OpenSSL QUIC: double free in QRX during channel init triggers DoS
CVE-2026-18798 Published on August 25, 2026
QUIC Server May Trigger Double Free When Processing INITIAL Packet
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.
Vulnerability Analysis
CVE-2026-18798 is exploitable with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity, and a high impact on availability.
Weakness Type
What is a Double-free Vulnerability?
The product calls free() twice on the same memory address, potentially leading to modification of unexpected memory locations. When a program calls free() twice with the same argument, the program's memory management data structures become corrupted. This corruption can cause the program to crash or, in some circumstances, cause two later calls to malloc() to return the same pointer. If malloc() returns the same value twice and the program later gives the attacker control over the data that is written into this doubly-allocated memory, the program becomes vulnerable to a buffer overflow attack.
CVE-2026-18798 has been classified to as a Double-free vulnerability or weakness.
Products Associated with CVE-2026-18798
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Affected Versions
OpenSSL:- Version 4.0.0 and below 4.0.2 is affected.
- Version 3.6.0 and below 3.6.4 is affected.
- Version 3.5.0 and below 3.5.8 is affected.