Description
The smtplib library in CPython (aka Python) before 2.7.12, 3.x before 3.4.5, and 3.5.x before 3.5.2 does not return an error when StartTLS fails, which might allow man-in-the-middle attackers to bypass the TLS protections by leveraging a network position between the client and the registry to block the StartTLS command, aka a "StartTLS stripping attack."
Fixed in Linux distributions
Distributions backport fixes, so the version that fixes this vulnerability differs per release — upgrade to at least the version shown. From the distributions' security trackers and advisories.
Debian
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian 11 | python2.7 | 2.7.12~rc1-1 | — |
| Debian 8 | python3.4 | 3.4.2-1+deb8u2 | DLA-1663-1 |
| Debian 7 | python2.7 | 2.7.3-6+deb7u3 | DLA-522-1 |
| Debian 7 | python3.2 | 3.2.3-7+deb7u1 | DLA-871-1 |
Ubuntu
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Ubuntu 16.04 LTS | python2.7 | 2.7.12-1ubuntu0~16.04.1 | USN-3134-1 |
| Ubuntu 16.04 LTS | python3.5 | 3.5.2-2ubuntu0~16.04.1 | USN-3134-1 |
| Ubuntu 14.04 LTS | python2.7 | 2.7.6-8ubuntu0.3 | USN-3134-1 |
| Ubuntu 14.04 LTS | python3.4 | 3.4.3-1ubuntu1~14.04.5 | USN-3134-1 |
| Ubuntu 14.04 LTS | python3.5 | 3.5.2-2ubuntu0~16.04.4~14.04.1 | — |
Red Hat
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| RHEL 7 | python +6 | 2.7.5-38.el7_2 | RHSA-2016:1626 |
| RHEL 6 | python +5 | 2.6.6-66.el6_8 | RHSA-2016:1626 |
| RHEL 2 | python27-python +6 | 2.7.8-16.el7 | RHSA-2016:1628 |
| RHEL 2 | python33-python +6 | 3.3.2-16.el7 | RHSA-2016:1629 |
| RHEL 2 | rh-python34-python +6 | 3.4.2-13.el7 | RHSA-2016:1630 |
| RHEL 2 | rh-python35-python +6 | 3.5.1-9.el7 | RHSA-2016:1627 |
SUSE
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| SLES 15 SP1 | python | 2.7.17-7.32.2 | SUSE-SU-2020:0234-1 |
| SLES 15 SP1 | python-base | 2.7.17-7.32.1 | SUSE-SU-2020:0234-1 |
| SLES 15 SP1 | python3 +1 | 3.6.10-3.42.2 | SUSE-SU-2020:0114-1 |
| SLES 15 | python | 2.7.17-7.32.2 | SUSE-SU-2020:0234-1 |
| SLES 15 | python-base | 2.7.17-7.32.1 | SUSE-SU-2020:0234-1 |
| SLES 15 | python3 +1 | 3.6.10-3.42.2 | SUSE-SU-2020:0114-1 |
| SLES 12 SP2 | python3 +1 | 3.4.5-19.1 | SUSE-SU-2016:2859-1 |
| SLES 12 SP1 | python | 2.7.9-24.1 | SUSE-SU-2016:2106-1 |
| SLES 12 SP1 | python-base | 2.7.9-24.2 | SUSE-SU-2016:2106-1 |
| SLES 12 SP1 | python-doc | 2.7.9-24.4 | SUSE-SU-2016:2106-1 |
| SLES 12 SP1 | python3 +1 | 3.4.5-17.1 | SUSE-SU-2016:2653-1 |
| SLES 12 | python | 2.7.9-16.7.1 | SUSE-SU-2019:0223-1 |
| SLES 12 | python-doc +1 | 2.7.9-16.7.2 | SUSE-SU-2019:0223-1 |
| SLES 12 | python3 +1 | 3.4.5-17.1 | SUSE-SU-2016:2653-1 |
Show 5 more (older releases, extended support, other products)Fewer releases
| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| SLES 11 SP4 | python +1 | 2.6.9-39.1 | SUSE-SU-2016:2270-1 |
| SLES 11 SP4 | python-doc | 2.6-8.39.1 | SUSE-SU-2016:2270-1 |
| SLES 2 | python | 2.7.17-7.32.2 | SUSE-SU-2020:0234-1 |
| SLES 2 | python-base | 2.7.17-7.32.1 | SUSE-SU-2020:0234-1 |
| SLES for SAP 12 SP2 | python3 +1 | 3.4.5-19.1 | SUSE-SU-2016:2859-1 |
| SLES for SAP 12 SP1 | python | 2.7.9-24.1 | SUSE-SU-2016:2106-1 |
| SLES for SAP 12 SP1 | python-base | 2.7.9-24.2 | SUSE-SU-2016:2106-1 |
| SLES for SAP 12 SP1 | python-doc | 2.7.9-24.4 | SUSE-SU-2016:2106-1 |
| SLES for SAP 12 SP1 | python3 +1 | 3.4.5-17.1 | SUSE-SU-2016:2653-1 |
| SLES for SAP 11 SP4 | python +1 | 2.6.9-39.1 | SUSE-SU-2016:2270-1 |
| SLES for SAP 11 SP4 | python-doc | 2.6-8.39.1 | SUSE-SU-2016:2270-1 |
openSUSE
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| openSUSE Tumbleweed | python | 2.7.12-1.5 | openSUSE-SU-2024:10450-1 |
| openSUSE Tumbleweed | python-base | 2.7.12-1.4 | openSUSE-SU-2024:10536-1 |
| openSUSE Tumbleweed | python-doc | 2.7.12-1.3 | openSUSE-SU-2024:10193-1 |
| openSUSE Tumbleweed | python36 | 3.6.15-1.1 | openSUSE-SU-2024:11284-1 |
| openSUSE Leap 15.1 | python3 +1 | 3.6.10-lp151.6.7.1 | openSUSE-SU-2020:0086-1 |
Azure Linux
| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Azure Linux 3 | python-py | No fix yet | — |
Public exploits
Exploit or proof-of-concept code for this vulnerability is publicly available, which lowers the bar for attackers. Treat it as a reason to patch sooner.
