Red Hat Bugzilla – Bug 1283635
CVE-2015-8239 sudo: Race condition when checking digests in sudoers
Last modified: 2016-11-08 11:20:23 EST
A vulnerability in functionality for adding support of SHA-2 digests along with the command was found. The sudoers plugin performs this digest verification while matching rules, and later independently calls execve() to execute the binary. This results in a race condition if the digest functionality is used as suggested (in fact, the rules are matched before the user is prompted for a password, so there is not negligible time frame to replace the binary from underneath sudo). Versions affected are since 1.8.7.
Created sudo tracking bugs for this issue:
Affects: fedora-all [bug 1283636]
(In reply to Adam Mariš from comment #2)
> Not vulnerable. This issue did not affect the versions of sudo as
> shipped with Red Hat Enterprise Linux 5, 6 and 7.
Not vulnerable in RHEL 7? What about this one:
Thanks, I'll update it.
sudo upstream updated the docs:
There are now additional changes applied upstream to make sudo use fexecve where available, so the program is executed using the same file descriptor that was used to read the file for checksumming purposes.
Note that fexecve documentation explicitly notes that fexecve can only help when file to be executed is in a user writeable directory, but not when the file is also user writeable.
The idea behind fexecve() is to allow the caller to verify (checksum) the
contents of an executable before executing it. Simply opening the file,
checksumming the contents, and then doing an execve(2) would not suffice,
since, between the two steps, the filename, or a directory prefix of the
pathname, could have been exchanged (by, for example, modifying the target
of a symbolic link). fexecve() does not mitigate the problem that the
contents of a file could be changed between the checksumming and the call
to fexecve(); for that, the solution is to ensure that the permissions on
the file prevent it from being modified by malicious users.
This is most likely not going to get any "full" fix upstream other than documenting that it's unsafe to execute user writeable programs even if the digest check is performed.