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Description of problem:sss allows extraneous @ characters at front of username

Custy has opened a upstream ticket as well: https://github.com/SSSD/sssd/issues/6635

Version-Release number of selected component (if applicable):
sssd-client-2.7.3-4.el8_7.3.x86_64 


How reproducible:
sss seems to ignore leading @ characters when looking up a username. For example:
# getent passwd '@test.a'
test.a:x:1000:1000:test admin service account:/home/test.a:/bin/bash

The username is 'test.a', not '@test.a'. It doesn't seem to matter how many @ signs there are:

# getent passwd '@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@test.a'
test.a:x:1000:1000:test admin service account:/home/test.a:/bin/bash

If I update nsswitch.conf to remove sss from the passwd line this behavior goes away.

When logging in, not only is this invalid username successfully authenticated, it is passed to other pam modules:


Steps to Reproduce:


Actual results:


Expected results:


Additional info:

Default re_expressions does not use "^" so they may skip/ignore some leading character (@ and \)
Changing

#define SSS_DEFAULT_RE "(?P<name>[^@]+)@?(?P<domain>[^@]*$)"
to

#define SSS_DEFAULT_RE "^(?P<name>[^@]+)@?(?P<domain>[^@]*$)"
and similar change for SSS_IPA_AD_DEFAULT_RE will solve the issue. It will make SSSD more strict about input values.

Consider that this regular expression is used also for group names and MS Windows group name can actually include @

though the possible solution may be :  re_expression parameter to "^(?P[^@]+)@?(?P[^@]*$)" and that seems to work, leading @'s result in username not found.

*** This bug has been marked as a duplicate of bug 2180981 ***