|
| 1 | +# CWE-252: Unchecked Return Value |
| 2 | + |
| 3 | +Return values of methods and functions should always be checked to ensure operations have been performed correctly. |
| 4 | + |
| 5 | +When immutable objects are used, methods that aim to modify them have to create a new object with the desired changed and return it. For the results of such methods to take place, the developer must remember to assign the new value to a variable, otherwise it won't be accessible. They can also be used to handle unexpected behaviors by returning specific values (such as `None` or a other default values) that may require additional safety checks. |
| 6 | + |
| 7 | +## Non-Compliant Code Example - Immutable objects |
| 8 | + |
| 9 | +This non-compliant code example shows a common mistake when trying to update an immutable object. Since `str` is an immutable type, `str.replace()` creates a new `str` object with the desired change [[Python Docs - str.replace](https://docs.python.org/3.9/library/stdtypes.html#str.replace)]. This object must be then assigned, typically in place of the original string. If not, the new value remains unused. |
| 10 | + |
| 11 | +*[noncompliant01.py](noncompliant01.py):* |
| 12 | + |
| 13 | +```python |
| 14 | +# SPDX-FileCopyrightText: OpenSSF project contributors |
| 15 | +# SPDX-License-Identifier: MIT |
| 16 | +""" Non-compliant Code Example """ |
| 17 | + |
| 18 | + |
| 19 | +def sanitize_string(user_input): |
| 20 | + """Function that ensure a given string is safe""" |
| 21 | + user_input.replace("un", "very ") |
| 22 | + |
| 23 | + |
| 24 | +my_string = "unsafe string" |
| 25 | +sanitize_string(my_string) |
| 26 | + |
| 27 | +##################### |
| 28 | +# exploiting above code example |
| 29 | +##################### |
| 30 | +print(my_string) |
| 31 | + |
| 32 | +``` |
| 33 | + |
| 34 | +Despite calling `sanitize_string()`, the value of my_string remains unchanged because the return value of str.replace() has been ignored. |
| 35 | + |
| 36 | +## Compliant Solution - Immutable objects |
| 37 | + |
| 38 | +This compliant solution correctly returns the value from `str.replace()` and assigns it to `my_string`: |
| 39 | + |
| 40 | +*[compliant01.py](compliant01.py):* |
| 41 | + |
| 42 | +```python |
| 43 | +# SPDX-FileCopyrightText: OpenSSF project contributors |
| 44 | +# SPDX-License-Identifier: MIT |
| 45 | +""" Compliant Code Example """ |
| 46 | + |
| 47 | + |
| 48 | +def sanitize_string(user_input): |
| 49 | + """Function that ensure a given string is safe""" |
| 50 | + return user_input.replace("un", "very ") |
| 51 | + |
| 52 | + |
| 53 | +my_string = "unsafe string" |
| 54 | +my_string = sanitize_string(my_string) |
| 55 | + |
| 56 | +##################### |
| 57 | +# exploiting above code example |
| 58 | +##################### |
| 59 | +print(my_string) |
| 60 | + |
| 61 | +``` |
| 62 | + |
| 63 | +## Non-Compliant Code Example - Invalid value handling |
| 64 | + |
| 65 | +Return values are also important when they may be used as an alternative to raising exceptions. `str.find()`, unlike `str.index()` return -1 [[Python Docs - str.find](https://docs.python.org/3/library/stdtypes.html#str.find)] instead of raising a `ValueError` [[Python Docs - str.index](https://docs.python.org/3/library/stdtypes.html#str.index)] when it cannot find the given sub-string. |
| 66 | +This non-compliant code example shows that using this value will point to the last element of the string regardless of what it is. |
| 67 | + |
| 68 | +*[noncompliant02.py](noncompliant02.py):* |
| 69 | + |
| 70 | +```python |
| 71 | +# SPDX-FileCopyrightText: OpenSSF project contributors |
| 72 | +# SPDX-License-Identifier: MIT |
| 73 | +""" Non-compliant Code Example """ |
| 74 | + |
| 75 | + |
| 76 | +def find_in_string(full_string, sub_string): |
| 77 | + """Function that searches for a sub-string in a given string""" |
| 78 | + index = full_string.find(sub_string) |
