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Date: Tue, 14 Jun 2016 17:46:18 +0200
From: Cedric Buissart <>
Subject: Python CVE-2016-0772: smtplib StartTLS stripping attack


This is to publicly disclose Python CVE-2016-0772: smtplib StartTLS
stripping attack.

Description :
A vulnerability in smtplib allowing MITM attacker to perform a startTLS
stripping attack. smtplib does not seem to raise an exception when the
remote end (smtp server) is capable of negotiating starttls but fails to
respond with 220 (ok) to an explicit call of SMTP.starttls(). This may
allow a malicious MITM to perform a startTLS stripping attack if the client
code does not explicitly check the response code for startTLS.

Upstream patch :
3.4 branch :
2.7 branch :

Red Hat Bugzilla :

Reported by: Tin (Team Oststrom)

Kind regards,

Cedric Buissart,
Product Security

Purkynova 99
Brno 612 45

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