Subject: [ASA-201906-5] pam-u2f: information disclosure

Arch Linux Security Advisory ASA-201906-5
=========================================

Severity: Medium
Date    : 2019-06-07
CVE-ID  : CVE-2019-12209 CVE-2019-12210
Package : pam-u2f
Type    : information disclosure
Remote  : No
Link    : https://security.archlinux.org/AVG-973

Summary
=======

The package pam-u2f before version 1.0.8-2 is vulnerable to information
disclosure.

Resolution
==========

Upgrade to 1.0.8-2.

# pacman -Syu "pam-u2f>=1.0.8-2"

The problems have been fixed upstream in version 1.0.8.

Workaround
==========

A major mitigation for both issues is to remove the `debug` and
`debug_file` options for `pam_u2f.so` in the PAM configuration.
Furthermore enabling the `openasuser` option will mitigate the symlink
attack in CVE-2019-12209.

Description
===========

- CVE-2019-12209 (information disclosure)

A symbolic link attack has been found in pam-u2f before 1.8.0. The file
`$HOME/.config/Yubico/u2f_keys` is blindly followed by the PAM module.
It can be a symlink pointing to an arbitrary file. The PAM module only
rejects non-regular files and files owned by other users than root or
the to-be-authenticated user. Even these checks are only made after
open()'ing the file, which may already trigger certain logic in the
kernel that is otherwise not reachable to regular users.

If the PAM modules' `debug` option is also enabled then most of the
content of the file is written either to stdout, stderr, syslog or to
the defined debug file. Therefore this can pose an information leak to
access e.g.  the contents of /etc/shadow, /root/.bash_history or
similar sensitive files. Furthermore the symlink attack can be used to
use other
users' u2f_keys files in the authentication process.

- CVE-2019-12210 (information disclosure)

A file descriptor leak has been found in pam-u2f before 1.8.0. If the
`debug` and `debug_file` options are set then the opened debug file
will be inherited to the successfully authenticated user's process.
Therefore this user can write further information to it, possibly
filling up a privileged file system or manipulating the information
found in the debug file.
This can leak sensitive information and also, if written to, be used to
fill the disk or plant misinformation.

Impact
======

An authenticated user can access sensitive information via a crafted
symlink or a leaked file descriptor.

References
==========

https://seclists.org/oss-sec/2019/q2/149
https://bugzilla.suse.com/show_bug.cgi?id=1087061
https://github.com/Yubico/pam-u2f/commit/7db3386fcdb454e33a3ea30dcfb8e8960d4c3aa3
https://github.com/Yubico/pam-u2f/commit/18b1914e32b74ff52000f10e97067e841e5fff62
https://security.archlinux.org/CVE-2019-12209
https://security.archlinux.org/CVE-2019-12210