Category Archives: Security

OpenSSH, PAM and user names

FreeBSD just published a security advisory for, amongst other issues, a piece of code in OpenSSH's PAM integration which could allow an attacker to use one user's credentials to impersonate another (original patch here). I would like to clarify two things, one that is already mentioned in the advisory and one that isn't.

The first is that in order to exploit this, the attacker must not only have valid credentials but also first compromise the unprivileged pre-authentication child process through a bug in OpenSSH itself or in a PAM service module.

The second is that this behavior, which is universally referred to in advisories and the trade press as a bug or flaw, is intentional and required by the PAM spec (such as it is). There are multiple legitimate use cases for this, such as:

  • Letting PAM, rather than the application, prompt for a user name; the spec allows passing NULL instead of a user name to pam_start(3), in which case it is the service module's responsibility (in pam_sm_authenticate(3)) to prompt for a user name using pam_get_user(3). Note that OpenSSH does not support this.

  • Mapping multiple users with different identities and credentials in the authentication backend to a single “template” user when the application they need to access does not need to distinguish between them, or when this determination is made through other means (e.g. environment variable, which service modules are allowed to set).

  • Mapping Windows user names (which can contain spaces and non-ASCII characters that would trip up most Unix applications) to Unix user names.

That being said, I do not object to the patch, only to its characterization. Regarding the first issue, it is absolutely correct to consider the unprivileged child as possibly hostile; this is, after all, the entire point of privilege separation. Regarding the second issue, there are other (and probably better) ways to achieve the same result—performing the translation in the identity service, i.e. nsswitch, comes to mind—and the percentage of users affected by the change lies somewhere between zero and negligible.

One could argue that instead of silently ignoring the user name set by PAM, OpenSSH should compare it to the original user name and either emit a warning or drop the connection if it does not match, but that is a design choice which is entirely up to the OpenSSH developers.

SSLv3

UPDATE 2014-10-14 23:40 UTC The details have been published: meet the SSL POODLE attack.

UPDATE 2014-10-15 11:15 UTC Simpler server test method, corrected info about browsers

UPDATE 2014-10-15 16:00 UTC More information about client testing

El Reg posted an article earlier today about a purported flaw in SSL 3.0 which may or may not be real, but it’s been a bad year for SSL, we’re all on edge, and we’d rather be safe than sorry. So let’s take it at face value and see what we can do to protect ourselves. If nothing else, it will force us to inspect our systems and make conscious decisions about their configuration instead of trusting the default settings. What can we do?

The answer is simple: there is no reason to support SSL 3.0 these days. TLS 1.0 is fifteen years old and supported by every browser that matters and over 99% of websites. TLS 1.1 and TLS 1.2 are eight and six years old, respectively, and are supported by the latest versions of all major browsers (except for Safari on Mac OS X 10.8 or older), but are not as widely supported on the server side. So let’s disable SSL 2.0 and 3.0 and make sure that TLS 1.0, 1.1 and 1.2 are enabled.

What to do next

Test your server

The Qualys SSL Labs SSL Server Test analyzes a server and calculates a score based on the list of supported protocols and algorithms, the strength and validity of the server certificate, which mitigation techniques are implemented, and many other factors. It takes a while, but is well worth it. Anything less than a B is a disgrace.

If you’re in a hurry, the following command will attempt to connect to your server using SSL 2.0 or 3.0:

:|openssl s_client -ssl3 -connect www.example.net:443

If the last line it prints is DONE, you have work to do.

Fix your server

Disable SSL 2.0 and 3.0 and enable TLS 1.0, 1.1 and 1.2 and forward secrecy (ephemeral Diffie-Hellman).

For Apache users, the following line goes a long way:

SSLProtocol ALL -SSLv3 -SSLv2

It disables SSL 2.0 and 3.0, but does not modify the algorithm preference list, so your server may still prefer older, weaker ciphers and hashes over more recent, stronger ones. Nor does it enable Forward Secrecy.

The Mozilla wiki has an excellent guide for the most widely used web servers and proxies.

Test your client

The Poodle Test website will show you a picture of a poodle if your browser is vulnerable and a terrier otherwise. It is the easiest, quickest way I know of to test your client.

Qualys SSL Labs also have an SSL Client Test which does much the same for your client as the SSL Server Test does for your server; unfortunately, it is not able to reliably determine whether your browser supports SSL 3.0.

Fix your client

On Windows, use the Advanced tab in the Internet Properties dialog (confusingly not searchable by that name, search for “internet options” or “proxy server” instead) to disable SSL 2.0 and 3.0 for all browsers.

On Linux and BSD:

  • Firefox: open and set security.tls.version.min to 1. You can force this setting for all users by adding lockPref("security.tls.version.min", 1); to your system-wide Mozilla configuration file. Support for SSL 3.0 will be removed in the next release.

  • Chrome: open and select “show advanced settings”. There should be an HTTP/SSL section which lets you disable SSL 3.0 is apparently no way to disable SSL 3.0. Support for SSL 3.0 will be removed in the next release.

I do not have any information about Safari and Opera. Please comment (or email me) if you know how to disable SSL 3.0 in these browsers.

Good luck, and stay safe.

Status crypto cards HOWTO: problems with the card reader (support could be better)

After hours (spread over weeks) I come to the conclusion that there is a lot of potential to improve the documentation of card readers (but I doubt the card reader vendors will do it) and of the pcsc documentation. It is not easy to arrive at a point where you understand everything. The compatibility list does not help much, as the card readers are partly past their end of life and the models which replace them are not listed. Respectively the one I bought does not support all the features I need. I even ported the driver to FreeBSD (not committed, I wanted to test everything first) and a lot of stuff works, but one critical part is that I can not store a certificate on the crypto card as the card reader or the driver  does not support extended APDUs (needed to transfer more than 255 bytes to the card reader).

