Description
The product uses external input to construct a pathname that should be within a restricted directory, but it does not properly neutralize '\..\filename' (leading backslash dot dot) sequences that can resolve to a location that is outside of that directory.
This allows attackers to traverse the file system to access files or directories that are outside of the restricted directory. This is similar to CWE-25, except using "\" instead of "/". Sometimes a program checks for "..\" at the beginning of the input, so a "\..\" can bypass that check. It is also useful for bypassing path traversal protection schemes that only assume that the "/" separator is valid.
Potential Impact
Confidentiality, Integrity
Read Files or Directories, Modify Files or Directories
Mitigations & Prevention
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 relat
Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.
Detection Methods
- Automated Static Analysis — 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 sea
Real-World CVE Examples
| CVE ID | Description |
|---|---|
| CVE-2002-1987 | Protection mechanism checks for "/.." but doesn't account for Windows-specific "\.." allowing read of arbitrary files. |
| CVE-2005-2142 | Directory traversal vulnerability in FTP server allows remote authenticated attackers to list arbitrary directories via a "\.." sequence in an LS command. |
Related Weaknesses
Taxonomy Mappings
- PLOVER: — '\..\filename' ('leading dot dot backslash')
- Software Fault Patterns: SFP16 — Path Traversal
Frequently Asked Questions
What is CWE-29?
CWE-29 (Path Traversal: '\..\filename') is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Variant-level weakness. The product uses external input to construct a pathname that should be within a restricted directory, but it does not properly neutralize '\..\filename' (leading backslash dot dot) sequences that can...
How can CWE-29 be exploited?
Attackers can exploit CWE-29 (Path Traversal: '\..\filename') to read files or directories, modify files or directories. This weakness is typically introduced during the Implementation phase of software development.
How do I prevent CWE-29?
Key mitigations include: 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 stric
What is the severity of CWE-29?
CWE-29 is classified as a Variant-level weakness (Low-Medium abstraction). It has been observed in 2 real-world CVEs.