CVE-2016-3739

CVE-2016-3739 is a medium-severity vulnerability in Haxx Curl with a CVSS 3.x base score of 5.3. 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-20.

Key facts

• Severity: Medium (CVSS 3.x base score 5.3)
• CVSS v2: 2.6
• EPSS exploit prediction: 6% (93rd percentile)
• Actively exploited: Not listed in CISA KEV
• Weakness: CWE-20
• Affected product: Haxx Curl
• Published: 2016-05-20
• Last modified: 2026-06-17

Description

The (1) mbed_connect_step1 function in lib/vtls/mbedtls.c and (2) polarssl_connect_step1 function in lib/vtls/polarssl.c in cURL and libcurl before 7.49.0, when using SSLv3 or making a TLS connection to a URL that uses a numerical IP address, allow remote attackers to spoof servers via an arbitrary valid certificate.

Frequently asked questions

What is CVE-2016-3739?

The (1) mbed_connect_step1 function in lib/vtls/mbedtls.c and (2) polarssl_connect_step1 function in lib/vtls/polarssl.c in cURL and libcurl before 7.49.0, when using SSLv3 or making a TLS connection to a URL that uses a numerical IP address, allow remote attackers to spoof servers via an arbitrary valid certificate.

How severe is CVE-2016-3739?

CVE-2016-3739 has a CVSS 3.x base score of 5.3, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and user interaction. Impact on confidentiality is none, integrity high, and availability none.

Is CVE-2016-3739 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 6% (93rd percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2016-3739?

CVE-2016-3739 primarily affects Haxx Curl. In total, 37 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2016-3739?

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

When was CVE-2016-3739 published?

CVE-2016-3739 was published on 2016-05-20 and last updated on 2026-06-17.

References

• http://www.openwall.com/lists/oss-security/2024/03/27/4
• http://www.oracle.com/technetwork/security-advisory/cpuoct2018-4428296.html
• http://www.securityfocus.com/bid/90726
• http://www.securitytracker.com/id/1035907
• http://www.slackware.com/security/viewer.php?l=slackware-security&y=2016&m=slackware-security.495349
• https://curl.haxx.se/CVE-2016-3739.patch
• https://curl.haxx.se/changes.html#7_49_0
• https://curl.haxx.se/docs/adv_20160518.html
• https://h20566.www2.hpe.com/portal/site/hpsc/public/kb/docDisplay?docId=emr_na-c05320149
• https://h20566.www2.hpe.com/portal/site/hpsc/public/kb/docDisplay?docId=emr_na-c05390722
• https://security.gentoo.org/glsa/201701-47

Affected products (37)

• cpe:2.3:a:haxx:curl:7.21.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.21.1:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.21.2:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.21.3:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.21.4:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.21.5:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.21.6:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.21.7:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.22.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.23.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.23.1:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.24.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.25.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.26.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.27.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.28.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.28.1:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.29.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.30.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.31.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.32.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.33.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.34.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.35.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.36.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.38.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.39.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.40.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.41.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.42.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.42.1:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.43.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.44.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.45.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.46.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.47.0:*:*:*:*:*:*:*
• cpe:2.3:a:haxx:curl:7.48.0:*:*:*:*:*:*:*

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