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Malformed SHA-512 TLS Session Ticket DoS in OpenSSL
CVE-2016-6302 is a denial-of-service vulnerability in OpenSSL's tls_decrypt_ticket() function. Ticket-length validation fails to account for the configured HMAC size. When a server uses SHA-512 for session-ticket authentication, an undersized malformed ticket can cause an integer underflow when the 64-byte HMAC length is subtracted from the ticket length. The resulting negative length is converted to a large size_t value for HMAC_Update(), triggering an out-of-bounds read that can crash the server process. The issue affects older OpenSSL releases with TLS session-ticket support and was fixed in OpenSSL 1.0.1u and 1.0.2i.
What it means and what to do now: patch path, interim mitigations, and the assume-compromise checklist.
What an attacker gets, and what they’ve been doing with it.
A remote unauthenticated attacker can crash an affected TLS server, interrupting connections and making services handled by the terminated process unavailable. The established impact is denial of service; confidentiality loss, integrity compromise, and arbitrary code execution are not established for this vulnerability.
If you can’t patch tonight, do this now.
Where supported, temporarily disable TLS session tickets or remove the custom SHA-512 session-ticket HMAC configuration to avoid the vulnerable processing path. Restrict network access to affected TLS services to trusted clients until patched. These measures do not replace installing the corrected library.
Patch, then assume compromise.
Install a vendor-supported OpenSSL update containing the fix. The historical upstream fixes are OpenSSL 1.0.1u and 1.0.2i for their respective branches. Red Hat supplied backported fixes through RHSA-2016:1940, including OpenSSL 1.0.1e-48.el6_8.3 for RHEL 6 and 1.0.1e-51.el7_2.7 for RHEL 7. Applicable JBoss Core Services fixes are available through RHSA-2018:2185, RHSA-2018:2186, and RHSA-2018:2187. Restart all services linked against OpenSSL, or reboot, after installing the update.
No public exploits tracked yet. Mallory keeps watching.
Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.
12 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.