Third-party code, not reviewed by us. Public PoCs are sometimes fake or contain malware. Read the code first and run it only in an isolated lab, against systems you are authorised to test.
Exploit-DB 1
- Python smtplib 2.7.11 / 3.4.4 / 3.5.1 - Man In The Middle StartTLS Stripping EDB-43500 · local · Jul 3, 2016
Sources: PoC-in-GitHub, Metasploit Framework, Exploit-DB — updated daily. Only links are listed; no exploit code is hosted here.
Advisories 21
- RHSA-2016:1630 Red Hat Security Advisory: rh-python34-python security update Red Hat Security Advisories (RHSA) · Jun 29, 2026
- RHSA-2016:1629 Red Hat Security Advisory: python33-python security update Red Hat Security Advisories (RHSA) · Jun 29, 2026
- RHSA-2016:1627 Red Hat Security Advisory: rh-python35-python security update Red Hat Security Advisories (RHSA) · Jun 29, 2026
- RHSA-2016:1628 Red Hat Security Advisory: python27-python security update Red Hat Security Advisories (RHSA) · Jun 29, 2026
- RHSA-2016:1626 Red Hat Security Advisory: python security update Red Hat Security Advisories (RHSA) · Jun 29, 2026
- USN-3134-1 python2.7, python3.2, python3.4, python3.5 vulnerabilities Ubuntu Security Notices (USN) · Apr 22, 2026
- DLA-522-1 python2.7 - security update Debian Security (DSA / DLA) · Mar 9, 2026
- DLA-871-1 python3.2 - security update Debian Security (DSA / DLA) · Mar 9, 2026
- DLA-1663-1 python3.4 - security update Debian Security (DSA / DLA) · Mar 9, 2026
- openSUSE-SU-2020:0086-1 Security update for python3 SUSE / openSUSE Security Updates · Feb 4, 2026
- openSUSE-SU-2024:10450-1 python-2.7.12-1.5 on GA media SUSE / openSUSE Security Updates · Feb 4, 2026
- SUSE-SU-2016:2106-1 Security update for python SUSE / openSUSE Security Updates · Feb 4, 2026
- SUSE-SU-2019:0223-1 Security update for python SUSE / openSUSE Security Updates · Feb 4, 2026
- openSUSE-SU-2024:10193-1 python-doc-2.7.12-1.3 on GA media SUSE / openSUSE Security Updates · Feb 4, 2026
- SUSE-SU-2020:0114-1 Security update for python3 SUSE / openSUSE Security Updates · Feb 4, 2026
- SUSE-SU-2016:2653-1 Security update for python3 SUSE / openSUSE Security Updates · Feb 4, 2026
- openSUSE-SU-2024:11284-1 python36-3.6.15-1.1 on GA media SUSE / openSUSE Security Updates · Feb 4, 2026
- SUSE-SU-2016:2270-1 Security update for python SUSE / openSUSE Security Updates · Feb 4, 2026
- SUSE-SU-2016:2859-1 Security update for python3 SUSE / openSUSE Security Updates · Feb 4, 2026
- openSUSE-SU-2024:10536-1 libpython2_7-1_0-2.7.12-1.4 on GA media SUSE / openSUSE Security Updates · Feb 4, 2026
- SUSE-SU-2020:0234-1 Security update for python SUSE / openSUSE Security Updates · May 2, 2025
References
- https://hg.python.org/cpython/rev/d590114c2394
- http://www.splunk.com/view/SP-CAAAPUE
- RHSA-2016:1630 vendor-advisory
- RHSA-2016:1627 vendor-advisory
- https://hg.python.org/cpython/rev/b3ce713fb9be
- RHSA-2016:1629 vendor-advisory
- http://www.splunk.com/view/SP-CAAAPSV
- [debian-lts-announce] 20190207 [SECURITY] [DLA 1663-1] python3.4 security update mailing-list
- https://bugzilla.redhat.com/show_bug.cgi?id=1303647
- https://docs.python.org/3.4/whatsnew/changelog.html#python-3-4-5
- [oss-security] 20160614 Python CVE-2016-0772: smtplib StartTLS stripping attack mailing-list
- https://hg.python.org/cpython/raw-file/v2.7.12/Misc/NEWS
- GLSA-201701-18 vendor-advisory
- https://docs.python.org/3.5/whatsnew/changelog.html#python-3-5-2
- 91225 vdb-entry
- RHSA-2016:1628 vendor-advisory
- RHSA-2016:1626 vendor-advisory
- openSUSE-SU-2020:0086 vendor-advisory