CVE-2026-42772 in OpenSSL
Summary
by MITRE • 09/29/2026
Issue summary: The QUIC stream reassembly algorithm performance deteriorates progressively as packets are arriving out of order. The worst case has a quadratic complexity proportional to the number of stream frames kept in the buffer for the received stream data.
Impact summary: A remote QUIC peer that completes the handshake can create a connection-scoped CPU pressure and potentially a Denial of Service using compliant STREAM frames inside the advertised receive window, with low attacker bandwidth.
CWE: CWE-407: Inefficient Algorithmic Complexity
Description: OpenSSL manages received QUIC stream fragments using a doubly-linked list. While it optimizes for append operations (at the end of the list), it falls back to a head-to-tail linear search for any fragment that does not immediately follow the current `tail`.
By manipulating the sequence of offsets, an attacker can force the server to perform O(n^2) operations, consuming excessive CPU time for the QUIC process.
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 identified in this advisory relates to a significant performance degradation within the QUIC stream reassembly algorithm implemented in OpenSSL, classified under CWE-407 for Inefficient Algorithmic Complexity. This flaw arises from the specific data structure chosen to manage received QUIC stream fragments, which utilizes a doubly-linked list optimized primarily for append operations at the end of the sequence. While this design choice improves performance when packets arrive sequentially or near-sequentially, it introduces a critical inefficiency when handling out-of-order packet delivery. In such scenarios, OpenSSL must perform a linear search from the head to the tail of the linked list to locate the correct insertion point for incoming fragments that do not immediately follow the current tail position. This operational mechanism means that as the number of unprocessed or partially processed stream frames in the buffer increases, the computational cost required to reassemble them grows disproportionately rather than proportionally.
The technical flaw manifests when an attacker manipulates the sequence offsets of QUIC packets sent to a vulnerable server. By deliberately sending STREAM frames with non-sequential offsets that fall within the advertised receive window but do not align with the current tail position, the remote peer forces the OpenSSL implementation to execute repeated linear scans through the existing buffer data for each new fragment. This results in quadratic time complexity relative to the number of stream frames kept in the buffer. Consequently, even a moderate volume of carefully crafted traffic can trigger excessive CPU consumption on the server side. The impact is particularly severe because it allows a remote QUIC peer that has successfully completed the cryptographic handshake to exert significant pressure on the system resources without requiring high bandwidth from the attacker's perspective.
From an operational standpoint, this vulnerability enables a Denial of Service attack with low resource requirements for the adversary. Since the flaw triggers during standard stream reassembly processes, it can be exploited by any compliant QUIC client that chooses to send out-of-order data. The resulting CPU exhaustion affects the specific QUIC process handling the connection, potentially leading to service degradation or complete unavailability for other services sharing those resources if not properly isolated. This aligns with ATT&CK technique T1499, Endpoint Denial of Service, specifically under sub-techniques involving resource exhaustion through algorithmic complexity attacks. The ability to cause such impact using compliant protocol frames makes detection and mitigation challenging, as the traffic appears legitimate from a network security monitoring perspective until system performance metrics indicate abnormal CPU usage patterns associated with QUIC connections.
Regarding compliance and regulatory frameworks, it is important to note that this vulnerability does not affect the FIPS module within OpenSSL. The QUIC implementation resides outside the boundary of the Federal Information Processing Standards compliant cryptographic modules, meaning that systems relying solely on FIPS-validated cryptography for their security posture are not directly impacted by this specific algorithmic flaw in terms of cryptographic validation failures. However, the operational availability impact remains a critical concern regardless of FIPS status. Mitigation strategies should focus on applying vendor-provided patches that update the QUIC stream reassembly logic to use more efficient data structures or algorithms with linear time complexity for insertion operations. Additionally, implementing rate limiting and connection throttling policies can help mitigate the immediate impact by restricting the volume of out-of-order packets a single peer can inject into the buffer before triggering defensive mechanisms.