CVE-2026-42772: Potential CPU DoS via O(n^2) Fragment Reassembly in QUIC

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-29 · updated 2026-10-08
Weakness: CWE-407

Description

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.

Affected products

OpenSSL: from 3.4.0 before 3.4.8; from 3.5.0 before 3.5.9; from 3.6.0 before 3.6.5; from 4.0.0 before 4.0.3 — fixed in 3.4.8, 3.5.9, 3.6.5, 4.0.3

How to fix

Upgrade to a fixed version: OpenSSL 3.4.8, 3.5.9, 3.6.5, 4.0.3. Check the vendor advisory for mitigations.

References

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