CWE-390: Detection of Error Condition without Action

docs/Secure-Coding-Guide-for-Python/CWE-703/CWE-390/REAME.md

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+# CWE-390: Detection of Error Condition without Action
+
+Allow exceptions to bubble up and handle exceptions at the right level in the stack.
+
+Each `except` block must ensure that the program continues only with formally specified behavior by either:
+
+* Recovering from the exceptional condition
+* Re-throwing the exception with additional information
+* Throwing custom exception only if it provides a benefit over Python's [Built-in Exceptions](https://docs.python.org/3.9/library/exceptions.html) [Python.org 2022].
+
+Invalid reasons for suppressing exceptions cause:
+
+* Excessive complexity
+* Print statements from lower levels
+* Incomplete trace-logs
+* Excessive logging
+
+Printing the stack trace can reveal details and sensitive data about an application such as the components on use, existing users, and other sensitive information such as keys or passwords, as described in [CWE-209: Generation of Error Message Containing Sensitive Information](https://cwe.mitre.org/data/definitions/209.html) and will not be handled in these examples.
+
+## Non-Compliant Code Example - Bare Exception
+
+In Python `Exception` extends from `BaseException`and a bare `except` will catch everything.
+
+For instance, catching a bare `except` causes a user to be unable to stop a script via `ctrl+c`, due to the base `except` catching all exceptions. In comparison, catching `except Exception` allows a `KeyboardInterrupt` to be the Python interpreter itself or other parts of the code. This is due to `KeyboardInterrupt` extending `BaseException` and not `Exception`.
+
+Note that using `except Exception` is still too broad as per [CWE-755: Improper Handling of Exceptional Conditions](https://github.com/ossf/wg-best-practices-os-developers/blob/main/docs/Secure-Coding-Guide-for-Python/CWE-703/CWE-755/README.md) and that a more specific exception handling is preferred.
+
+The `noncompliant01.py` code demonstrates a bare except on a `ZeroDivisionError` and must be run on the command line in order to experience the issue.
+
+*[noncompliant01.py](noncompliant01.py):*
+
+```py
+# SPDX-FileCopyrightText: OpenSSF project contributors
+# SPDX-License-Identifier: MIT
+""" Non-compliant Code Example """
+
+from time import sleep
+
+
+def exception_example():
+    """Non-compliant Code Example using bare except"""
+    while True:
+        try:
+            sleep(1)
+            _ = 1 / 0
+        except:
+            print("Don't care")
+
+
+#####################
+# exploiting above code example
+#####################
+exception_example()
+```
+
+The `noncompliant01.py` will continue to run when launched via terminal even when using `CTRL+C`.
+
+The process will have to be terminated or killed in order to stop it. A programming IDE will allow stopping the `noncompliant01.py`as IDEs tend to kill the process rather than sending `CTRL+C`.
+
+## Compliant Code Example - Bare Exception
+
+The `compliant01.py` code example can be stopped via `CTRL+C` on the command line as it is catching the self created `ZeroDivisionError` instead of using a bare exception.
+
+*[compliant01.py](compliant01.py):*
+
+```py
+# SPDX-FileCopyrightText: OpenSSF project contributors
+# SPDX-License-Identifier: MIT
+""" Compliant Code Example """
+from time import sleep
+
+
+def exception_example():
+    """Compliant Code Example catching a specific exception"""
+    while True:
+        sleep(1)
+        try:
+            _ = 1 / 0
+        except ZeroDivisionError:
+            print("How is it now?")
+
+
+#####################
+# exploiting above code example
+#####################
+exception_example()
+```
+
+## Exceptions
+
+The following two exceptions, highlighted in [SEI Cert's Oracle Coding Standard for Java](https://wiki.sei.cmu.edu/confluence/display/java/SEI+CERT+Oracle+Coding+Standard+for+Java), are important to understand when to attempt to handle exceptions at the right level in the stack in Python also.
+
+We wrote a code example in Python in order to assist in the understanding of these exceptions [SEI CERT ERR00-J 2025](https://wiki.sei.cmu.edu/confluence/display/java/ERR00-J.+Do+not+suppress+or+ignore+checked+exceptions).
+
