OpenSSL High-Severity Flaw Lets Attackers Leak Heap Memory in Plaintext

OpenSSL has announced a high-severity vulnerability in its Datagram Transport Layer Security (DTLS) implementation that could allow a remote peer to read unintended plaintext heap memory during handshake data transmission or trigger a denial-of-service condition.

This vulnerability, tracked as CVE-2026-84782, stems from an out-of-bounds read when handling DTLS handshake message retransmissions.

The issue affects various OpenSSL versions, including 4.0, 3.6, 3.5, 3.4, 3.0, 1.1.1, and 1.0.2, as noted in the OpenSSL Security Advisory released on September 29, 2026.

OpenSSL High-Severity Flaw

DTLS is specifically designed for datagram-based transports and incorporates retransmission mechanisms to recover handshake packets that may be lost in transit.

The vulnerability occurs when an application sends a DTLS handshake message in multiple fragments, and the transport temporarily cannot accept more data.

In this case, OpenSSL may return a WANT_WRITE status, which suspends the handshake message write midway through transmission.

While this operation is suspended, a DTLS retransmission timer can expire and attempt to resend an earlier handshake message from the retransmission queue.

OpenSSL’s retransmission logic mistakenly reuses the same internal buffer and tracking state associated with the suspended handshake write.

Instead of resetting its read position to the beginning of the queued message, retransmission could start at the offset reached during the in-progress write.

Consequently, a peer might receive a malformed or mislabeled DTLS handshake message containing leftover bytes from an unrelated, larger message still in transit. In some cases, this can lead to reading beyond the intended message buffer, which could expose heap-resident data in plaintext.

This vulnerability is classified as CWE-125, indicating an out-of-bounds read weakness. Its primary security concern is the unintended disclosure of heap memory to the remote DTLS peer.

The leaked data will vary based on the process’s memory contents and runtime conditions, potentially including fragments of previously allocated application or library data. However, the advisory has not confirmed any reliable arbitrary-memory disclosure capability.

Moreover, the flaw can also cause a process to crash when the retransmission logic attempts to read from an unmapped memory region, potentially causing a denial-of-service condition for affected applications handling attacker-controlled DTLS traffic.

A related flaw within the same retransmission path could corrupt the bookkeeping necessary to resume the suspended handshake write. If retransmission occurs while another write is still paused, any subsequent calls to SSL_read(), SSL_write(), SSL_accept(), or SSL_connect() may encounter inconsistent states, triggering an abort in debugging builds.

Affected Versions and Fixes

OpenSSL has addressed the issue by resetting the retransmission read position before resending a message. The project also ensures that retransmissions are skipped while a handshake write is suspended, deferring them until the application resumes the pending operation.

Organizations should identify internet-facing services, VPNs, VoIP systems, IoT deployments, and custom applications using DTLS, and promptly upgrade their bundled or system-provided OpenSSL packages. This issue does not affect the OpenSSL FIPS module, as the vulnerable DTLS code lies outside its boundary.

The vulnerability was reported on August 17, 2026, by Laurent Gaffie of SecuRizon, and Ryan Hooper developed the fix.

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