MediumCVE-2026-42772

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

Published 29.09.2026Last modified 08.10.2026Sources: Red Hat, Debian, Ubuntu, BSI, CVE.org, EUVD, Msrc Update Guide

What is affected, what to do

A flaw was found in OpenSSL. A remote attacker who establishes a QUIC network connection can cause a Denial of Service (DoS) by sending specially sequenced, out-of-order stream frames.

Update OpenSSL to version 4.0.3, OpenSSL to version 3.6.5, OpenSSL to version 3.5.9.

Affected: OpenSSL lt 4.0.0..4.0.3
OpenSSL lt 3.6.0..3.6.5
OpenSSL lt 3.5.0..3.5.9
OpenSSL lt 3.4.0..3.4.8
Fixed in: OpenSSL 4.0.3
OpenSSL 3.6.5
OpenSSL 3.5.9
OpenSSL 3.4.8

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Metrics

5.3

XIEM Effective CVSS

Highest available CVSS rating from NVD and vendor data.

Source: Vendor (CNA), CVSS v3

5.3

CVSS v3

Severity Medium on the scale from 0 to 10.

Source: Red Hat, Debian, Ubuntu, BSI, CVE.org, EUVD, Msrc Update Guide

0.5 %

EPSS score

Low — model estimates < 1% exploitation likelihood.

Source: FIRST EPSS, percentile 43.1

Show all metrics

Severity: Medium
Priority (0–325) The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first? Read in glossary: 68.50 Methodology
Exploit maturity An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack. Read in glossary: no public PoC known
XIEM Effective CVSS XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command. Read in glossary: 5.3 Methodology
CVSS v3 CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions. Read in glossary: 5.3
EPSS score The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“. Read in glossary: 0.5 %; Low — model estimates < 1% exploitation likelihood.
EPSS percentile The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking. Read in glossary: 43.1; High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
KEV status The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching. Read in glossary: —
Published: 29.09.2026
CISA assessment (SSVC): Exploitation: none known · automatable · Technical impact: partial; Source: CISA Vulnrichment, as of 30 September 2026
CWE CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect. Read in glossary: CWE-407

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2 NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management. Read in glossary: Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
CRA The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates. Read in glossary: From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001 ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security. Read in glossary: Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

A flaw was found in OpenSSL. A remote attacker who establishes a QUIC network connection can cause a Denial of Service (DoS) by sending specially sequenced, out-of-order stream frames. Because the stream reassembly mechanism handles non-sequential data fragments inefficiently, processing these frames forces the server to consume excessive CPU resources. Consequently, an attacker can exhaust system processing capacity using minimal network bandwidth.

Source: redhat_csaf, Debian Security Advisories, Ubuntu Security Notices, BSI WID Portal, BSI CSAF, NVD (NIST), cvelistv5, ENISA EUVD, Microsoft Security Update Guide

Affected products

The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

Affected products
		Affected versions	Fixed in
OpenSSL	OpenSSL	`3.4.0 – < 3.4.8` `3.5.0 – < 3.5.9` `3.6.0 – < 3.6.5` `4.0.0 – < 4.0.3`	`3.4.8` `3.5.9` `3.6.5` `4.0.3`
openssl	openssl	`4.0.0 – < 4.0.3` `3.5.0 – < 3.5.9` `3.6.0 – < 3.6.5` `3.4.0 – < 3.4.8`	`4.0.3` `3.5.9` `3.6.5` `3.4.8`

Vendors and products

Affected operating systems

Affected operating systems

Linux	debian	openssl	`trixie`

Weakness classes (CWE)

CWE-407Inefficient Algorithmic Complexity
Class
An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.
Entry at MITRE CWE-407

References & sources

openssl-library.orgOpenSSL AdvisoryVendor advisory, euvd
github.com4.0.3 git commitPatch, euvd
github.com3.6.5 git commitPatch, euvd
github.com3.5.9 git commitPatch, euvd
github.com3.4.8 git commitPatch, euvd

Changes to this CVE

Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

08.10.2026Initial analysis by NVDNVD

Newly listed as affected:

OpenSSL (3.4.0 – before 3.4.8; 3.5.0 – before 3.5.9; 3.6.0 – before 3.6.5; 4.0.0 – before 4.0.3)

New references:

and 1 more reference

Show 6 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Product configuration	added	—	OR cpe:2.3:a:openssl:openssl::::::::* versions from (including) 3.4.0 up to (excluding) 3.4.8 cpe:2.3:a:openssl:openssl::::::::* versions from (including) 3.5.0 up to (excluding) 3.5.9 cpe:2.3:a:openssl:openssl::::::::* versions from (including) 3.6.0 up to (excluding) 3.6.5 cpe:2.3:a:openssl:openssl::::::::* versions from (including) 4.0.0 up to (excluding) 4.0.3
Reference type	added	—	OpenSSL Software Foundation: https://github.com/openssl/openssl/commit/32d0ed8afe1b8c3e7ece725b44663da3d7087a09 Types: Patch
Reference type	added	—	OpenSSL Software Foundation: https://github.com/openssl/openssl/commit/ca8402e273af4de5b3f04fa61a0f0c02ce3ae20e Types: Patch
Reference type	added	—	OpenSSL Software Foundation: https://github.com/openssl/openssl/commit/eb2becc0a4baea7f3050a247834d0e5c2ebe1773 Types: Patch
Reference type	added	—	OpenSSL Software Foundation: https://github.com/openssl/openssl/commit/f42ae513bbda513b3c121d54834040ee4a0eae1a Types: Patch
Reference type	added	—	OpenSSL Software Foundation: https://openssl-library.org/news/secadv/20260929.txt Types: Vendor Advisory

Source identifier: nvd@nist.gov

30.09.2026Record modifiedCISA ADP

CVSS V3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

CISA assessment (SSVC): exploitation: none known, automatable: yes

Show 2 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
CVSS	added	—	AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
SSVC assessment	added	—	{"id":"CVE-2026-42772","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-09-30T19:35:54.157005Z"}

Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

29.09.2026Receivedopenssl.org

Weakness type: CWE-407

Description added or changed

New references:

and 1 more reference

Show 8 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Description	added	—	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.
Weakness type (CWE)	added	—	CWE-407
Affected versions (publisher)	added	—	New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/42xxx/CVE-2026-42772.json">CVE-2026-42772</a>
Reference	added	—	https://github.com/openssl/openssl/commit/32d0ed8afe1b8c3e7ece725b44663da3d7087a09
Reference	added	—	https://github.com/openssl/openssl/commit/ca8402e273af4de5b3f04fa61a0f0c02ce3ae20e
Reference	added	—	https://github.com/openssl/openssl/commit/eb2becc0a4baea7f3050a247834d0e5c2ebe1773
Reference	added	—	https://github.com/openssl/openssl/commit/f42ae513bbda513b3c121d54834040ee4a0eae1a
Reference	added	—	https://openssl-library.org/news/secadv/20260929.txt

Source identifier: openssl-security@openssl.org

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