class Brakeman::CheckExecute

Checks for string interpolation and parameters in calls to Kernel#system, Kernel#exec, Kernel#syscall, and inside backticks.

Examples of command injection vulnerabilities:

system("rf -rf #{params[:file]}")
exec(params[:command])
`unlink #{params[:something}`

Constants

KNOWN_SHELL_COMMANDS

These are common shells that are known to allow the execution of commands via a -c flag. See dash_c_shell_command? for more info.

SAFE_VALUES
SHELLWORDS
SHELL_ESCAPE_MIXIN_METHODS
SHELL_ESCAPE_MODULE_METHODS

Public Instance Methods

run_check() click to toggle source

Check models, controllers, and views for command injection.

# File lib/brakeman/checks/check_execute.rb, line 31
def run_check
  Brakeman.debug "Finding system calls using ``"
  check_for_backticks tracker

  check_open_calls

  Brakeman.debug "Finding other system calls"
  calls = tracker.find_call :targets => [:IO, :Open3, :Kernel, :'POSIX::Spawn', :Process, nil],
    :methods => [:capture2, :capture2e, :capture3, :exec, :pipeline, :pipeline_r,
      :pipeline_rw, :pipeline_start, :pipeline_w, :popen, :popen2, :popen2e,
      :popen3, :spawn, :syscall, :system], :nested => true

  Brakeman.debug "Processing system calls"
  calls.each do |result|
    process_result result
  end
end

Private Instance Methods

check_for_backticks(tracker) click to toggle source

Looks for calls using backticks such as

`rm -rf #{params[:file]}`
# File lib/brakeman/checks/check_execute.rb, line 178
def check_for_backticks tracker
  tracker.find_call(:target => nil, :method => :`).each do |result|
    process_backticks result
  end
end
check_open_calls() click to toggle source
# File lib/brakeman/checks/check_execute.rb, line 133
def check_open_calls
  tracker.find_call(:targets => [nil, :Kernel], :method => :open).each do |result|
    if match = dangerous_open_arg?(result[:call].first_arg)
      warn :result => result,
        :warning_type => "Command Injection",
        :warning_code => :command_injection,
        :message => msg("Possible command injection in ", msg_code("open")),
        :user_input => match,
        :confidence => :high
    end
  end
end
dangerous?(exp) click to toggle source

This method expects a :dstr or :evstr node

# File lib/brakeman/checks/check_execute.rb, line 208
def dangerous? exp
  exp.each_sexp do |e|
    if call? e and e.method == :to_s
      e = e.target
    end

    next if node_type? e, :lit, :str
    next if SAFE_VALUES.include? e
    next if shell_escape? e
    next if temp_file_path? e

    if node_type? e, :if
      # If we're in a conditional, evaluate the `then` and `else` clauses to
      # see if they're dangerous.
      if res = dangerous?(e.sexp_body.sexp_body)
        return res
      end
    elsif node_type? e, :or, :evstr, :dstr
      if res = dangerous?(e)
        return res
      end
    else
      return e
    end
  end

  false
end
dangerous_interp?(exp) click to toggle source
# File lib/brakeman/checks/check_execute.rb, line 237
def dangerous_interp? exp
  match = include_interp? exp
  return unless match
  interp = match.match

  interp.each_sexp do |e|
    if res = dangerous?(e)
      return Match.new(:interp, res)
    end
  end

  false
end
dangerous_open_arg?(exp) click to toggle source
# File lib/brakeman/checks/check_execute.rb, line 164
def dangerous_open_arg? exp
  if string_interp? exp
    # Check for input at start of string
    exp[1] == "" and
      node_type? exp[2], :evstr and
      has_immediate_user_input? exp[2]
  else
    has_immediate_user_input? exp
  end
end
dangerous_string_building?(exp) click to toggle source
# File lib/brakeman/checks/check_execute.rb, line 260
def dangerous_string_building? exp
  if string_building?(exp) && res = dangerous?(exp)
    return Match.new(:interp, res)
  end

  false
end
dash_c_shell_command?(first_arg, second_arg) click to toggle source

@return [Boolean] true iff the command given by `first_arg`, `second_arg`

invokes a new shell process via `<shell_command> -c` (like `bash -c`)
# File lib/brakeman/checks/check_execute.rb, line 126
def dash_c_shell_command?(first_arg, second_arg)
  string?(first_arg) &&
  KNOWN_SHELL_COMMANDS.include?(first_arg.value) &&
  string?(second_arg) &&
  second_arg.value == "-c"
end
include_interp?(exp) click to toggle source

Checks if an expression contains string interpolation.

