OpenSSL Patches High-Severity DTLS Bug That Can Leak Heap Memory in the Clear

OpenSSL disclosed a High-severity flaw in its DTLS implementation on September 29 and released fixes the same day. The bug, tracked as CVE-2026-84782, can leak heap memory to the other side of a connection as unencrypted data, or crash the program outright.

DTLS is the TLS variant used for UDP traffic. It resends a handshake message if no reply arrives before the timer expires, splitting large messages into fragments that each fit in a single UDP datagram. If the connection cannot accept more data for the moment, sending can pause part-way through a message and resume later — and while it is paused, the resend timer can still fire and send an earlier message again.

What goes wrong

Before the fix, the resend used the paused message's position in the buffer instead of returning to the start of the message being resent. The packet went out with the wrong label, its body was leftover bytes from the larger message, and reading it could overrun the buffer. The mislabeled message thus carried heap memory to the peer as plaintext handshake data; if the read reached unmapped memory, the program crashed.

OpenSSL does not say whether an attacker can force a resend at the right moment, and reports no known exploitation. Only software that uses OpenSSL for DTLS is exposed — for example when protecting WebRTC data channels or setting up keys for internet calls. Both clients and servers are affected.

Who is affected and what to do

Fixes ship in OpenSSL 4.0.3, 3.6.5, 3.5.9 and 3.4.8. The older 3.0, 1.1.1 and 1.0.2 branches receive fixes only for premium-support customers; public 3.0 updates ended on September 7. OpenSSL lists no workaround for those who cannot update yet.

Laurent Gaffie of Secorizon reported the flaw on August 17, and Ryan Hooper developed the fix. OpenSSL rates it High, one step below Critical. CISA gave it a CVSS score of 8.2, rating confidentiality impact Low and availability High. Ubuntu and Debian have already pushed updated packages; Ubuntu users must reboot after installing.

The release also fixes 13 other flaws. The most serious, CVE-2026-84783, is Moderate and affects only OpenSSL 4.0, letting a remote peer crash a multi-threaded TLS client or server. The rest are rated Low, including five in OpenSSL's QUIC code and three timing side-channels in ECDSA and SM2.

Source: The Hacker News