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PEP 672 -- Unicode-related Security Considerations for Python

PEP:672
Title:Unicode-related Security Considerations for Python
Author:Petr Viktorin <encukou at gmail.com>
Status:Active
Type:Informational
Created:01-Nov-2021
Post-History:01-Nov-2021

Abstract

This document explains possible ways to misuse Unicode to write Python programs that appear to do something else than they actually do.

This document does not give any recommendations and solutions.

Introduction

Unicode [1] is a system for handling all kinds of written language. It aims to allow any character from any human language to be used. Python code may consist of almost all valid Unicode characters. While this allows programmers from all around the world to express themselves, it also allows writing code that is potentially confusing to readers.

It is possible to misuse Python's Unicode-related features to write code that appears to do something else than what it does. Evildoers could take advantage of this to trick code reviewers into accepting malicious code.

The possible issues generally can't be solved in Python itself without excessive restrictions of the language. They should be solved in code edirors and review tools (such as diff displays), by enforcing project-specific policies, and by raising awareness of individual programmers.

This document purposefully does not give any solutions or recommendations: it is rather a list of things to keep in mind.

This document is specific to Python. For general security considerations in Unicode text, see [tr36] and [tr39].

Acknowledgement

Investigation for this document was prompted by CVE-2021-42574 [4], Trojan Source Attacks, reported by Nicholas Boucher and Ross Anderson, which focuses on Bidirectional override characters and homoglyphs in a variety of programming languages.

Confusing Features

This section lists some Unicode-related features that can be surprising or misusable.

ASCII-only Considerations

ASCII is a subset of Unicode, consisting of the most common symbols, numbers, Latin letters and control characters.

While issues with the ASCII character set are generally well understood, the're presented here to help better understanding of the non-ASCII cases.

Confusables and Typos

Some characters look alike. Before the age of computers, many mechanical typewriters lacked the keys for the digits 0 and 1: users typed O (capital o) and l (lowercase L) instead. Human readers could tell them apart by context only. In programming languages, however, distinction between digits and letters is critical -- and most fonts designed for programmers make it easy to tell them apart.

Similarly, in fonts designed for human languages, the uppercase “I” and lowercase “l” can look similar. Or the letters “rn” may be virtually indistinguishable from the single letter “m”. Again, programmers' fonts make these pairs of confusables noticeably different.

However, what is “noticeably” different always depends on the context. Humans tend to ignore details in longer identifiers: the variable name accessibi1ity_options can still look indistinguishable from accessibility_options, while they are distinct for the compiler. The same can be said for plain typos: most humans will not notice the typo in responsbility_chain_delegate.

Control Characters

Python generally considers all CR (\r), LF (\n), and CR-LF pairs (\r\n) as an end of line characters. Most code editors do as well, but there are editors that display “non-native” line endings as unknown characters (or nothing at all), rather than ending the line, displaying this example:

# Don't call this function:
fire_the_missiles()

as a harmless comment like:

# Don't call this function:⬛fire_the_missiles()

CPython may treat the control character NUL (\0) as end of input, but many editors simply skip it, possibly showing code that Python will not run as a regular part of a file.

Some characters can be used to hide/overwrite other characters when source is listed in common terminals. For example:

  • BS (\b, Backspace) moves the cursor back, so the character after it will overwrite the character before.
  • CR (\r, carriage return) moves the cursor to the start of line, subsequent characters overwrite the start of the line.
  • SUB (\x1A, Ctrl+Z) means “End of text” on Windows. Some programs (such as type) ignore the rest of the file after it.
  • ESC (\x1B) commonly initiates escape codes which allow arbitrary control of the terminal.

Confusable Characters in Identifiers

Python is not limited to ASCII. It allows characters of all scripts – Latin letters to ancient Egyptian hieroglyphs – in identifiers (such as variable names). See PEP 3131 for details and rationale. Only “letters and numbers” are allowed, so while γάτα is a valid Python identifier, 🐱 is not. (See Identifiers and keywords [6] for details.)

Non-printing control characters are also not allowed in identifiers.

However, within the allowed set there is a large number of “confusables”. For example, the uppercase versions of the Latin b, Greek β (Beta), and Cyrillic в (Ve) often look identical: B, Β and В, respectively.

This allows identifiers that look the same to humans, but not to Python. For example, all of the following are distinct identifiers:

  • scope (Latin, ASCII-only)
  • scоpe (with a Cyrillic о)
  • scοpe (with a Greek ο)
  • ѕсоре (all Cyrillic letters)

Additionally, some letters can look like non-letters:

  • The letter for the Hawaiian ʻokina looks like an apostrophe; ʻHelloʻ is a Python identifier, not a string.
  • The East Asian word for ten looks like a plus sign, so 十= 10 is a complete Python statement. (The “十” is a word: “ten” rather than “10”.)

Note

The converse also applies – some symbols look like letters – but since Python does not allow arbitrary symbols in identifiers, this is not an issue.

Confusable Digits

Numeric literals in Python only use the ASCII digits 0-9 (and non-digits such as . or e).

However, when numbers are converted from strings, such as in the int and float constructors or by the str.format method, any decimal digit can be used. For example ߅ (NKO DIGIT FIVE) or (TAMIL DIGIT FIVE) work as the digit 5.

Some scripts include digits that look similar to ASCII ones, but have a different value. For example:

>>> int('৪୨')
42
>>> '{٥}'.format('zero', 'one', 'two', 'three', 'four', 'five')
five

Bidirectional Text

Some scripts, such as Hebrew or Arabic, are written right-to-left. Phrases in such scripts interact with nearby text in ways that can be surprising to people who aren't familiar with these writing systems and their computer representation.

The exact process is complicated, and explained in Unicode Standard Annex #9, Unicode Bidirectional Algorithm [2].

