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pySCG: addign 2nd 1335 rule doc and code (#879)
* pySCG: adding 2nd 1335 rule doc and code as part of #531 * Update docs/Secure-Coding-Guide-for-Python/CWE-682/CWE-1335/README.md * fixing links --------- Signed-off-by: Helge Wehder <[email protected]> Signed-off-by: myteron <[email protected]> Co-authored-by: Hubert Daniszewski <[email protected]>
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‎docs/Secure-Coding-Guide-for-Python/CWE-682/CWE-1335/README.md

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# CWE-1335: Incorrect Bitwise Shift of Integer
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Ensure to know what bit-wise shift operators do in case you can not avoid them as recommended in *NUM01-J. Do not perform bitwise and arithmetic operations on the same data* [[SEI CERT JAVA 2024]](https://wiki.sei.cmu.edu/confluence/display/java/NUM01-J.+Do+not+perform+bitwise+and+arithmetic+operations+on+the+same+data) and use math instead.
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A need to use bit-wise operations in Python can be due to translations or dealings with `C`, `C++` or `Java`, system libraries, raw binary data, or cryptographic algorithms. Existing Python modules hooking into system `C` libraries for cryptographic functions or math all to avoid the need to implement bit-shifting on a Python level. Bit-shifting can have unexpected outcomes. Python's ctypes module allows integration of `C` based system libraries into Python and direct access to C-type variables that can have different behavior than using high-level Python.
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For the sake of simplicity, we only want to look at whole numbers and only understand the output of provided code examples illustrating Python's behavior.
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## Bit-shifting Positive vs Negative Numbers
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Python tries to avoid issues around signed numbers by storing the sign in a separate data field. Bit-shifting a whole negative number will always remain negative in Python. Positive whole numbers will remain positive in Python or zero. Shifting a positive whole number to the right will never go below zero and shifting a negative number to the left will never go above minus one. Shifting to the left behaves differently between positive and negative numbers.
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[*example01.py:*](example01.py)
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```py
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# SPDX-FileCopyrightText: OpenSSF project contributors
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# SPDX-License-Identifier: MIT
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"""example code"""
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positive_int: int = 1
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print(f"+{positive_int} = ", end="")
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print(positive_int.to_bytes(positive_int.__sizeof__(), byteorder="big", signed=True))
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negative_int: int = -1
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print(f"{negative_int} = ", end="")
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print(negative_int.to_bytes(negative_int.__sizeof__(), byteorder="big", signed=True))
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```
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**Output of example01.py:**
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```bash
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+1 = b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01'
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-1 = b'\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff'
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```
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Code `example01.py` shows that a `-1` is stored as hexadecimal `\xff` or binary `11111111`s. Storing a decimal `-2` ends with Hex `xFE` or binary `1110`. Storing decimal `+1` ends as binary `0001`. Knowing this allows understanding of some of the side effects.
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A right shift `>>=` on a positive number fills zeros from the left and a negative number would fill one's. A continuation of right shifts will reach a point where there is no more change in both cases.
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This behaves differently for a left shift `<<=` operation. As a positive number can reach zero it can remain at zero. A negative whole number can never reach zero and is therefore bound to stay at `-1`.
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[*example02.py:*](example02.py)
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```py
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# SPDX-FileCopyrightText: OpenSSF project contributors
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# SPDX-License-Identifier: MIT
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"""example code"""
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print("\nstay positive")
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positive_int: int = 1
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print(f"+{positive_int} = ", end="")
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print(positive_int.to_bytes(positive_int.__sizeof__(), byteorder="big", signed=True))
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positive_int >>= 1
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positive_int >>= 1
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print(f"+{positive_int} = ", end="")
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print(positive_int.to_bytes(positive_int.__sizeof__(), byteorder="big", signed=True))
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positive_int <<= 1
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positive_int <<= 1
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positive_int <<= 1000000000
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print(f"+{positive_int} = ", end="")
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print(positive_int.to_bytes(positive_int.__sizeof__(), byteorder="big", signed=True))
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print("\nstaying negative")
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negative_int: int = -1
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print(f"{negative_int} = ", end="")
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print(negative_int.to_bytes(negative_int.__sizeof__(), byteorder="big", signed=True))
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negative_int >>= 1
