[Dovecot] limiting number of incorrect logins per connection
Hi Guys,
Running Dovecot 2 on my server. It is regularly getting dictionary auth attacked. What I have noticed is that once connected to a pop3/imap login session, you can send endless incorrect usernames+passwords attempts. This is a problem for me... I use fail2ban to try and stop these script kiddies. The problem is that fail2ban detects the bad auths, firewalls the IP, however, since it's an "established" session, the attacker can keep authing away... It's only on a subsequent (new) connection that the firewalling will take effect.
Why is there no configuration option such as "max auth attempts per connection"? This would be useful, so once the limit is reached, the connection is dropped.
is there a patch/workaround?
Am 26.08.2011 09:25, schrieb Alex:
Hi Guys,
Running Dovecot 2 on my server. It is regularly getting dictionary auth attacked. What I have noticed is that once connected to a pop3/imap login session, you can send endless incorrect usernames+passwords attempts. This is a problem for me... I use fail2ban to try and stop these script kiddies. The problem is that fail2ban detects the bad auths, firewalls the IP, however, since it's an "established" session, the attacker can keep authing away... It's only on a subsequent (new) connection that the firewalling will take effect.
Why is there no configuration option such as "max auth attempts per connection"? This would be useful, so once the limit is reached, the connection is dropped.
is there a patch/workaround?
there where equal questions in the past i.e read http://comments.gmane.org/gmane.mail.imap.dovecot/46204
-- Best Regards
MfG Robert Schetterer
Germany/Munich/Bavaria
On 26.8.2011, at 10.25, Alex wrote:
Running Dovecot 2 on my server. It is regularly getting dictionary auth attacked. What I have noticed is that once connected to a pop3/imap login session, you can send endless incorrect usernames+passwords attempts. This is a problem for me... I use fail2ban to try and stop these script kiddies. The problem is that fail2ban detects the bad auths, firewalls the IP, however, since it's an "established" session, the attacker can keep authing away... It's only on a subsequent (new) connection that the firewalling will take effect.
Umm. If client hasn't managed to log in in 3 minutes, it's disconnected (no matter what it does with the connection).
3 minutes! I think that's too long, how can I drop that down to about 45 seconds?
On Fri, 26 Aug 2011 11:44:45 +0300, Timo Sirainen wrote:
On 26.8.2011, at 10.25, Alex wrote:
Running Dovecot 2 on my server. It is regularly getting dictionary auth attacked. What I have noticed is that once connected to a pop3/imap login session, you can send endless incorrect usernames+passwords attempts. This is a problem for me... I use fail2ban to try and stop these script kiddies. The problem is that fail2ban detects the bad auths, firewalls the IP, however, since it's an "established" session, the attacker can keep authing away... It's only on a subsequent (new) connection that the firewalling will take effect.
Umm. If client hasn't managed to log in in 3 minutes, it's disconnected (no matter what it does with the connection).
I am happy to recompile if there is no config option. I gather it's in the src/auth dir somewhere in one of the C source files. Just need to be pointed in the right dir.
On Fri, 26 Aug 2011 19:07:08 +1000, Alex wrote:
3 minutes! I think that's too long, how can I drop that down to about 45 seconds?
On Fri, 26 Aug 2011 11:44:45 +0300, Timo Sirainen wrote:
On 26.8.2011, at 10.25, Alex wrote:
Running Dovecot 2 on my server. It is regularly getting dictionary auth attacked. What I have noticed is that once connected to a pop3/imap login session, you can send endless incorrect usernames+passwords attempts. This is a problem for me... I use fail2ban to try and stop these script kiddies. The problem is that fail2ban detects the bad auths, firewalls the IP, however, since it's an "established" session, the attacker can keep authing away... It's only on a subsequent (new) connection that the firewalling will take effect.
Umm. If client hasn't managed to log in in 3 minutes, it's disconnected (no matter what it does with the connection).
Alex, I've not personally done it (so just speculating here, bear with me) but you can customize Fail2Ban's actions if needed. So, if you can match the attemps through some regex (and since you're seeing them in the logs, that should be quite possible), then you can edit one of the 'actions' to drop the connection for <ip>.