Consider the following code, which assigns a 100-character string to the variable s:

s = "X" * 100 #    "X" is assigned

When the X is replaced by the Hebrew letter א, the line becomes:

s = "א" * 100 #    "א" is assigned

This command still assigns a 100-character string to s, but when displayed as general text following the Bidirectional Algorithm (e.g. in a browser), it appears as s = "א" followed by a comment.

Other surprising examples include:

  • In the statement ערך = 23, the variable ערך is set to the integer 23.
  • In the statement قيمة = ערך, the variable قيمة is set to the value of ערך.
  • In the statement قيمة - (ערך ** 2), the value of ערך is squared and then subtracted from قيمة. The opening parenthesis is displayed as ).

Bidirectional Marks, Embeddings, Overrides and Isolates

Default reordering rules do not always yield the intended direction of text, so Unicode provides several ways to alter it.

The most basic are directional marks, which are invisible but affect text as a left-to-right (or right-to-left) character would. Continuing with the s = "X" example above, in the next example the X is replaced by the Latin x followed or preceded by a right-to-left mark (U+200F). This assigns a 200-character string to s (100 copies of x interspersed with 100 invisible marks), but under Unicode rules for general text, it is rendered as s = "x" followed by an ASCII-only comment:

s = "x‏" * 100 #    "‏x" is assigned

The directional embedding, override and isolate characters are also invisible, but affect the ordering of all text after them until either ended by a dedicated character, or until the end of line. (Unicode specifies the effect to last until the end of a “paragraph” (see Unicode Bidirectional Algorithm [2]), but allows tools to interpret newline characters as paragraph ends (see Unicode Newline Guidelines [3]). Most code editors and terminals do so.)

These characters essentially allow arbitrary reordering of the text that follows them. Python only allows them in strings and comments, which does limit their potential (especially in combination with the fact that Python's comments always extend to the end of a line), but it doesn't render them harmless.

Normalizing identifiers

Python strings are collections of Unicode codepoints, not “characters”.

For reasons like compatibility with earlier encodings, Unicode often has several ways to encode what is essentially a single “character”. For example, all are these different ways of writing Å as a Python string, each of which is unequal to the others.

  • "\N{LATIN CAPITAL LETTER A WITH RING ABOVE}" (1 codepoint)
  • "\N{LATIN CAPITAL LETTER A}\N{COMBINING RING ABOVE}" (2 codepoints)
  • "\N{ANGSTROM SIGN}" (1 codepoint, but different)

For another example, the ligature has a dedicated Unicode codepoint, even though it has the same meaning as the two letters fi.

Also, common letters frequently have several distinct variations. Unicode provides them for contexts where the difference has some semantic meaning, like mathematics. For example, some variations of n are:

  • n (LATIN SMALL LETTER N)
  • 𝐧 (MATHEMATICAL BOLD SMALL N)
  • 𝘯 (MATHEMATICAL SANS-SERIF ITALIC SMALL N)
  • (FULLWIDTH LATIN SMALL LETTER N)
  • (SUPERSCRIPT LATIN SMALL LETTER N)

Unicode includes algorithms to normalize variants like these to a single form, and Python identifiers are normalized. (There are several normal forms; Python uses NFKC.)

For example, xn and xⁿ are the same identifier in Python:

>>> xⁿ = 8
>>> xn
8

… as is and fi, and as are the different ways to encode Å.

This normalization applies only to identifiers, however. Functions that treat strings as identifiers, such as getattr, do not perform normalization:

>>> class Test:
...     def finalize(self):
...         print('OK')
...
>>> Test().finalize()
OK
>>> Test().finalize()
OK
>>> getattr(Test(), 'finalize')
Traceback (most recent call last):
  ...
AttributeError: 'Test' object has no attribute 'finalize'

This also applies when importing:

  • import finalization performs normalization, and looks for a file named finalization.py (and other finalization.* files).
  • importlib.import_module("finalization") does not normalize, so it looks for a file named finalization.py.

Some filesystems independently apply normalization and/or case folding. On some systems, finalization.py, finalization.py and FINALIZATION.py are three distinct filenames; on others, some or all of these name the same file.

Source Encoding

The encoding of Python source files is given by a specific regex on the first two lines of a file, as per Encoding declarations [5]. This mechanism is very liberal in what it accepts, and thus easy to obfuscate.

This can be misused in combination with Python-specific special-purpose encodings (see Text Encodings [7]). For example, with encoding: unicode_escape, characters like quotes or braces can be hidden in an (f-)string, with many tools (syntax highlighters, linters, etc.) considering them part of the string. For example:

# For writing Japanese, you don't need an editor that supports
# UTF-8 source encoding: unicode_escape sequences work just as well.

import os

message = '''
This is "Hello World" in Japanese:
\u3053\u3093\u306b\u3061\u306f\u7f8e\u3057\u3044\u4e16\u754c

This runs `echo WHOA` in your shell:
\u0027\u0027\u0027\u002c\u0028\u006f\u0073\u002e
\u0073\u0079\u0073\u0074\u0065\u006d\u0028
\u0027\u0065\u0063\u0068\u006f\u0020\u0057\u0048\u004f\u0041\u0027
\u0029\u0029\u002c\u0027\u0027\u0027
'''

Here, encoding: unicode_escape in the initial comment is an encoding declaration. The unicode_escape encoding instructs Python to treat \u0027 as a single quote (which can start/end a string), \u002c as a comma (punctuator), etc.

Open Issues

We should probably write and publish:

  • Recommendations for Text Editors and Code Tools
  • Recommendations for Programmers and Teams
  • Possible Improvements in Python
Source: https://github.com/python/peps/blob/master/pep-0672.rst