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negative_int >>= 1
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negative_int >>= 1
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print(f"{negative_int} = ", end="")
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print(negative_int.to_bytes(negative_int.__sizeof__(), byteorder="big", signed=True))
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negative_int <<= 1
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print(f"{negative_int} = ", end="")
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print(negative_int.to_bytes(negative_int.__sizeof__(), byteorder="big", signed=True))
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negative_int <<= 1
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print(f"{negative_int} = ", end="")
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print(negative_int.to_bytes(negative_int.__sizeof__(), byteorder="big", signed=True))
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```
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**Output of example02.py:**
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```bash
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stay positive
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+1 = b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01'
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+0 = b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
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+0 = b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
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staying negative
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-1 = b'\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff'
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-1 = b'\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff'
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-2 = b'\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfe'
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-4 = b'\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfc'
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```
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Code `example02.py` illustrates the different behavior of positive and negative whole number's during shifting. A continuation of shifting a positive whole number to the right and back to the left can remain at zero. This is not true for a negative whole number as they can only reach `-1` and never zero. Moving a negative number back and forth can continue to change.
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## Overflow Protection
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Continuously shifting a Python `int`class variable will change the storage size while keeping the variable type as class int as demonstrated in the following code `example03.py`:
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[*example03.py:*](example03.py)
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```py
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# SPDX-FileCopyrightText: OpenSSF project contributors
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# SPDX-License-Identifier: MIT
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"""example code"""
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positive_int: int = 1
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print(f"Before: {type(positive_int)}")
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print(positive_int.to_bytes(positive_int.__sizeof__(), byteorder="big", signed=True))
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print("\nShifting 1000x\n")
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positive_int <<= 1000
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print(f"After: {type(positive_int)}")
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print(positive_int.to_bytes(positive_int.__sizeof__(), byteorder="big", signed=True))
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```
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**Output of example03.py:**
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```bash
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Before: <class 'int'>
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b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01'
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Shifting 1000x
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After: <class 'int'>
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b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
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```
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Code `example03.py` demonstrates how Python is changing required storage in the background while keeping the class of type `int`. While this does prevent overflow in Python it can have some unexpected issues interacting with other languages. A mistake in a loop can cause Python to just continue to eat up memory while finding a natural ending in other languages.
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## Non-Compliant Code Example
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Method `shift_right` in `noncompliant01.py` can reach zero for positive numbers but loops forever on negative inputs.
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*[noncompliant01.py](noncompliant01.py):*
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```python
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# SPDX-FileCopyrightText: OpenSSF project contributors
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# SPDX-License-Identifier: MIT
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"""Non-compliant Code Example."""
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from time import sleep
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def shift_right(value: int) -> int:
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while (value != 0):
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print(f"{value}", flush=True)
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value >>= 1
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sleep(1)
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return value
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value = shift_right(10)
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print("Returned", value)
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shift_right(-10)
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print("Will never reach here")
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```
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## Compliant Solution