Returns Match with :interp type if found.

# File lib/brakeman/checks/check_execute.rb, line 254
def include_interp? exp
  @string_interp = false
  process exp
  @string_interp
end
include_user_input?(exp) click to toggle source
Calls superclass method Brakeman::BaseCheck#include_user_input?
# File lib/brakeman/checks/check_execute.rb, line 146
def include_user_input? exp
  if node_type? exp, :arglist, :dstr, :evstr, :dxstr
    exp.each_sexp do |e|
      if res = include_user_input?(e)
        return res
      end
    end

    false
  else
    if shell_escape? exp
      false
    else
      super exp
    end
  end
end
process_backticks(result) click to toggle source

Processes backticks.

# File lib/brakeman/checks/check_execute.rb, line 185
def process_backticks result
  return unless original? result

  exp = result[:call]

  if input = include_user_input?(exp)
    confidence = :high
  elsif input = dangerous?(exp)
    confidence = :medium
  else
    return
  end

  warn :result => result,
    :warning_type => "Command Injection",
    :warning_code => :command_injection,
    :message => "Possible command injection",
    :code => exp,
    :user_input => input,
    :confidence => confidence
end
process_result(result) click to toggle source

Processes results from Tracker#find_call.

# File lib/brakeman/checks/check_execute.rb, line 52
def process_result result
  call = result[:call]
  args = call.arglist
  first_arg = call.first_arg

  case call.method
  when :popen
    # Normally, if we're in a `popen` call, we only are worried about shell
    # injection when the argument is not an array, because array elements
    # are always escaped by Ruby. However, an exception is when the array
    # contains two values are something like "bash -c" because then the third
    # element is effectively the command being run and might be a malicious
    # executable if it comes (partially or fully) from user input.
    if !array?(first_arg)
      failure = include_user_input?(first_arg) ||
                dangerous_interp?(first_arg) ||
                dangerous_string_building?(first_arg)
    elsif dash_c_shell_command?(first_arg[1], first_arg[2])
      failure = include_user_input?(first_arg[3]) ||
                dangerous_interp?(first_arg[3]) ||
                dangerous_string_building?(first_arg[3])
    end
  when :system, :exec
    # Normally, if we're in a `system` or `exec` call, we only are worried
    # about shell injection when there's a single argument, because comma-
    # separated arguments are always escaped by Ruby. However, an exception is
    # when the first two arguments are something like "bash -c" because then
    # the third argument is effectively the command being run and might be
    # a malicious executable if it comes (partially or fully) from user input.
    if dash_c_shell_command?(first_arg, call.second_arg)
      failure = include_user_input?(args[3]) ||
                dangerous_interp?(args[3]) ||
                dangerous_string_building?(args[3])
    else
      failure = include_user_input?(first_arg) ||
                dangerous_interp?(first_arg) ||
                dangerous_string_building?(first_arg)
    end
  when :capture2, :capture2e, :capture3
    # Open3 capture methods can take a :stdin_data argument which is used as the
    # the input to the called command so it is not succeptable to command injection.
    # As such if the last argument is a hash (and therefore execution options) it
    # should be ignored

    args.pop if hash?(args.last) && args.length > 2
    failure = include_user_input?(args) ||
              dangerous_interp?(args) ||
              dangerous_string_building?(args)
  else
    failure = include_user_input?(args) ||
              dangerous_interp?(args) ||
              dangerous_string_building?(args)
  end

  if failure and original? result

    if failure.type == :interp #Not from user input
      confidence = :medium
    else
      confidence = :high
    end

    warn :result => result,
      :warning_type => "Command Injection",
      :warning_code => :command_injection,
      :message => "Possible command injection",
      :code => call,
      :user_input => failure,
      :confidence => confidence
  end
end
shell_escape?(exp) click to toggle source
# File lib/brakeman/checks/check_execute.rb, line 268
def shell_escape? exp
  return false unless call? exp

  if exp.target == SHELLWORDS and SHELL_ESCAPE_MODULE_METHODS.include? exp.method
    true
  elsif SHELL_ESCAPE_MIXIN_METHODS.include?(exp.method)
    true
  else
    false
  end
end