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Vulnerability record · CVE-2021-3449 · published 25 March 2021

CVE-2021-3449: OpenSSL TLS server NULL pointer dereference via renegotiation ClientHello

OOpenssl · Openssl

An OpenSSL TLS server can be crashed by a maliciously crafted TLSv1.2 renegotiation ClientHello that omits the signature_algorithms extension while including signature_algorithms_cert, triggering a NULL pointer dereference. Only servers with TLSv1.2 and renegotiation enabled (the default) are affected; OpenSSL TLS clients are not impacted. All OpenSSL 1.1.1 versions through 1.1.1j are affected, and 1.0.2 is not.

5.9 CVSS 3.1 Medium EPSS 64% · top 0.8% CWE-476 · NULL pointer dereference
5.9CVSS 3.1 base score, v2 4.3
64%EPSS exploitation probability, 30 days
NoNot in CISA KEV
106Affected product versions listed by NVD
58References
17 Jun 2026Last modified by NVD

Description

An OpenSSL TLS server may crash if sent a maliciously crafted renegotiation ClientHello message from a client. If a TLSv1.2 renegotiation ClientHello omits the signature_algorithms extension (where it was present in the initial ClientHello), but includes a signature_algorithms_cert extension then a NULL pointer dereference will result, leading to a crash and a denial of service attack. A server is only vulnerable if it has TLSv1.2 and renegotiation enabled (which is the default configuration). OpenSSL TLS clients are not impacted by this issue. All OpenSSL 1.1.1 versions are affected by this issue. Users of these versions should upgrade to OpenSSL 1.1.1k. OpenSSL 1.0.2 is not impacted by this issue. Fixed in OpenSSL 1.1.1k (Affected 1.1.1-1.1.1j).

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

Automated analysis

Generated by VULONE's analysis model from the NVD record, CISA KEV and EPSS data on 19 September 2026. Confidence: high.

high priorityRemote unauthenticated denial of service against a widely deployed library, with a high EPSS score despite no KEV listing.

What it is

An OpenSSL TLS server can be crashed by a maliciously crafted TLSv1.2 renegotiation ClientHello that omits the signature_algorithms extension while including signature_algorithms_cert, triggering a NULL pointer dereference. Only servers with TLSv1.2 and renegotiation enabled (the default) are affected; OpenSSL TLS clients are not impacted. All OpenSSL 1.1.1 versions through 1.1.1j are affected, and 1.0.2 is not.

Impact

An unauthenticated remote attacker can crash the TLS server process, causing a denial of service. There is no confidentiality or integrity impact; only availability is affected.

Attack surface

Reached over the network by sending a crafted renegotiation ClientHello to a TLSv1.2 server with renegotiation enabled. No authentication or user interaction is required, though the CVSS vector rates attack complexity as high.

Exploitation

Not listed in CISA KEV and no ransomware usage documented, but EPSS is high at roughly 0.64 (99th percentile), indicating elevated likelihood of exploitation activity. References are advisories and patches only, with no public exploit tags.

What to do

  • Upgrade OpenSSL to 1.1.1k or later; 1.0.2 is not affected
  • Apply vendor patches for bundled OpenSSL in Debian, FreeBSD, NetApp, Oracle, Siemens, SonicWall, McAfee, Node.js and other listed products
  • Disable TLSv1.2 renegotiation where the application permits it
  • Restrict network exposure of TLS services that cannot be patched immediately
  • Track vendor advisories for downstream products that embed the affected OpenSSL versions

Detection

  • Monitor TLS server logs and process supervisors for repeated unexpected crashes or restarts of services linked against OpenSSL 1.1.1
  • Inspect TLS handshake traffic for renegotiation ClientHello messages that omit signature_algorithms but include signature_algorithms_cert
  • Alert on high rates of renegotiation attempts from single sources against TLSv1.2 endpoints
  • Correlate crash telemetry with inbound connection source IPs to identify repeat offenders

This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.

Affected products

106 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
http://www.openwall.com/lists/oss-security/2021/03/27/1 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2021/03/27/2 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2021/03/28/3 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2021/03/28/4 Mailing ListThird Party Advisory
https://cert-portal.siemens.com/productcert/pdf/ssa-389290.pdf Third Party Advisory
https://cert-portal.siemens.com/productcert/pdf/ssa-772220.pdf PatchThird Party Advisory
https://git.openssl.org/gitweb/?p=openssl.git%3Ba=commitdiff%3Bh=fb9fa6b51defd48157eeb207f52181f735d96148
https://kb.pulsesecure.net/articles/Pulse_Security_Advisories/SA44845 Third Party Advisory
https://kc.mcafee.com/corporate/index?page=content&id=SB10356 Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/08/msg00029.html Mailing ListThird Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/CCBFLLVQVILIVGZMBJL3IXZ
https://psirt.global.sonicwall.com/vuln-detail/SNWLID-2021-0013 Third Party Advisory
https://security.FreeBSD.org/advisories/FreeBSD-SA-21:07.openssl.asc Third Party Advisory
https://security.gentoo.org/glsa/202103-03 Third Party Advisory
https://security.netapp.com/advisory/ntap-20210326-0006/ Third Party Advisory
https://security.netapp.com/advisory/ntap-20210513-0002/ Third Party Advisory
https://security.netapp.com/advisory/ntap-20240621-0006/
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-openssl-2021-GHY28dJd Third Party Advisory
https://www.debian.org/security/2021/dsa-4875 Third Party Advisory
https://www.openssl.org/news/secadv/20210325.txt Vendor Advisory
https://www.oracle.com//security-alerts/cpujul2021.html PatchThird Party Advisory
https://www.oracle.com/security-alerts/cpuApr2021.html PatchThird Party Advisory
https://www.oracle.com/security-alerts/cpuapr2022.html PatchThird Party Advisory
https://www.oracle.com/security-alerts/cpujul2022.html Third Party Advisory
https://www.oracle.com/security-alerts/cpuoct2021.html Third Party Advisory
https://www.tenable.com/security/tns-2021-05 Third Party Advisory
https://www.tenable.com/security/tns-2021-06 Third Party Advisory
https://www.tenable.com/security/tns-2021-09 Third Party Advisory
https://www.tenable.com/security/tns-2021-10 Third Party Advisory
http://www.openwall.com/lists/oss-security/2021/03/27/1 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2021/03/27/2 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2021/03/28/3 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2021/03/28/4 Mailing ListThird Party Advisory
https://cert-portal.siemens.com/productcert/pdf/ssa-389290.pdf Third Party Advisory
https://cert-portal.siemens.com/productcert/pdf/ssa-772220.pdf PatchThird Party Advisory
https://git.openssl.org/gitweb/?p=openssl.git%3Ba=commitdiff%3Bh=fb9fa6b51defd48157eeb207f52181f735d96148
https://kb.pulsesecure.net/articles/Pulse_Security_Advisories/SA44845 Third Party Advisory
https://kc.mcafee.com/corporate/index?page=content&id=SB10356 Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/08/msg00029.html Mailing ListThird Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/CCBFLLVQVILIVGZMBJL3IXZ

Track CVE-2021-3449 inside VULONE

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Source: NIST National Vulnerability Database (record CVE-2021-3449), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.