I'm just not entirely sure that iptables (or pf, or whatever firewall you've got) can do it to active connections, 'cause that problem hasn't arised for me so far.
On Fri, Aug 26, 2011 at 06:14, Alex <alex@ahhyes.net> wrote:
I am happy to recompile if there is no config option. I gather it's in the src/auth dir somewhere in one of the C source files. Just need to be pointed in the right dir.
On Fri, 26 Aug 2011 19:07:08 +1000, Alex wrote:
3 minutes! I think that's too long, how can I drop that down to about 45 seconds?
On Fri, 26 Aug 2011 11:44:45 +0300, Timo Sirainen wrote:
On 26.8.2011, at 10.25, Alex wrote:
Running Dovecot 2 on my server. It is regularly getting dictionary auth
attacked. What I have noticed is that once connected to a pop3/imap login session, you can send endless incorrect usernames+passwords attempts. This is a problem for me... I use fail2ban to try and stop these script kiddies. The problem is that fail2ban detects the bad auths, firewalls the IP, however, since it's an "established" session, the attacker can keep authing away... It's only on a subsequent (new) connection that the firewalling will take effect.
Umm. If client hasn't managed to log in in 3 minutes, it's disconnected (no matter what it does with the connection).
Yeah, I had read about half of that thread, and after I sent my mail kept reading and stumbled upon this: "(...) using the recent module needs dovecotto close the connection upon authentication failure, as iptables only (normally) comes in to play for new connections (...)".
So, yeah, my suggestion probably won't work.
On Fri, Aug 26, 2011 at 09:15, Felipe Scarel <fbscarel@gmail.com> wrote:
Alex, I've not personally done it (so just speculating here, bear with me) but you can customize Fail2Ban's actions if needed. So, if you can match the attemps through some regex (and since you're seeing them in the logs, that should be quite possible), then you can edit one of the 'actions' to drop the connection for <ip>.
I'm just not entirely sure that iptables (or pf, or whatever firewall you've got) can do it to active connections, 'cause that problem hasn't arised for me so far.
On Fri, Aug 26, 2011 at 06:14, Alex <alex@ahhyes.net> wrote:
I am happy to recompile if there is no config option. I gather it's in the src/auth dir somewhere in one of the C source files. Just need to be pointed in the right dir.
On Fri, 26 Aug 2011 19:07:08 +1000, Alex wrote:
3 minutes! I think that's too long, how can I drop that down to about 45 seconds?
On Fri, 26 Aug 2011 11:44:45 +0300, Timo Sirainen wrote:
On 26.8.2011, at 10.25, Alex wrote:
Running Dovecot 2 on my server. It is regularly getting dictionary auth
attacked. What I have noticed is that once connected to a pop3/imap login session, you can send endless incorrect usernames+passwords attempts. This is a problem for me... I use fail2ban to try and stop these script kiddies. The problem is that fail2ban detects the bad auths, firewalls the IP, however, since it's an "established" session, the attacker can keep authing away... It's only on a subsequent (new) connection that the firewalling will take effect.
Umm. If client hasn't managed to log in in 3 minutes, it's disconnected (no matter what it does with the connection).
fail2ban will work as soon as dovecot have closed a none-authenticated connection: 3mins->180sec If tarpit delay for auth failures in a connection is set to 15s (which seems to be the default unless i missunderstood).... this let an attackers only 12 tries (at most) before IP gets blacklisted by fail2ban... Far enough to circumvent bruteforce and even dictionnary based attacks... unless the attacker has a botnet and uses non agressives retry policy. But in the last case, even if you blacklist IP at first failed tried, you're still vuln to such attacks.
regards.
Le 26/08/2011 14:22, Felipe Scarel a écrit :
Yeah, I had read about half of that thread, and after I sent my mail kept reading and stumbled upon this: "(...) using the recent module needs dovecotto close the connection upon authentication failure, as iptables only (normally) comes in to play for new connections (...)".
So, yeah, my suggestion probably won't work.