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Bit-shifting is an optimization pattern that works better for languages closer to the CPU than Python. Math in Python is better done by arithmetical functions in Python as stated by *CWE-1335: Promote readability and compatibility by using mathematical written code with arithmetic operations instead of bit-wise operations* [[OpenSSF Secure Coding in Python 2025]](https://github.com/ossf/wg-best-practices-os-developers/blob/main/docs/Secure-Coding-Guide-for-Python/CWE-682/CWE-1335/01/README.md).
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Understanding `ctypes` or `C` requires understanding the *CERT C Coding Standard* [[SEI CERT C 2025]](https://www.securecoding.cert.org/confluence/display/seccode/CERT+C+Coding+Standard)and setting boundaries manually in Python.
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## Automated Detection
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Not available
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## Related Guidelines
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<table>
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<tr>
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<td>
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<a href="https://github.com/ossf/wg-best-practices-os-developers/tree/main/docs/Secure-Coding-Guide-for-Python">[OpenSSF Secure Coding in Python 2025]</a>
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</td>
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<td>
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<a href="https://github.com/ossf/wg-best-practices-os-developers/blob/main/docs/Secure-Coding-Guide-for-Python/CWE-682/CWE-1335/01/README.md">CWE-1335: Promote readability and compatibility by using mathematical written code with arithmetic operations instead of bit-wise operations</a>
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</td>
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</tr>
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<tr>
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<tr>
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<td>
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<a href="http://cwe.mitre.org/">MITRE CWE</a>
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</td>
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<td>
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Pillar: <a href="https://cwe.mitre.org/data/definitions/682.html"> [CWE-682: Incorrect Calculation]</a>
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</td>
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</tr>
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<tr>
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<td>
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<a href="http://cwe.mitre.org/">MITRE CWE</a>
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</td>
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<td>
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Base: <a href="https://cwe.mitre.org/data/definitions/1335.html">[CWE-1335: Incorrect Bitwise Shift of Integer (4.12)]</a>
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</td>
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</tr>
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<tr>
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<td>
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<a href="https://wiki.sei.cmu.edu/confluence/display/java/SEI+CERT+Oracle+Coding+Standard+for+Java">[SEI CERT Oracle Coding Standard for Java]</a>
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</td>
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<td>
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<a href="https://wiki.sei.cmu.edu/confluence/display/java/NUM14-J.+Use+shift+operators+correctly">[NUM14-J. Use shift operators correctly]</a>
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</td>
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</tr>
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<tr>
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<td>
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<a href="https://www.securecoding.cert.org/confluence/display/seccode/CERT+C+Coding+Standard">[CERT C Coding Standard]</a>
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</td>
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<td>
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<a href="https://wiki.sei.cmu.edu/confluence/display/c/INT34-C.+Do+not+shift+an+expression+by+a+negative+number+of+bits+or+by+greater+than+or+equal+to+the+number+of+bits+that+exist+in+the+operand">[INT34-C. Do not shift an expression by a negative number of bits or by greater than or equal to the number of
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bits that exist in the operand]</a>
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</td>
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</tr>
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</table>
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## Bibliography
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<table>
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<tr>
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<td>
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[SEI CERT JAVA 2024]
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</td>
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<td>
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NUM01-J. Do not perform bitwise and arithmetic operations on the same data [online]. Available from: <a href="https://wiki.sei.cmu.edu/confluence/display/java/NUM01-J.+Do+not+perform+bitwise+and+arithmetic+operations+on+the+same+data">https://wiki.sei.cmu.edu/confluence/display/java/NUM01-J.+Do+not+perform+bitwise+and+arithmetic+operations+on+the+same+data</a>, [Accessed 6 May 2025]
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</td>
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</tr>
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<tr>
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<td>
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[SEI CERT C 2025]
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</td>
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<td>
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CERT C Coding Standard [online]. Available from: <a href=https://www.securecoding.cert.org/confluence/display/seccode/CERT+C+Coding+Standard>https://www.securecoding.cert.org/confluence/display/seccode/CERT+C+Coding+Standard</a> [Accessed 6 May 2025]
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</td>
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</tr>
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<table>

