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Date: Thu, 08 Dec 2011 12:47:56 -0700
From: Kurt Seifried <>
CC: Jeff Mitchell <>
Subject: Re: Disputing CVE-2011-4122

On 12/08/2011 07:11 AM, Jeff Mitchell wrote:
> On 12/07/2011 11:26 AM, Kurt Seifried wrote:
>>> One could assume that kcheckpass should do the validation. However, the
>>> PAM documentation makes no mention of what a service name is supposed to
>>> look like, and consequently it must be treated as opaque by the
>>> application code. Therefore all validation must be expected to be done
>>> by the library, and failure to do so must be seen as a bug in the
>>> library exclusively.
>> Can you provide a link to the documentation?
> Thanks,
> Jeff
Looking around I did find:

15.2.1. PAM Service Files

Each PAM-aware application or service has a file within the /etc/pam.d/
directory. Each file within this directory bears the name of the service
for which it controls access.

It is up to the PAM-aware program to define its service name and install
its own PAM configuration file in the /etc/pam.d/ directory. For
example, the login program defines its service name as login and
installs the /etc/pam.d/login PAM configuration file.

so to some degree it is defined: the service name must fit legal file
name constraints, but this means things like length, but on ext4 for
example this means 256 chars max, and only NULL and "/" are disallowed,
to say nothing of other file systems like xfs (any bytes except null)
and Joliet (CDFS, max 64 characters, unicode supported[1])

So perhaps going for a lowest common denominator of common filesystems
you'd expect to find /etc/ on (so ext4, xfs, maybe Joliet for cd based
systems?) as a filter would be appropriate? And poking the PAM people to
refine the specification a little bit? Thoughts or comments anyone?



-Kurt Seifried / Red Hat Security Response Team

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