+* __ERR00-J-EX0:__ You may suppress exceptions during the release of non-reusable resources, such as closing files, network sockets, or shutting down threads, if they don't affect future program behavior.
+* __ERR00-J-EX1:__ Allow higher-level code to catch and attempt recovery from exceptions. If recovery is not possible, log the exception, add information if needed, and rethrow it.
+
+*[example01.py](example01.py):*
+
+```py
+# SPDX-FileCopyrightText: OpenSSF project contributors
+# SPDX-License-Identifier: MIT
+""" Compliant Code Example """
+from time import sleep
+
+
+def exception_example():
+    """Compliant Code Example catching a specific exception"""
+    while True:
+        sleep(1)
+        try:
+            _ = 1 / 0
+        except ZeroDivisionError:
+            print("How is it now?")
+
+
+#####################
+# exploiting above code example
+#####################
+exception_example()
+```
+
+If recovery remains impossible, wrap the checked exception in an unchecked exception and rethrow it.
+
+## Automated Detection
+
+|Tool|Version|Checker|Description|
+|:----|:----|:----|:----|
+|[Ruff](https://docs.astral.sh/ruff/)|v0.4.5|[bare-except (E722)](https://docs.astral.sh/ruff/rules/bare-except/)|Use lazy % formatting in logging functions|
+|[Pylint](https://pylint.pycqa.org/)|3.2.7|[W0702:bare-except](https://pylint.pycqa.org/en/latest/user_guide/messages/warning/bare-except.html)|No exception type(s) specified|
+|[flake8](https://www.flake8rules.com/)|7.1.1|[E722](https://www.flake8rules.com/rules/E722.html)|do not use bare 'except'|
+
+## Related Guidelines
+
+|||
+|:---|:---|
+|[MITRE](https://github.com/ossf/wg-best-practices-os-developers/tree/main/docs/Secure-Coding-Guide-for-Python)|[CWE-209: Generation of Error Message Containing Sensitive Information](https://cwe.mitre.org/data/definitions/209.html)|
+|[SEI CERT Oracle Coding Standard for Java](https://wiki.sei.cmu.edu/confluence/display/java/SEI+CERT+Oracle+Coding+Standard+for+Java)|[ERR00-J. Do not suppress or ignore checked exceptions](https://wiki.sei.cmu.edu/confluence/display/java/ERR00-J.+Do+not+suppress+or+ignore+checked+exceptions)|
+|[MITRE CWE Base](http://cwe.mitre.org/)|[CWE-703](https://cwe.mitre.org/data/definitions/703.html), Improper Check or Handling of Exceptional Conditions|
+|[MITRE CWE Pillar](http://cwe.mitre.org/)|[CWE-390](http://cwe.mitre.org/data/definitions/390.html), Detection of Error Condition without Action|
+
+## Biblography
+
+|||
+|:---|:---|
+|[[Python.org](https://docs.python.org/3.9/) 2022]| python.org. (2022). Built-in Exceptions [online]. Available from: [https://docs.python.org/3.9/library/exceptions.html](https://docs.python.org/3.9/library/exceptions.html) [accessed 08 February 2023]|
+|[[Bloch 2008](https://wiki.sei.cmu.edu/confluence/display/java/Rule+AA.+References#RuleAA.References-Bloch08)]|Item 62, "Document All Exceptions Thrown by Each Method"|
+|[[Bloch 2008](https://wiki.sei.cmu.edu/confluence/display/java/Rule+AA.+References#RuleAA.References-Bloch08)]| Item 65, "Don't Ignore Exceptions"|
+|[[Goetz](https://wiki.sei.cmu.edu/confluence/display/java/Rule+AA.+References#RuleAA.References-Goetz06)]|Section 5.4, "Blocking and Interruptible Methods"|
+|[SEI CERT ERR00-J 2025]|ERR00-J. Do not suppress or ignore checked exceptions [online]. Available from: [https://wiki.sei.cmu.edu/confluence/display/java/ERR00-J.+Do+not+suppress+or+ignore+checked+exceptions](https://wiki.sei.cmu.edu/confluence/display/java/ERR00-J.+Do+not+suppress+or+ignore+checked+exceptions) [Accessed Februrary 2025]|

docs/Secure-Coding-Guide-for-Python/CWE-703/CWE-390/example01.py

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+# SPDX-FileCopyrightText: OpenSSF project contributors
+# SPDX-License-Identifier: MIT
+"""Code Example"""
+
+
+def slice_cake(cake: int, plates: int) -> float:
+    """Calculates size of each slice per plate for a cake
+    Args:
+        cake (int)  : Size of the cake
+        guests (int): Amount of guests
+    Returns:
+        (float): Size of each slice
+    """
+
+    try:
+        return cake / plates
+    except ZeroDivisionError as zero_division_error:
+        raise ZeroDivisionError(
+            "slice_cake:You got to give me plates"
+        ) from zero_division_error
+
+
+#####################
+# exploiting above code example
+#####################
+slice_cake(cake=100, plates=0)