‎docs/Secure-Coding-Guide-for-Python/CWE-682/CWE-1335/example01.py

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# SPDX-FileCopyrightText: OpenSSF project contributors
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# SPDX-License-Identifier: MIT
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"""example code"""
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positive_int: int = 1
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print(f"+{positive_int} = ", end="")
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print(positive_int.to_bytes(positive_int.__sizeof__(), byteorder="big", signed=True))
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negative_int: int = -1
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print(f"{negative_int} = ", end="")
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print(negative_int.to_bytes(negative_int.__sizeof__(), byteorder="big", signed=True))

‎docs/Secure-Coding-Guide-for-Python/CWE-682/CWE-1335/example02.py

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# SPDX-FileCopyrightText: OpenSSF project contributors
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# SPDX-License-Identifier: MIT
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"""example code"""
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print("\nstay positive")
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positive_int: int = 1
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print(f"+{positive_int} = ", end="")
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print(positive_int.to_bytes(positive_int.__sizeof__(), byteorder="big", signed=True))
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positive_int >>= 1
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positive_int >>= 1
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print(f"+{positive_int} = ", end="")
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print(positive_int.to_bytes(positive_int.__sizeof__(), byteorder="big", signed=True))
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positive_int <<= 1
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positive_int <<= 1
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positive_int <<= 1000000000
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print(f"+{positive_int} = ", end="")
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print(positive_int.to_bytes(positive_int.__sizeof__(), byteorder="big", signed=True))
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print("\nstaying negative")
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negative_int: int = -1
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print(f"{negative_int} = ", end="")
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print(negative_int.to_bytes(negative_int.__sizeof__(), byteorder="big", signed=True))
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negative_int >>= 1
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negative_int >>= 1
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negative_int >>= 1
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print(f"{negative_int} = ", end="")
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print(negative_int.to_bytes(negative_int.__sizeof__(), byteorder="big", signed=True))
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negative_int <<= 1
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print(f"{negative_int} = ", end="")
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print(negative_int.to_bytes(negative_int.__sizeof__(), byteorder="big", signed=True))
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negative_int <<= 1
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print(f"{negative_int} = ", end="")
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print(negative_int.to_bytes(negative_int.__sizeof__(), byteorder="big", signed=True))

‎docs/Secure-Coding-Guide-for-Python/CWE-682/CWE-1335/example03.py

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# SPDX-FileCopyrightText: OpenSSF project contributors
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# SPDX-License-Identifier: MIT
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"""example code"""
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positive_int: int = 1
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print(f"Before: {type(positive_int)}")
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print(positive_int.to_bytes(positive_int.__sizeof__(), byteorder="big", signed=True))
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print("\nShifting 1000x\n")
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positive_int <<= 1000
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print(f"After: {type(positive_int)}")
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print(positive_int.to_bytes(positive_int.__sizeof__(), byteorder="big", signed=True))

‎docs/Secure-Coding-Guide-for-Python/CWE-682/CWE-1335/noncompliant01.py

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# SPDX-FileCopyrightText: OpenSSF project contributors
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# SPDX-License-Identifier: MIT
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"""Non-compliant Code Example."""
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from time import sleep
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def shift_right(value: int) -> int:
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while (value != 0):
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print(f"{value}", flush=True)
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value >>= 1
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sleep(1)
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return value
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value = shift_right(10)
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print("Returned", value)
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shift_right(-10)
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print("Will never reach here")

‎docs/Secure-Coding-Guide-for-Python/readme.md

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|[CWE-682: Incorrect Calculation](https://cwe.mitre.org/data/definitions/682.html)|Prominent CVE|
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|:---------------------------------------------------------------------------------------------------------------|:----|
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|[CWE-191: Integer Underflow (Wrap or Wraparound)](CWE-682/CWE-191/README.md)||
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|[# CWE-1335: Incorrect Bitwise Shift of Integer](CWE-682/CWE-1335/README.md)||
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|[CWE-1335: Promote readability and compatibility by using mathematical written code with arithmetic operations instead of bit-wise operations](CWE-682/CWE-1335/01/README.md)||
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|[CWE-1339: Insufficient Precision or Accuracy of a Real Number](CWE-682/CWE-1339/.) ||
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