| 79 | + print(f"""Sub-string '{sub_string}' appears for the first time in |
| 80 | + '{full_string}' at index {index}: '{full_string[index:]}'""") |
| 81 | + |
| 82 | + |
| 83 | +##################### |
| 84 | +# exploiting above code example |
| 85 | +##################### |
| 86 | +my_string = "Secure Python coding" |
| 87 | +find_in_string(my_string, "Python") |
| 88 | +find_in_string(my_string, "I'm evil") |
| 89 | + |
| 90 | +``` |
| 91 | + |
| 92 | +Even though `I'm evil` is clearly not a part of Secure Python coding, the `find_in_string()` method will suggest otherwise. |
| 93 | + |
| 94 | +## Compliant Solution - Invalid value handling |
| 95 | + |
| 96 | +Since `str.find()` indicates the fact that the sub-string couldn't be found with a negative index, a simple `if` check is enough to tackle the issue from the previous code example. |
| 97 | + |
| 98 | +*[compliant02.py](compliant02.py):* |
| 99 | + |
| 100 | +```python |
| 101 | +# SPDX-FileCopyrightText: OpenSSF project contributors |
| 102 | +# SPDX-License-Identifier: MIT |
| 103 | +""" Non-compliant Code Example """ |
| 104 | + |
| 105 | + |
| 106 | +def find_in_string(full_string, sub_string): |
| 107 | + """Function that searches for a sub-string in a given string""" |
| 108 | + index = full_string.find(sub_string) |
| 109 | + if index >= 0: |
| 110 | + print(f"""Sub-string '{sub_string}' appears for the first time in |
| 111 | + '{full_string}' at index {index}: '{full_string[index:]}'""") |
| 112 | + else: |
| 113 | + print(f"There is no '{sub_string}' in '{full_string}'") |
| 114 | + |
| 115 | + |
| 116 | +##################### |
| 117 | +# exploiting above code example |
| 118 | +##################### |
| 119 | +my_string = "Secure Python coding" |
| 120 | +find_in_string(my_string, "Python") |
| 121 | +find_in_string(my_string, "I'm evil") |
| 122 | + |
| 123 | +``` |
| 124 | + |
| 125 | +Now, the latter print will correctly indicate the lack of `I'm evil` in `Secure Python coding`. |
| 126 | + |
| 127 | +## Automated Detection |
| 128 | + |
| 129 | +|Tool|Version|Checker|Description| |
| 130 | +|:---|:---|:---|:---| |
| 131 | +|Bandit|1.7.4 on Python 3.10.4|Not Available|| |
| 132 | +|Flake8|8-4.0.1 on Python 3.10.4|Not Available|| |
| 133 | + |
| 134 | +## Related Guidelines |
| 135 | + |
| 136 | +||| |
| 137 | +|:---|:---| |
| 138 | +|[MITRE CWE](http://cwe.mitre.org/)|Pillar: [CWE-703: Improper Check or Handling of Exceptional Conditions (4.13) (mitre.org)](https://cwe.mitre.org/data/definitions/703.html)| |
| 139 | +|[MITRE CWE](http://cwe.mitre.org/)|Base: [CWE-252: Unchecked Return Value](https://cwe.mitre.org/data/definitions/252.html)| |
| 140 | +|[SEI CERT Coding Standard for Java](https://wiki.sei.cmu.edu/confluence/display/java/SEI+CERT+Oracle+Coding+Standard+for+Java)|[EXP00-J. Do not ignore values returned by methods](https://wiki.sei.cmu.edu/confluence/display/java/EXP00-J.+Do+not+ignore+values+returned+by+methods)| |
| 141 | +|[SEI CERT C Coding Standard](https://wiki.sei.cmu.edu/confluence/display/c/SEI+CERT+C+Coding+Standard)|[EXP12-C. Do not ignore values returned by functions](https://wiki.sei.cmu.edu/confluence/display/c/EXP12-C.+Do+not+ignore+values+returned+by+functions)| |
| 142 | +|ISO/IEC TR 24772:2019|Passing Parameters and Return Values [CSJ]| |
| 143 | + |
| 144 | +## Bibliography |
| 145 | + |
| 146 | +||| |
| 147 | +|:---|:---| |
| 148 | +|[[Python Docs - str.replace](https://docs.python.org/3.9/library/stdtypes.html#str.replace)]<br>[[Python Docs - str.find](https://docs.python.org/3/library/stdtypes.html#str.find)]<br>[[Python Docs - str.index](https://docs.python.org/3/library/stdtypes.html#str.index)]|Python Software Foundation. (2025). Built-in Types [online]. Available from: [https://docs.python.org/3.9/library/stdtypes.html](https://docs.python.org/3.9/library/stdtypes.html) [accessed 17 June 2025] | |
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