Name Using Escaped Slashes in Alternate Encoding
Summary This attack targets the use of the backslash in alternate encoding. An attacker can provide a backslash as a leading character and causes a parser to believe that the next character is special. This is called an escape. By using that trick, the attacker tries to exploit alternate ways to encode the same character which leads to filter problems and opens avenues to attack.
Prerequisites The application accepts the backlash character as escape character. The application server does incomplete input data decoding, filtering and validation.
Solutions Verify that the user-supplied data does not use backslash character to escape malicious characters. Assume all input is malicious. Create a white list that defines all valid input to the software system based on the requirements specifications. Input that does not match against the white list should not be permitted to enter into the system. Be aware of the threat of alternative method of data encoding. Regular expressions can be used to filter out backslash. Make sure you decode before filtering and validating the untrusted input data. In the case of path traversals, use the principle of least privilege when determining access rights to file systems. Do not allow users to access directories/files that they should not access. Any security checks should occur after the data has been decoded and validated as correct data format. Do not repeat decoding process, if bad character are left after decoding process, treat the data as suspicious, and fail the validation process. Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.
Related Weaknesses
CWE ID Description
CWE-20 Improper Input Validation
CWE-22 Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
CWE-73 External Control of File Name or Path
CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
CWE-172 Encoding Error
CWE-173 Improper Handling of Alternate Encoding
CWE-180 Incorrect Behavior Order: Validate Before Canonicalize
CWE-181 Incorrect Behavior Order: Validate Before Filter
CWE-697 Insufficient Comparison
CWE-707 Improper Enforcement of Message or Data Structure
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