Path Traversal: '....//'
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 '....//' (doubled dot dot slash) 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. The '....//' manipulation is useful for bypassing some path traversal protection schemes. If "../" is filtered in a sequential fashion, as done by some regular expression engines, then "....//" can collapse into the "../" unsafe value (CWE-182). It could also be useful when ".." is removed, if the operating system treats "//" and "/" as equivalent.
Common Consequences 1
Scope: ConfidentialityIntegrity
Impact: Read Files or DirectoriesModify Files or Directories
Detection Methods 2
Automated Static Analysis - Source CodeSOAR Partial
According to SOAR [REF-1479], the following detection techniques may be useful:
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Cost effective for partial coverage:
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Source code Weakness Analyzer
Context-configured Source Code Weakness AnalyzerArchitecture or Design ReviewHigh
According to SOAR [REF-1479], the following detection techniques may be useful:
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Highly cost effective:
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Inspection (IEEE 1028 standard) (can apply to requirements, design, source code, etc.)
Formal Methods / Correct-By-ConstructionPotential 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 1
CVE-2004-1670Mail server allows remote attackers to create arbitrary directories via a ".." or rename arbitrary files via a "....//" in user supplied parameters.
References 1
State-of-the-Art Resources (SOAR) for Software Vulnerability Detection, Test, and Evaluation
Gregory Larsen, E. Kenneth Hong Fong, David A. Wheeler, and Rama S. Moorthy
07-2014
ID: REF-1479
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
Notes
RelationshipThis could occur due to a cleansing error that removes a single "../" from "....//"