Well, the status so far:

  • I have a HOWTO what to install to use crypto cards in FreeBSD
  • I have a HOWOT what to install / configure in Windows
  • I have a HOWTO regarding creating keys on a openpgp v2 card and how to use this key with ssh on FreeBSD (or any other unix-like OS which can run pcsc)
  • I have a card reader which does not support extended APDUs
  • I want to make sure what I write in the HOWTOs is also suitable for the use with Windows / PuTTY
  • it seems Windows needs a certificate and not only a key when using the Windows CAPI (using the vendor supplied card reader driver) in PuTTY-CSC (works at work with a USB token)
  • the pcsc pkcs11 Windows DLL is not suitable yet for use on Windows 8 64bit
  • I contacted the card reader vendor if the card reader or the driver is the problem regarding the extended APDUs
  • I found problems in gpg4win / pcsc on Windows 8
  • I have send some money to the developers of gpg4win to support their work (if you use gnupg on Windows, try to send a few units of money to them, the work stagnated as they need to spend their time for paid work)

So either I need a new card reader, or have to wait for an update of the linux driver of the vendor… which probably means it may be a lot faster to buy a new card reader. When looking for one with at least a PIN pad, I either do not find anything which is listed as supported by pcsc on the vendor pages (it is incredible how hard it is to navigate the websites of some companies… a lot of buzzwords but no way to get to the real products), or they only list updated models where I do not know if they will work.

When I have something which works with FreeBSD and Windows, I will publish all the HOWTOs here at once.

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OpenPGP crypto cards ordered

I wrote in a previous blog post that I want to switch to crypto cards for use with ssh and GnuPG. After some research I settled on the OpenPGP cryto cards. I ordered them from kernelconcepts. As soon as they arrive (and I have some free time), I will start to use them and write down how to work with them with FreeBSD.

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Which crypto card to use with FreeBSD (ssh/gpg)

The recent security incident triggered a discussion how to secure ssh/gpg keys.

One way I want to focus on here (because it is the way I want to use at home), is to store the keys on a crypto card. I did some research for suitable crypto cards and found one which is called Feitian PKI Smartcard, and one which is called OpenPGP card. The OpenPGP card also exists in a USB version (basically a small version of the card is already integrated into a small USB card reader).

The Feitian card is reported to be able to handle RSA keys upto 2048 bits. They do not seem to handle DSA (or ECDSA) keys. The smartcard quick starter guide they have  (the Tuning smartcard file system part) tells how to change the parameters of the card to store upto 9 keys on it.

The spec of the OpenPGP card tells that it supports RSA keys upto 3072 bits, but there are reports that it is able to handle RSA keys upto 4096 bits (you need to have at least GPG 2.0.18 to handle that big keys on the crypto card). It looks to me like the card is not handle DSA (or ECDSA) cards. There are only slots for upto 3 keys on it.

If I go this way, I would also need a card reader. It seems a class 3 one (hardware PIN pad and display) would be the most “future-proof” way to go ahead. I found a Reiner SCT cyberJack secoder card reader, which is believed to be supported by OpenSC and seems to be a good balance between cost and features of the Reiner SCT card readers.

If anyone reading this can suggest a better crypto card (keys upto 4096 bits, more than 3 slots, and/or DSA/ECDSA  support), or a better card reader, or has any practical experience with any of those components on FreeBSD, please add a comment.

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Hats off to the people handling the recent security incident

I pull my hat to the people handling the recent security incident on the FreeBSD infrastructure.

Guys:

  • Thanks a lot for the countless hours you invested to find and close the initial attack vector.
  • Thanks a lot for the countless hours you invested to get the machines back to a well known state.
  • Thanks a lot for the countless hours you invested to verify the source repository.
  • Thanks a lot for the countless hours you invested to get back to a trusted package building environment.
  • Thanks a lot for the countless hours you invested to get the “remaining” infrastructure (and everything else I forgot to mention) back into a good state.

Or in short: Thanks a lot for the countless hours you invested to get us from “we’re busted” to “we’re back”.

And last but not least, thanks for the decision to be better safe than sorry regarding our userbase (while it is the only way to handle something like this in a OSS project, I unfortunately think it has to be mentioned instead of taking it as an obvious decision).

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Spawning Fetchmail with a Minimal Environment

I often ran fetchmail in the background, in “daemon mode”, to keep fetching my email from multiple accounts and piping it all through the Sendmail instance running as the local MTA of my laptop.

But I don’t always remember to run fetchmail before launching GNOME or before “polluting” my shell’s environment with dozens of environment variables that may be either useless or even mildly dangerous for a long running process like fetchmail.

So I started making a habit out of starting fetchmail under a relatively minimalistic env(1) invocation like this one:

% env -i LANG='C' LC_ALL='C' \
    HOME="${HOME}" TERM='dumb' \
    MAIL="${MAIL}" TMPDIR="${TMPDIR:-/tmp}" \
    PATH='/bin:/usr/bin:/usr/local/bin' fetchmail -a -K -d 101

The env -i start of this command clears all environment variables from the current shell, and then I copy only a small set of environment values from the current shell to the env-subprocess that fetchmail will run under.

Now I feel much safer about a fetchmail process running hours or even days in the background. Even if I restart my desktop session a few times, or I decide to experiment a bit with alternate environments, I know that fetchmail will be running with a working and very minimal set of environment options.


Posted in Computers, Email, Free software, FreeBSD, GNU/Linux, Linux, Open source, Security, Software Tagged: Computers, Email, Free software, FreeBSD, GNU/Linux, Linux, Open source, Security, Software