Class: Scryer::Rules::PathTraversalRule

Inherits:
Scryer::Rule show all
Defined in:
lib/scryer/rules/path_traversal_rule.rb

Overview

Flags a filesystem operation (File.join/read/open/new/write/delete, Dir.glob/entries, send_file) where an argument references params directly — without sanitization, ../../etc/passwd-style path segments in the request let an attacker read (or write/delete) files outside whatever directory the code intended to restrict access to.

Constant Summary collapse

DANGEROUS_CALLS = 
{
  "File" => %w[join read open new write delete binread binwrite],
  "Dir" => %w[glob entries]
}.freeze
BARE_METHODS = 
%w[send_file send_data].freeze

Instance Attribute Summary

Attributes inherited from Scryer::Rule

#file, #sexp, #source

Instance Method Summary collapse

Methods inherited from Scryer::Rule

inherited, #initialize

Constructor Details

This class inherits a constructor from Scryer::Rule

Instance Method Details

#scan ⇒ Object 



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# File 'lib/scryer/rules/path_traversal_rule.rb', line 23

def scan
  findings = []

  Ast.each_node(sexp) do |node|
    next unless Ast.tagged?(node, :method_add_arg, :command, :command_call)

    inner = Ast.tagged?(node, :method_add_arg) ? node[1] : node
    receiver_and_name = Ast.call_name(inner)
    next unless receiver_and_name

    receiver, method_name = receiver_and_name
    next unless dangerous_call?(receiver, method_name)

    args = Ast.call_arguments(node)
    # `send_file`/`send_data`'s keyword options (`filename:`, `type:`,
    # `disposition:`) control response metadata (the Content-Disposition
    # filename the browser shows, the content-type header, inline vs.
    # attachment) — none of them ever touch the filesystem. Only the
    # first *positional* argument (the actual path for send_file, the
    # raw bytes for send_data) is ever used as a path, so a `params`
    # reference confined to the keyword-args hash isn't a path
    # traversal risk and would otherwise be a false positive on the
    # extremely common `send_file path, filename: params[:filename]`
    # idiom. This exclusion is scoped to BARE_METHODS specifically —
    # File.join/Dir.glob/etc. never take a keyword-args hash in
    # practice, so they're unaffected.
    path_args =
      if receiver.nil? && BARE_METHODS.include?(method_name)
        args.reject { |a| Ast.tagged?(a, :bare_assoc_hash) }
      else
        args
      end
    next unless path_args.any? { |a| Ast.references_params?(a) && !sanitized_via_basename?(a) }

    line = Ast.line_of(node)
    findings << finding(
      line: line,
      message: "`#{describe_call(receiver, method_name)}` is called with a path/argument " \
                "that references `params` — a value like `../../config/master.key` reaches " \
                "the filesystem unchanged, letting an attacker read or write files outside " \
                "whatever directory this was meant to be scoped to.",
      suggested_fix: "Reduce the input to a safe basename before using it " \
                      "(`File.basename(params[:name])`), and/or verify the resolved path " \
                      "stays inside the intended directory (compare " \
                      "`File.expand_path(...)` against the allowed root) before touching " \
                      "the filesystem."
    )
  end

  findings
end