CVE-2026-18798: OpenSSL QUIC Server DOS Vulnerability

CVE-2026-18798 Overview

CVE-2026-18798 is a double free vulnerability [CWE-415] in the OpenSSL QUIC server implementation. The flaw resides in port_default_packet_handler(), where the QUIC record layer receive (QRX) object can be freed twice when channel creation fails for an initial packet. A remote attacker can trigger the condition by sending a malformed INITIAL packet carrying a Destination Connection ID (DCID) shorter than 8 bytes, which is non-compliant with RFC 9000. The resulting heap corruption typically terminates the QUIC server process, leading to Denial of Service. The OpenSSL FIPS module is not affected because the QUIC implementation lies outside the FIPS module boundary.

Critical Impact
Remote unauthenticated attackers can crash OpenSSL-based QUIC servers by sending a single malformed INITIAL packet, resulting in service disruption.

Affected Products

OpenSSL versions containing the QUIC server stack with port_default_packet_handler()
Applications and services that expose OpenSSL QUIC server functionality over the network
The OpenSSL FIPS module is not affected

Discovery Timeline

2026-08-25 - CVE CVE-2026-18798 published to NVD
2026-08-25 - OpenSSL Security Advisory published
2026-08-25 - Last updated in NVD database

Technical Details for CVE-2026-18798

Vulnerability Analysis

The OpenSSL QUIC stack validates incoming initial packets inside the default packet handler port_default_packet_handler(). To perform validation, the handler allocates a QRX (QUIC record layer RX) object. When validation succeeds, the handler proceeds to create a connection object by invoking port_bind_channel(), passing ownership of the QRX object to the newly formed channel.

If port_bind_channel() fails after taking the QRX pointer, it frees the QRX internally before returning an error. The caller, port_default_packet_handler(), does not know the QRX has already been released. It enters its error branch and calls ossl_qrx_free() on the same pointer, resulting in a double free of a heap object.

Double free conditions corrupt heap metadata, typically causing the QUIC server process to abort. Remote code execution is considered highly improbable, but process termination is a reliable outcome and yields a denial of service.

Root Cause

The root cause is unclear ownership semantics for the QRX object across port_default_packet_handler() and port_bind_channel(). Both functions assume responsibility for freeing the QRX on failure, so any failure path inside port_bind_channel() produces overlapping free operations on the same heap allocation.

Attack Vector

The failure inside port_bind_channel() is easy to induce remotely. An attacker sends a malformed INITIAL packet whose DCID is shorter than the 8-byte minimum required by RFC 9000. The function ossl_quic_lcidm_enrol_odcid() rejects the invalid DCID length, port_bind_channel() jumps to its error path and frees the QRX, and control returns to the caller which frees the QRX again. No authentication or user interaction is required, and a single crafted UDP datagram is sufficient.

/* Patch: ssl/quic/quic_port.c
 * Ensures QRX is freed only in the caller by preventing double ownership
 * when channel allocation succeeds but a later step fails.
 */
     * start by allocation and provisioning as much of the channel as we can
     */
    ch = ossl_quic_channel_alloc(&args);
-    if (ch == NULL)
+    if (ch == NULL) {
+        ossl_qrx_free(qrx);
        return NULL;
+    }

    /*
     * Fixup the channel tls connection here before we init the channel

Source: OpenSSL commit 70cebd74

The upstream fix also introduces reference counting on the QRX object via a new ossl_qrx_newref() API, so multiple owners can safely hold and release references without producing a double free.

/* Patch: include/internal/quic_record_rx.h */
 OSSL_QRX *ossl_qrx_new(const OSSL_QRX_ARGS *args);

/*
- * Frees the QRX. All packets obtained using ossl_qrx_read_pkt must already
- * have been released by calling ossl_qrx_release_pkt.
+ * Frees the QRX/reference to QRX. Frees the QRX object, if all references are
+ * gone. All packets obtained using ossl_qrx_read_pkt must already have been
+ * released by calling ossl_qrx_release_pkt.
 */
 void ossl_qrx_free(OSSL_QRX *qrx);

+/*
+ * Obtains a new reference to QRX object. Returns NULL if reference can not
+ * be obtained.
+ */
+OSSL_QRX *ossl_qrx_newref(OSSL_QRX *qrx);

Source: OpenSSL commit 967582d5

Detection Methods for CVE-2026-18798

Indicators of Compromise

Unexpected termination or repeated crash-restart cycles of OpenSSL QUIC server processes
Heap corruption signatures in core dumps referencing ossl_qrx_free or port_default_packet_handler
Bursts of QUIC INITIAL packets from a single source containing DCIDs shorter than 8 bytes
Glibc double free or corruption errors in application or systemd journal logs

Detection Strategies

Inspect UDP/443 (or other QUIC listener ports) for INITIAL packets whose DCID length field is below 8, which violates RFC 9000
Correlate QUIC server process aborts (SIGABRT, SIGSEGV) with inbound QUIC traffic timestamps
Enable AddressSanitizer or malloc hardening in test environments to surface double free events on QRX allocations

Monitoring Recommendations

Alert on repeated restarts of OpenSSL-linked QUIC services within a short interval
Deploy network IDS signatures for malformed QUIC INITIAL packets with undersized DCIDs
Ingest QUIC server logs and crash telemetry into a centralized data lake for correlation with source IP reputation

How to Mitigate CVE-2026-18798

Immediate Actions Required

Upgrade OpenSSL to the fixed release identified in the OpenSSL Security Advisory dated 2026-08-25
Inventory all applications statically or dynamically linked against OpenSSL with QUIC server support enabled
Restrict exposure of QUIC listeners to trusted networks until patches are deployed
Enable automatic process supervision so crashed QUIC services restart, reducing sustained downtime during exploitation attempts

Patch Information

The fix is delivered in the following upstream commits: OpenSSL commit 70cebd74, OpenSSL commit 967582d5, and OpenSSL commit a14a1dea. Consult the OpenSSL Security Advisory 2026-08-25 for the specific fixed versions applicable to each supported branch.

Workarounds

Disable the OpenSSL QUIC server code path in applications that do not require QUIC
Front QUIC endpoints with a validating proxy or load balancer that drops INITIAL packets whose DCID length is less than 8 bytes
Apply network-layer rate limiting on UDP QUIC ports to reduce crash-restart amplification

bash

# Example nftables rule to drop QUIC INITIAL packets with undersized DCIDs
# Note: adjust offset/length to match your traffic profile; this is illustrative
nft add rule inet filter input udp dport 443 \
    meta l4proto udp \
    @th,64,8 < 8 drop

# Verify OpenSSL version after patching
openssl version -a