OpenSSL’s DTLS State Handling Error Opens the Door to Heap Data Exposure
OpenSSL has disclosed CVE-2026-84782, a high-severity out-of-bounds read vulnerability affecting the Datagram Transport Layer Security (DTLS) handshake retransmission mechanism. The flaw occurs when a DTLS handshake write is suspended part-way through and the retransmission timer attempts to resend an earlier handshake message. Due to incorrect state handling, OpenSSL can reuse a stale buffer offset from the suspended write, causing data outside the intended message boundary to be read and transmitted to the remote peer.
Successful exploitation can result in heap memory disclosure, exposing unintended application or library data as plaintext DTLS handshake content. If the out-of-bounds read reaches an unmapped memory region, the affected process can crash, resulting in a denial of service (DoS). OpenSSL released patched versions on September 29, 2026, and organizations using DTLS should prioritize updates, particularly for internet-facing services, VPN infrastructure, VoIP systems, embedded devices, and applications that bundle their own OpenSSL libraries.
Vulnerability Details
| CVE ID | Severity | Weakness | Affected Component | Impact |
|---|---|---|---|---|
| CVE-2026-84782 | High | CWE-125: Out-of-bounds Read | OpenSSL DTLS Handshake Retransmission | Heap Memory Disclosure / Denial of Service |
Technical Analysis
The vulnerability originates in OpenSSL's handling of DTLS handshake message retransmissions. DTLS handshake messages can be divided into multiple fragments before being transmitted over the underlying datagram transport.
When OpenSSL attempts to transmit a large handshake message, the underlying transport may temporarily be unable
to accept additional data. In this situation, the write operation can stop part-way through the handshake message
and return a WANT_WRITE condition.
OpenSSL keeps internal state describing how much of the message has already been transmitted so that the operation can continue from the correct point when the application retries the write.
Impact
-
Heap Memory Disclosure:
OpenSSL may read data beyond the intended DTLS handshake message buffer and transmit portions of adjacent heap memory to the remote peer. -
Sensitive Information Exposure:
Depending on the application's memory layout, leaked heap content could include fragments of application data, OpenSSL internal structures, or other information located near the affected memory allocation. -
Denial of Service:
If the out-of-bounds read reaches an unmapped or inaccessible memory region, the OpenSSL process may crash, disrupting the application or service relying on DTLS communication. -
Incorrect DTLS Handshake Behavior:
Overwritten retransmission state may prevent the suspended handshake write from resuming correctly, causing malformed handshake traffic or failed DTLS connections. -
Impact on Network-Facing Services:
Applications using OpenSSL for DTLS-based network communication may be exposed remotely when an attacker can interact with the vulnerable DTLS endpoint.
Affected OpenSSL Versions
| OpenSSL Branch | Affected Versions | Fixed Version |
|---|---|---|
| OpenSSL 4.0 | 4.0.0 through versions before 4.0.3 | 4.0.3 |
| OpenSSL 3.6 | 3.6.0 through versions before 3.6.5 | 3.6.5 |
| OpenSSL 3.5 | 3.5.0 through versions before 3.5.9 | 3.5.9 |
| OpenSSL 3.4 | 3.4.0 through versions before 3.4.8 | 3.4.8 |
| OpenSSL 3.0 | 3.0.0 through versions before 3.0.23 | 3.0.23 |
| OpenSSL 1.1.1 | 1.1.1 through versions before 1.1.1zj | 1.1.1zj |
| OpenSSL 1.0.2 | 1.0.2 through versions before 1.0.2zs | 1.0.2zs |
MITRE ATT&CK Mapping
| Technique ID | Technique Name | Tactic |
|---|---|---|
| T1005 | Data from Local System | Exfiltration |
| T1499 | Endpoint Denial of Service | Impact |
Mitigation
-
Upgrade OpenSSL to a fixed version.
Systems using affected OpenSSL branches should upgrade to OpenSSL 4.0.3, 3.6.5, 3.5.9, 3.4.8, 3.0.23, 1.1.1zj, or 1.0.2zs, depending on the deployed branch and vendor support arrangements. -
Prioritize DTLS-enabled applications.
Identify applications and services that actively use DTLS rather than TLS-only communication. Internet-facing VPN gateways, VoIP systems, communication services, and UDP-based applications should receive priority. -
Apply operating-system vendor security updates.
Where OpenSSL is provided through the operating system, use vendor-provided security packages. Ubuntu users should apply the updates provided through USN-8847-1. -
Inventory bundled OpenSSL libraries.
Do not rely exclusively on system package inventories. Third-party applications, containers, embedded products, security appliances, and network devices may contain private copies of OpenSSL that require separate vendor updates.
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