OpenSSL up to 3.5.7/3.6.3/4.0.1 QUIC port_default_packet_handler double free

Summaryinfo

A vulnerability was found in OpenSSL up to 3.5.7/3.6.3/4.0.1. It has been rated as critical. This affects the function port_default_packet_handler of the component QUIC. The manipulation leads to double free. This vulnerability is referenced as CVE-2026-18798. Remote exploitation of the attack is possible. No exploit is available.

Detailsinfo

A vulnerability was found in OpenSSL up to 3.5.7/3.6.3/4.0.1. It has been rated as critical. This issue affects the function port_default_packet_handler of the component QUIC. The manipulation with an unknown input leads to a double free vulnerability. Using CWE to declare the problem leads to CWE-415. The product calls free() twice on the same memory address, potentially leading to modification of unexpected memory locations. Impacted is integrity, and availability. The summary by CVE is:

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.

The advisory is shared at openssl-library.org. The identification of this vulnerability is CVE-2026-18798 since 08/04/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details are known, but no exploit is available. The price for an exploit might be around USD $5k-$25k at the moment (estimation calculated on 08/25/2026).

Upgrading to version 3.5.8, 3.6.4 or 4.0.2 eliminates this vulnerability.

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Productinfo

Type

• Network Encryption Software

Name

• OpenSSL

Version

• 3.5.0
• 3.5.1
• 3.5.2
• 3.5.3
• 3.5.4
• 3.5.5
• 3.5.6
• 3.5.7
• 3.6.0
• 3.6.1
• 3.6.2
• 3.6.3
• 4.0.0
• 4.0.1

License

• open-source

Website

• Product: https://www.openssl.org/

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