On Fri, Aug 26, 2011 at 09:15, Felipe Scarel<fbscarel@gmail.com> wrote:
Alex, I've not personally done it (so just speculating here, bear with me) but you can customize Fail2Ban's actions if needed. So, if you can match the attemps through some regex (and since you're seeing them in the logs, that should be quite possible), then you can edit one of the 'actions' to drop the connection for<ip>.
I'm just not entirely sure that iptables (or pf, or whatever firewall you've got) can do it to active connections, 'cause that problem hasn't arised for me so far.
On Fri, Aug 26, 2011 at 06:14, Alex<alex@ahhyes.net> wrote:
I am happy to recompile if there is no config option. I gather it's in the src/auth dir somewhere in one of the C source files. Just need to be pointed in the right dir.
On Fri, 26 Aug 2011 19:07:08 +1000, Alex wrote:
3 minutes! I think that's too long, how can I drop that down to about 45 seconds?
On Fri, 26 Aug 2011 11:44:45 +0300, Timo Sirainen wrote:
On 26.8.2011, at 10.25, Alex wrote:
Running Dovecot 2 on my server. It is regularly getting dictionary auth
attacked. What I have noticed is that once connected to a pop3/imap login session, you can send endless incorrect usernames+passwords attempts. This is a problem for me... I use fail2ban to try and stop these script kiddies. The problem is that fail2ban detects the bad auths, firewalls the IP, however, since it's an "established" session, the attacker can keep authing away... It's only on a subsequent (new) connection that the firewalling will take effect.
Umm. If client hasn't managed to log in in 3 minutes, it's disconnected (no matter what it does with the connection).
On Fri, Aug 26, 2011 at 10:14 AM, Alexandre Chapellon <a.chapellon@horoa.net
wrote:
fail2ban will work as soon as dovecot have closed a none-authenticated connection: 3mins->180sec If tarpit delay for auth failures in a connection is set to 15s (which seems to be the default unless i missunderstood).... this let an attackers only 12 tries (at most) before IP gets blacklisted by fail2ban... Far enough to circumvent bruteforce and even dictionnary based attacks... unless the attacker has a botnet and uses non agressives retry policy. But in the last case, even if you blacklist IP at first failed tried, you're still vuln to such attacks.
regards.
Le 26/08/2011 14:22, Felipe Scarel a écrit :
Yeah, I had read about half of that thread, and after I sent my mail kept
reading and stumbled upon this: "(...) using the recent module needs dovecotto close the connection upon authentication failure, as iptables only (normally) comes in to play for new connections (...)".
So, yeah, my suggestion probably won't work.
On Fri, Aug 26, 2011 at 09:15, Felipe Scarel<fbscarel@gmail.com> wrote:
Alex, I've not personally done it (so just speculating here, bear with
me) but you can customize Fail2Ban's actions if needed. So, if you can match the attemps through some regex (and since you're seeing them in the logs, that should be quite possible), then you can edit one of the 'actions' to drop the connection for<ip>.
I'm just not entirely sure that iptables (or pf, or whatever firewall you've got) can do it to active connections, 'cause that problem hasn't arised for me so far.
On Fri, Aug 26, 2011 at 06:14, Alex<alex@ahhyes.net> wrote:
I am happy to recompile if there is no config option. I gather it's in
the src/auth dir somewhere in one of the C source files. Just need to be pointed in the right dir.
On Fri, 26 Aug 2011 19:07:08 +1000, Alex wrote:
3 minutes! I think that's too long, how can I drop that down to about
45 seconds?
On Fri, 26 Aug 2011 11:44:45 +0300, Timo Sirainen wrote:
On 26.8.2011, at 10.25, Alex wrote:
Running Dovecot 2 on my server. It is regularly getting dictionary auth
> attacked. What I have noticed is that once connected to a pop3/imap > login > session, you can send endless incorrect usernames+passwords attempts. > This > is a problem for me... I use fail2ban to try and stop these script > kiddies. > The problem is that fail2ban detects the bad auths, firewalls the IP, > however, since it's an "established" session, the attacker can keep > authing > away... It's only on a subsequent (new) connection that the > firewalling will > take effect. > > Umm. If client hasn't managed to log in in 3 minutes, it's disconnected (no matter what it does with the connection).
