: Confidentiality

Impact: Read Files or Directories

The attacker may be able read the contents of unexpected files and expose sensitive data. If the targeted file is used for a security mechanism, then the attacker may be able to bypass that mechanism. For example, by reading a password file, the attacker could conduct brute force password guessing attacks in order to break into an account on the system.

Scope: Availability

Impact: DoS: Crash, Exit, or Restart

The attacker may be able to overwrite, delete, or corrupt unexpected critical files such as programs, libraries, or important data. This may prevent the product from working at all and in the case of a protection mechanisms such as authentication, it has the potential to lockout every user of the product.

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 3

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.

Phase: Operation

Strategy: Firewall

Use an application firewall that can detect attacks against this weakness. It can be beneficial in cases in which the code cannot be fixed (because it is controlled by a third party), as an emergency prevention measure while more comprehensive software assurance measures are applied, or to provide defense in depth [REF-1481].

Effectiveness: Moderate

Demonstrative Examples 2

ID : DX-18

In the example below, the path to a dictionary file is read from a system property and used to initialize a File object.

Code Example:Bad
Java
java

However, the path is not validated or modified to prevent it from containing relative or absolute path sequences before creating the File object. This allows anyone who can control the system property to determine what file is used. Ideally, the path should be resolved relative to some kind of application or user home directory.

ID : DX-159

This script intends to read a user-supplied file from the current directory. The user inputs the relative path to the file and the script uses Python's os.path.join() function to combine the path to the current working directory with the provided path to the specified file. This results in an absolute path to the desired file. If the file does not exist when the script attempts to read it, an error is printed to the user.

Code Example:Bad
Python
python

However, if the user supplies an absolute path, the os.path.join() function will discard the path to the current working directory and use only the absolute path provided. For example, if the current working directory is /home/user/documents, but the user inputs /etc/passwd, os.path.join() will use only /etc/passwd, as it is considered an absolute path. In the above scenario, this would cause the script to access and read the /etc/passwd file.

Code Example:Good
Python
python

The constructed path string uses os.sep to add the appropriate separation character for the given operating system (e.g. '\' or '/') and the call to os.path.normpath() removes any additional slashes that may have been entered - this may occur particularly when using a Windows path. The path is checked against an expected directory (/home/cwe/documents); otherwise, an attacker could provide relative path sequences like ".." to cause normpath() to generate paths that are outside the intended directory (Relative Path Traversal). By putting the pieces of the path string together in this fashion, the script avoids a call to os.path.join() and any potential issues that might arise if an absolute path is entered. With this version of the script, if the current working directory is /home/cwe/documents, and the user inputs /etc/passwd, the resulting path will be /home/cwe/documents/etc/passwd. The user is therefore contained within the current working directory as intended.

Observed Examples 18

CVE-2024-0520Product for managing datasets for AI model training and evaluation allows both relative (Relative Path Traversal) and absolute (Absolute Path Traversal) path traversal to overwrite files via the Content-Disposition header

CVE-2022-31503Python package constructs filenames using an unsafe os.path.join call on untrusted input, allowing absolute path traversal because os.path.join resets the pathname to an absolute path that is specified as part of the input.

CVE-2002-1345Multiple FTP clients write arbitrary files via absolute paths in server responses

CVE-2001-1269ZIP file extractor allows full path

CVE-2002-1818Path traversal using absolute pathname

CVE-2002-1913Path traversal using absolute pathname

CVE-2005-2147Path traversal using absolute pathname

CVE-2000-0614Arbitrary files may be overwritten via compressed attachments that specify absolute path names for the decompressed output.

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.

CVE-2001-0038Remote attackers can read arbitrary files by specifying the drive letter in the requested URL.

CVE-2001-0255FTP server allows remote attackers to list arbitrary directories by using the "ls" command and including the drive letter name (e.g. C:) in the requested pathname.

CVE-2001-0933FTP server allows remote attackers to list the contents of arbitrary drives via a ls command that includes the drive letter as an argument.

CVE-2002-0466Server allows remote attackers to browse arbitrary directories via a full pathname in the arguments to certain dynamic pages.

CVE-2002-1483Remote attackers can read arbitrary files via an HTTP request whose argument is a filename of the form "C:" (Drive letter), "//absolute/path", or ".." .

CVE-2004-2488FTP server read/access arbitrary files using "C:\" filenames

CVE-2001-0687FTP server allows a remote attacker to retrieve privileged web server system information by specifying arbitrary paths in the UNC format (\\computername\sharename).

References 3

The Art of Software Security Assessment

Mark Dowd, John McDonald, and Justin Schuh

Addison Wesley

2006

ID: REF-62

Secure by Design Alert: Eliminating Directory Traversal Vulnerabilities in Software

Cybersecurity and Infrastructure Security Agency

02-05-2024

https://www.cisa.gov/resources-tools/resources/secure-design-alert-eliminating-directory-traversal-vulnerabilities-software(2024-07-14)

ID: REF-1448

D3FEND: Application Layer Firewall