Ensure that integer overflow is properly handled in order to avoid unexpected behavior. Python data types can be divided into two categories:
Built-in types such as int, float, or complex [Python 2024]. These types are provided by classes and are protected against overflows.
Primitive types share issues known from C , or C++ and appear in Python to:
time.numpy or ctype.Developers should follow the C guidelines when using or interacting wth C type variables.
Using a numpy.int64 can cause an unintentional flip of its sign when reaching the maximum number that can be stored as demonstrated in noncompliant01.py.
""" Non-compliant Code Example """
import numpy
a = numpy.int64(numpy.iinfo(numpy.int64).max)
print(a + 1) # RuntimeWarning and continues
print()
b = numpy.int64(numpy.iinfo(numpy.int64).max + 1) # OverflowError and stops
print(b) # we will never reach this
Adding +1 to 9223372036854775807 results in a negative number -9223372036854775808 and throws a RuntimeWarning but continues.
An attempt to create int from a too big number causes an OverflowError and stops.
[!NOTE] It has been observed that different results may occur depending on the version of
numpy. For reference, we are usingnumpy 1.23.1and Python3.9.12.
The compliant01.py code detects the integer overflow by catching the appropriate Exception.
""" Compliant Code Example """
import warnings
import numpy
warnings.filterwarnings("error")
a = numpy.int64(numpy.iinfo(numpy.int64).max)
with warnings.catch_warnings():
try:
print(a + 1)
except Warning as _:
print("Failed to increment " + str(a) + " due to overflow error")
# RuntimeWarning and continues
try:
b = numpy.int64(numpy.iinfo(numpy.int64).max + 1) # OverflowError and stops
except OverflowError as e:
print("Failed to assign value to B due to overflow error")
The noncompliant02.py example tries to use time.localtime() to get x hours in the future but causes integer overflow as the given Python int is too large to convert to C long. This is possible because time implements C representations of integers with all the security vulnerabilities as if you were using C.
""" Non-compliant Code Example """
import time
def get_time_in_future(hours_in_future):
"""Gets the time n hours in the future"""
currtime = [tm for tm in time.localtime()]
currtime[3] = currtime[3] + hours_in_future
if currtime[3] + hours_in_future > 24:
currtime[3] = currtime[3] - 24
return time.asctime(tuple(currtime)).split(" ")[3]
#####################
# exploiting above code example
#####################
print(get_time_in_future(23**74))
This compliant02.py solution handles OverflowError Exception when a too large value is given to get_time_in_future.
""" Compliant Code Example """
import time
def get_time_in_future(hours_in_future):
"""Gets the time n hours in the future"""
try:
currtime = list(time.localtime())
currtime[3] = currtime[3] + hours_in_future
if currtime[3] + hours_in_future > 24:
currtime[3] = currtime[3] - 24
return time.asctime(tuple(currtime)).split(" ")[3]
except OverflowError as _:
return "Number too large to set time in future " + str(hours_in_future)
#####################
# attempting to exploit above code example
#####################
print(get_time_in_future(23**74))
The noncompliant03.py code example results in a OverflowError: math range error. This is due to math.exp being a C implementation behind the scenes for better performance. So while it returns a Python float it does use C type of variables internally for the calculation in mathmodule.c [cpython 2024].
""" Non-compliant Code Example """
import math
def calculate_exponential_value(number):
"""Return 'E' raised to the power of different numbers:"""
return math.exp(number)
#####################
# attempting to exploit above code example
#####################
print(calculate_exponential_value(1000))
This compliant03.py solution detects the integer overflow by catching the appropriate Exception on overflow:
""" Compliant Code Example """
import math
def calculate_exponential_value(number):
"""Return 'E' raised to the power of different numbers:"""
try:
return math.exp(number)
except OverflowError as _:
return "Number " + str(number) + " caused an integer overflow"
#####################
# attempting to exploit above code example
#####################
print(calculate_exponential_value(710))
| [Python 2024] | Format String Syntax. Available from: https://docs.python.org/3.9/library/stdtypes.html [Accessed 20 June 2024] |
| [cpython 2024] | mathmodule.c. Available from: https://github.com/python/cpython/blob/main/Modules/mathmodule.c) [Accessed 20 June 2024] |