CVE-2026-84784 in OpenSSL
Summary
by MITRE • 09/29/2026
Issue summary: A malicious remote peer may flood the local QUIC stack with NEW_CONNECTION_ID frames by avoiding a limit check on how many connection IDs the remote QUIC stack can use.
Impact summary: The local QUIC stack sends a RETIRE_CONN_ID frame for every NEW_CONNECTION_ID frame it receives. The RETIRE_CONN_ID frame is dispatched via the Control Frame Queue (CFQ). If the remote peer also withholds ACKs, then it can force the local stack to allocate ~400MB (depending on ACK delay).
CWE: CWE-770: Allocation of Resources Without Limits or Throttling
Description: RFC 9000 sections 5.1.1 and 5.1.2 [1] describe the mechanism
by which a remote peer can notify the local QUIC stack to change the destination connection ID (a.k.a. CID) the local stack uses to identify the connection at the remote peer. Each CID is associated with a sequence number. The sequence number is transmitted in NEW_CONNECTION_ID and RETIRE_CONNECTION_ID frames to identify the CID which is being either associated with a connection or retired.
The remote peer sends a NEW_CONNECTION_ID frame to let the local stack know a new CID is being associated with an existing connection. The NEW_CONNECTION_ID frame carries the new CID, its sequence number, and the retire-prior-to number. The retire-prior-to identifies existing CIDs that are to be retired. The local QUIC stack must send a RETIRE_CONNECTION_ID for every destination CID whose sequence number is less than retire-prior-to. The CID becomes retired after the local stack receives an ACK for its RETIRE_CONNECTION_ID frame.
Although the OpenSSL QUIC stack supports at most one destination CID for every connection, it can be tricked into processing more than one RETIRE_CONNECTION_ID frame per connection. The OpenSSL QUIC stack currently retires the destination CID as soon as it receives the NEW_CONNECTION_ID, while in fact the destination CID must be retired after an ACK for the RETIRE_CONNECTION_ID frame is received. Correcting the flawed logic also fixes the backlog growth.
[1] https://datatracker.ietf.org/doc/html/rfc9000#name-issuing-connection-ids
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 • 09/29/2026
The vulnerability described involves a resource exhaustion flaw within the OpenSSL QUIC stack, specifically related to the handling of connection identifier management during QUIC protocol operations. According to RFC 9000 sections 5.1.1 and 5.1.2, remote peers can notify local stacks to change destination connection IDs using NEW_CONNECTION_ID frames. Each connection ID is associated with a sequence number transmitted in these frames as well as RETIRE_CONNECTION_ID frames. The protocol dictates that upon receiving a NEW_CONNECTION_ID frame containing a retire-prior-to value, the local stack must send a RETIRE_CONNECTION_ID for every existing destination CID whose sequence number is less than this threshold. Crucially, under standard compliance, the connection ID should only be considered fully retired after an acknowledgment of the RETIRE_CONNECTION_ID frame is received from the peer.
The core technical flaw lies in the OpenSSL implementation's deviation from this expected behavior. The vulnerable stack processes and retires destination CIDs immediately upon receiving a NEW_CONNECTION_ID frame rather than waiting for the corresponding ACK to confirm receipt by the remote peer. This premature retirement logic creates a state mismatch where the local stack believes it has successfully retired old IDs, while the remote peer may not have received or processed those retire requests if acknowledgments are withheld. Consequently, when a malicious remote peer floods the connection with NEW_CONNECTION_ID frames and simultaneously withholds ACKs for any RETIRE_CONNECTION_ID frames sent by the victim, the local stack continues to allocate resources for managing these pending retirement operations without proper cleanup.
This misalignment leads to significant resource consumption on the affected system. The OpenSSL QUIC stack dispatches each required RETIRE_CONNECTION_ID frame through a Control Frame Queue. As the remote peer continuously sends NEW_CONNECTION_ID frames and ignores acknowledgments, the local queue accumulates entries for every connection ID that needs retirement. This unbounded growth in memory allocation can result in approximately 400MB of RAM being consumed per affected connection, depending on the ACK delay settings configured within the stack. Such behavior constitutes a classic example of CWE-770, Allocation of Resources Without Limits or Throttling, where an attacker exploits a lack of resource constraints to degrade system performance or cause denial of service conditions for legitimate users sharing those resources.
From an operational impact perspective, this vulnerability allows a remote adversary to trigger a local denial of service by exhausting available memory on the host running the vulnerable OpenSSL QUIC implementation. The attack does not require authentication and can be executed over standard network connections utilizing the QUIC protocol. While the FIPS module itself remains unaffected because the QUIC implementation resides outside its boundary, the overall security posture of applications relying on this library is compromised due to potential crashes or instability caused by memory exhaustion. This aligns with ATT&CK techniques related to resource hijacking and denial of service via application layer flooding.
Mitigation strategies should focus on correcting the logical error in connection ID retirement timing. The implementation must be updated to ensure that destination CIDs are only marked as retired after an ACK for the corresponding RETIRE_CONNECTION_ID frame is received, thereby aligning with RFC 9000 specifications and preventing the accumulation of unacknowledged retire requests. Additionally, implementing strict limits on the number of pending connection ID operations or enforcing throttling mechanisms within the Control Frame Queue can provide defense-in-depth against such resource exhaustion attacks until a full patch is deployed. Administrators should monitor for unusual memory usage patterns in QUIC-enabled services and apply vendor-provided updates that address this specific logic flaw to restore proper protocol compliance and system stability.