==========================
logging-formatter-anticrlf
Python logging Formatter for CRLF Injection (CWE-93 / CWE-117) prevention
logging Formatter to sanitize CRLF errors (CWE-93, some forms of CWE-117)
This class is a drop-in replacement for logging.Formatter
, and has the
exact same construction arguments. However, as a final step of formatting a
log line, it escapes carriage returns (\r) and linefeeds (\n).
By default, these are replaced with their escaped equivalents (see Examples
_),
but the replacements
dictionary can be modified to change this behavior.
This sanitization should solve CWE-93 errors and CRLF-based versions of
CWE-117. Some CWE-117 errors are concerns about e.g. XSS flaws in logs that
are likely to be viewed in a browser; this formatter can't handle every
form of CWE-117.
Installation
::
pip install logging-formatter-anticrlf
Examples
::
import sys
import logging
import anticrlf
handler = logging.StreamHandler(sys.stderr)
handler.setFormatter(anticrlf.LogFormatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s'))
logger = logging.getLogger(__name__)
logger.addHandler(handler)
logger.setLevel(logging.INFO)
logger.info("Example text with a newline\nhere")
This results in::
2017-02-03 08:43:52,557 - __main__ - INFO - Example text with a newline\nhere
Whereas with the default Formatter
, it would be::
2017-02-03 08:43:52,557 - __main__ - INFO - Example text with a newline
here
If you wanted newlines to be replaced with \x0A instead, you could::
formatter = anticrlf.LogFormatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')
formatter.replacements["\n"] = "\\x0A" # Note the double backslash for literal!
handler.setFormatter(formatter)
Changing Replacements
The replacements
field of the formatter is a SubstitutionMap
object that behaves
like a dict
with a few exceptions designed to help developers avoid making insecure mistakes.
Specifically:
- an "empty"
SubstitutionMap
object will contain the default mappings for CR and LF chars - calling
del
on either the CR or LF key will reset the value rather than delete the key - any attempt to create a key-value pair that results in any value containing any of the keys
will raise an
UnsafeSubstitutionError
The rationale for the last item is that the keys of the replacements
field are strings
that are considered unsafe. Replacing one unsafe string with another defeats the purpose of
using this module.
Additionally, if you assign a regular dict
to the replacements
field, and try to log
something using that configuration, anticrlf.LogFormatter
will reset the replacements
field to its default value and issue a UserWarning
to that effect.
That means the following::
formatter.replacements["\n"] = "\\x0A" # replace LF chars with '\x0A'
del formatter.replacements["\n"] # return to replacing LF with '\n'
formatter.replacements["\t"] = "\\t" # replace tabs with '\t'
formatter.replacements["\n"] = "<\t>" # raises UnsafeSubstitutionError
The last occurs because the value <\t>
contains \t
, which was previously created as a key.
And::
formatter.replacements = { "\n": "\r" } # this is a mistake!
logger.info("example")
Will result, if that logger is using that formatter, in replacements
being returned to its
safe default value and a UserWarning
about that being issued.