CVE-2016-5699

CVE-2016-5699 is a medium-severity vulnerability in Python with a CVSS 3.x base score of 6.1. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-113.

Key facts

• Severity: Medium (CVSS 3.x base score 6.1)
• CVSS v2: 4.3
• EPSS exploit prediction: 10% (95th percentile)
• Actively exploited: Not listed in CISA KEV
• Weakness: CWE-113
• Affected product: Python
• Published: 2016-09-02
• Last modified: 2026-06-17

Description

CRLF injection vulnerability in the HTTPConnection.putheader function in urllib2 and urllib in CPython (aka Python) before 2.7.10 and 3.x before 3.4.4 allows remote attackers to inject arbitrary HTTP headers via CRLF sequences in a URL.

Frequently asked questions

What is CVE-2016-5699?

CRLF injection vulnerability in the HTTPConnection.putheader function in urllib2 and urllib in CPython (aka Python) before 2.7.10 and 3.x before 3.4.4 allows remote attackers to inject arbitrary HTTP headers via CRLF sequences in a URL.

How severe is CVE-2016-5699?

CVE-2016-5699 has a CVSS 3.x base score of 6.1, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is low, integrity low, and availability none.

Is CVE-2016-5699 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 10% (95th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2016-5699?

CVE-2016-5699 primarily affects Python. In total, 27 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2016-5699?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

When was CVE-2016-5699 published?

CVE-2016-5699 was published on 2016-09-02 and last updated on 2026-06-17.

References

• http://blog.blindspotsecurity.com/2016/06/advisory-http-header-injection-in.html
• http://lists.opensuse.org/opensuse-security-announce/2020-01/msg00040.html
• http://rhn.redhat.com/errata/RHSA-2016-1626.html
• http://rhn.redhat.com/errata/RHSA-2016-1627.html
• http://rhn.redhat.com/errata/RHSA-2016-1628.html
• http://rhn.redhat.com/errata/RHSA-2016-1629.html
• http://rhn.redhat.com/errata/RHSA-2016-1630.html
• http://www.openwall.com/lists/oss-security/2016/06/14/7
• http://www.openwall.com/lists/oss-security/2016/06/15/12
• http://www.openwall.com/lists/oss-security/2016/06/16/2
• http://www.oracle.com/technetwork/topics/security/bulletinjul2016-3090568.html
• http://www.securityfocus.com/bid/91226
• http://www.splunk.com/view/SP-CAAAPSV
• http://www.splunk.com/view/SP-CAAAPUE
• https://docs.python.org/3.4/whatsnew/changelog.html#python-3-4-4
• https://hg.python.org/cpython/raw-file/v2.7.10/Misc/NEWS
• https://hg.python.org/cpython/rev/1c45047c5102
• https://hg.python.org/cpython/rev/bf3e1c9b80e9
• https://lists.debian.org/debian-lts-announce/2019/02/msg00011.html

Affected products (27)

• cpe:2.3:a:python:python:*:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.0:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.0.1:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.1.0:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.1.1:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.1.2:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.1.3:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.1.4:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.1.5:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.2.0:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.2.1:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.2.2:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.2.3:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.2.4:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.2.5:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.2.6:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.3.0:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.3.1:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.3.2:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.3.3:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.3.4:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.3.5:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.3.6:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.4.0:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.4.1:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.4.2:*:*:*:*:*:*:*
• cpe:2.3:a:python:python:3.4.3:*:*:*:*:*:*:*

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Threat intelligence

Threat-intel indicators referencing this CVE:

• 203.192.232.180 (ipv4-addr)
• 177.53.215.134 (ipv4-addr)
• 5.99.196.202 (ipv4-addr)
• 121.164.135.251 (ipv4-addr)