CVE-2020-1967: OpenSSL TLS 1.3 NULL pointer dereference denial of service

Automated analysis

Generated by VULONE's analysis model from the NVD record, CISA KEV and EPSS data on 20 September 2026. Confidence: high.

high priorityRemote, unauthenticated, low-complexity availability impact with a public exploit and very high EPSS score, though not observed in KEV or ransomware campaigns.

What it is

OpenSSL 1.1.1d through 1.1.1f mishandles the signature_algorithms_cert TLS extension, causing a NULL pointer dereference in SSL_check_chain() when an invalid or unrecognised signature algorithm is received from a peer. Applications that call SSL_check_chain() during or after a TLS 1.3 handshake can be crashed by a malicious peer. The flaw is a remote denial of service affecting both server and client roles.

Impact

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

Attack surface

Reachable over the network by any peer that can complete or participate in a TLS 1.3 handshake and send a crafted signature_algorithms_cert extension. No authentication or user interaction is required per the CVSS vector (AV:N/AC:L/PR:N/UI:N).

Exploitation

Not listed in CISA KEV, but EPSS is high (0.533, ~98.9th percentile) and a public exploit reference exists (github.com/irsl/CVE-2020-1967), indicating practical exploitability.

What to do

• Upgrade OpenSSL to 1.1.1g or later; versions prior to 1.1.1d are not affected.
• Patch or update dependent products (Debian, FreeBSD, Fedora, Oracle, NetApp, Broadcom, openSUSE, JD Edwards, Tenable) using their vendor advisories.
• Where immediate patching is not possible, restrict or monitor TLS 1.3 exposure and consider disabling TLS 1.3 if the application calls SSL_check_chain().
• Track vendor advisories for embedded or bundled OpenSSL copies that may not be covered by OS package updates.

Detection

• Monitor for process crashes or restarts in TLS-terminating services (web servers, load balancers, VPN gateways) that correlate with TLS 1.3 handshakes.
• Inspect TLS handshake logs or packet captures for malformed or unrecognised signature_algorithms_cert values.
• Correlate crash telemetry with source IPs initiating repeated TLS 1.3 connections to identify DoS attempts.
• Inventory OpenSSL versions across hosts and containers to find systems still running 1.1.1d-1.1.1f.

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