If you substitute (create a wrap to) the "imap-login" binary with an script? The script can create a "fail attempt/ip" file into home dir and return ok or not to dovecot main process based on this information. This will solve you problem with established connections and will ban the "badguy" in realtime.
I know this is possible in 1.x version. Timo, this is possible on 2.x version?
Regards.
-- Use cópia oculta (BCC ou CCO) e apague dados pessoais no campo da mensagem ao encaminhar qualquer e-mail.
On 26.8.2011, at 18.27, Allan Cassaro wrote:
If you substitute (create a wrap to) the "imap-login" binary with an script? The script can create a "fail attempt/ip" file into home dir and return ok or not to dovecot main process based on this information.
imap-login is typically chrooted and running with nonprivileged account that can't access user's home dir. I guess you could change those, but wrapping imap-login won't help because you don't know the username at that point..
Either auth or anvil process could do something like this.
Slightly advanced, but note that you CAN block in progress connections using iptables
What is happening is that the usual top (ish) rule in your iptable script says something like "continue if connection established", this can be seen as a performance optimisation to avoid running your full rulebase after the connection exists, however, it's not a limitation of iptables. So you have two main ways to tackle this:
Add your blacklist rules *before* the continue rule. One of the fastest/simplest ways to achieve this is with the new ipset functionality of recent kernels. You can then have a single iptables (block) rule, which references a dynamically updated ipset that contains all your blacklisted IPs. Curiously ipsets can have a timeout value which appears to cause entries to fall out after a set time period. Requires some changes to the "actions" in fail2ban
Alternatively note that you can purge specific connections from conntrack (subsequent packets will either be treated as "invalid" or "new" depending on whether you have loose tracking set in /proc/sys/... Something like "conntrack -D -n a.b.c.d" should do it? This still requires some tweaking to fail2ban, but the iptables rules stay the same
Just saying...
Good luck
Ed W
On 26/08/2011 13:22, Felipe Scarel wrote:
Yeah, I had read about half of that thread, and after I sent my mail kept reading and stumbled upon this: "(...) using the recent module needs dovecotto close the connection upon authentication failure, as iptables only (normally) comes in to play for new connections (...)".
So, yeah, my suggestion probably won't work.
On Fri, Aug 26, 2011 at 09:15, Felipe Scarel <fbscarel@gmail.com> wrote:
Alex, I've not personally done it (so just speculating here, bear with me) but you can customize Fail2Ban's actions if needed. So, if you can match the attemps through some regex (and since you're seeing them in the logs, that should be quite possible), then you can edit one of the 'actions' to drop the connection for <ip>.
login-common/client-common.h :
#define CLIENT_LOGIN_TIMEOUT_MSECS (MASTER_LOGIN_TIMEOUT_SECS*1000)
So set it to (45*60*1000)
But I don't think there's much of a practical difference between these.
On 26.8.2011, at 12.07, Alex wrote:
3 minutes! I think that's too long, how can I drop that down to about 45 seconds?
On Fri, 26 Aug 2011 11:44:45 +0300, Timo Sirainen wrote:
On 26.8.2011, at 10.25, Alex wrote:
Running Dovecot 2 on my server. It is regularly getting dictionary auth attacked. What I have noticed is that once connected to a pop3/imap login session, you can send endless incorrect usernames+passwords attempts. This is a problem for me... I use fail2ban to try and stop these script kiddies. The problem is that fail2ban detects the bad auths, firewalls the IP, however, since it's an "established" session, the attacker can keep authing away... It's only on a subsequent (new) connection that the firewalling will take effect.
Umm. If client hasn't managed to log in in 3 minutes, it's disconnected (no matter what it does with the connection).
participants (7)
-
Alex
-
Alexandre Chapellon
-
Allan Cassaro
-
Ed W
-
Felipe Scarel
-
Robert Schetterer
-
Timo Sirainen