Description of problem:

pam_xauth's pam_sm_open_session() calls putenv() to put a string into the
environment, and then frees that string, leaving the environment with a pointer
to free memory.  It also calls pam_putenv(), so the value it sets normally makes
it into the environment anyway, but the presence of the pointer to freed memory
can result in either complete garbage or a reallocated buffer finding its way
back into the environment.  It can also result in the same variable appearing
twice in the environment vector (see below), though the shell generally corrects
this by discarding all but one of the duplicates.

In our case, we noticed that when su called pam_getenvlist, one of the resulting
malloc'd strings was occupying the block freed by pam_xauth.  This meant the
string in question appeared already in the environment; in our case, prior to
another existing value, which meant that both values were exported.  Of course,
this is only one manifestation of the problem; however, we noticed debugging su
that the environment corruption first appeared while pam_open_session() was
running, and temporarily removing the entry in /etc/pam.d/su that invokes
pam_xauth made the problem go away.

We noticed this in pam-0.99.7.1-5.fc7 on i386 F7, but the problem is not
platform specific and CVS shows that the bug has been present in upstream
pam_xauth since at least Linux-PAM 0.80.

This problem was recently fixed upstream; the fix appears in Linux-PAM 0.99.9.


I am giving this severity "high" because it affects privileged code including
suid programs such as "su", and while I can't prove that an attacker can exploit
the bug to scribble on such a program's memory, I also cannot rule out that
possibility.