CVE-2026-18798 in OpenSSL
Summary
by MITRE • 08/25/2026
Issue summary: QUIC server may double free QRX (QUIC record layer RX) object when channel creation fails for initial packet.
Impact summary: Double free leads to heap corruption, which typically results in termination of QUIC server process, leading to Denial of Service. There is so far no evidence that this double free is exploitable for remote code execution, thus it is considered highly improbable.
CWE: CWE-415: Double Free
Description: In order to validate initial packet, OpenSSL QUIC stack default packet handler (port_default_packet_handler()) creates a so-called QRX object. If the initial packet validates successfully with QRX object, the default packet handler proceeds to channel (connection object) creation. The QRX object used for packet validation is passed to port_bind_channel(), so it becomes part of the newly created connection. If port_bind_channel() fails, then it also frees the QRX object. Once port_bind_channel() returns, the port_default_packet_handler() detects the failure and proceeds to the error branch, where the same QRX object is freed for the second time.
The failure in port_bind_channel() function can be induced with a relatively low effort by a malformed (non RFC 9000 compliant) INITIAL packet. If the packet carries DCID (destination connection ID) which is shorter than 8 bytes, then port_bind_channel() jumps to the error path after ossl_quic_lcidm_enrol_odcid() detects that the DCID has invalid length.
FIPS impact: no The FIPS module is not affected, as the QUIC implementation is outside of the OpenSSL FIPS module boundary.
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Analysis
by VulDB Data Team • 08/25/2026
The vulnerability identified in the OpenSSL QUIC stack represents a critical memory management flaw within the packet handling logic, specifically categorized under CWE-415 for Double Free errors. This issue arises during the initialization phase of a QUIC connection when the server attempts to process an incoming initial packet. The core mechanism involves the creation and subsequent release of a QRX object, which serves as the record layer receive context necessary for validating packets before establishing a full channel or connection state. Under normal operational conditions, this workflow proceeds without incident; however, specific failure paths within the binding logic introduce a race condition in resource deallocation that compromises memory integrity.
The technical flaw is rooted in the interaction between the default packet handler and the channel binding function. When port_default_packet_handler receives an initial packet, it first instantiates a QRX object to perform validation checks on the packet structure. If this validation succeeds, the QRX object is passed as an argument to port_bind_channel, which attempts to create or bind the corresponding connection channel. In scenarios where port_bind_channel fails—often due to invalid parameters such as a Destination Connection ID that does not meet the minimum length requirements specified in RFC 9000—the function executes its own error handling routine and frees the QRX object passed to it. Crucially, control then returns to port_default_packet_handler, which detects the failure of channel creation and proceeds to an error branch where it independently attempts to free the same QRX object again. This results in a double free condition because the pointer is deallocated twice without being reset or nullified after the first release.
The operational impact of this vulnerability is primarily centered on service availability rather than confidentiality or integrity, although memory corruption inherently carries risk. The immediate consequence of triggering this double free is heap corruption within the OpenSSL process running the QUIC server. This corruption typically causes the application to crash, resulting in a Denial of Service for clients attempting to establish connections via the affected endpoint. While heap corruption can theoretically be leveraged for arbitrary code execution through sophisticated exploitation techniques involving allocator manipulation and control flow hijacking, current analysis indicates that this specific instance is highly improbable to exploit remotely for such purposes. The primary vector remains limited to causing process termination rather than gaining shell access or escalating privileges on the host system.
From a threat modeling perspective, an attacker can induce this failure with relatively low effort by crafting malformed INITIAL packets that violate RFC 9000 compliance standards. Specifically, sending initial packets containing Destination Connection IDs shorter than eight bytes will trigger the error path in ossl_quic_lcidm_enrol_odcid, leading directly to the double free scenario within port_bind_channel and subsequently in the handler's cleanup routine. This aligns with ATT&CK techniques related to resource exhaustion or application crashes, as the attacker does not need complex payloads but rather specific structural violations of the protocol specification to disrupt service continuity.
Regarding compliance and security boundaries, it is important to note that this vulnerability resides outside the OpenSSL FIPS module boundary. Consequently, systems relying on strict Federal Information Processing Standards validation for cryptographic operations are not directly impacted by this memory management flaw in terms of their certified status. However, the availability impact remains significant regardless of FIPS certification because the underlying QUIC implementation handles network I/O and connection state management independently of the core cryptographic engine's compliance scope. Mitigation strategies should focus on applying vendor-provided patches that correct the error handling logic to ensure pointers are nullified after freeing or restructuring the control flow to prevent dual deallocation. Additionally, implementing strict input validation at the protocol level can help filter out malformed packets before they reach the vulnerable code paths, thereby reducing the attack surface for potential denial-of-service attempts.