Thanks for your report.

In general:

The last field on the line is special: adding colons to it does not add any field splitting ambiguity.  libuser therefore explicitly allows ':' in the last field.

glibc's parsers do not recognize the ':' as a separator in that case (in particular, you can have a ':' in user's shell).  On the other hand, shadow-utils's parsers do recognize the ':' as terminating even the last field on the line, so the situation is ambiguous.

Therefore, libuser will always differ from one of glibc and shadow-utils.

Given that the field splitting does not have any security impact, and the file format is not really strictly defined, I think leaving the existing code is fine.  We can't promise that it will work, but we don't have a really good reason to prohibit it, either.


In this specific case:
As mentioned above, glibc will parse those lines as an user with shell "a:b" and "c:d", respectively.  

Please reopen this if I have overlooked a reason to prohibit the ':' in the last field.