QUIC Connection-Level Flow Control is Not Enforced for Streams (CVE-2026-75804)

What is affected, what to do

A flaw was found in OpenSSL. A remote attacker can cause a Denial of Service (DoS) due to missing connection-level flow control enforcement in the QUIC protocol implementation.

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

Not sure whether and how you are affected? Ask a question about this vulnerability

Metrics

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 can cause a Denial of Service (DoS) due to missing connection-level flow control enforcement in the QUIC protocol implementation. By opening multiple streams that respect individual stream limits while preventing data consumption, the attacker can force the system to buffer far more data than permitted by the connection limit. This excessive memory allocation can exhaust available system resources and degrade or terminate the service.

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

Vendors and products

• Debian Openssl
• OpenSSL OpenSSL

Weakness classes (CWE)

• CWE-770Allocation of Resources Without Limits or Throttling

  Base

  The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.

  Entry at MITRE CWE-770

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).

1. 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:

   • https://github.com/openssl/openssl/commit/2e8f54666b3fb7b05ff5f58aa6cac9285163654e
   • https://github.com/openssl/openssl/commit/4533ee8a5686c953ed3b644738ac4bdf20806538
   • https://github.com/openssl/openssl/commit/64d3102fb5b54311e92517f26ba00169d719e74a
   • https://github.com/openssl/openssl/commit/f9eaecf5bdd6692da052bc65b0332af2a938ac03

   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/2e8f54666b3fb7b05ff5f58aa6cac9285163654e Types: Patch
   Reference type	added	—	OpenSSL Software Foundation: https://github.com/openssl/openssl/commit/4533ee8a5686c953ed3b644738ac4bdf20806538 Types: Patch
   Reference type	added	—	OpenSSL Software Foundation: https://github.com/openssl/openssl/commit/64d3102fb5b54311e92517f26ba00169d719e74a Types: Patch
   Reference type	added	—	OpenSSL Software Foundation: https://github.com/openssl/openssl/commit/f9eaecf5bdd6692da052bc65b0332af2a938ac03 Types: Patch
   Reference type	added	—	OpenSSL Software Foundation: https://openssl-library.org/news/secadv/20260929.txt Types: Vendor Advisory

   Source identifier: nvd@nist.gov

2. 29.09.2026Record 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-2026-75804","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-09-29T17:15:09.218928Z"}

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

3. 29.09.2026Receivedopenssl.org

   Weakness type: CWE-770

   Description added or changed

   New references:

   • https://github.com/openssl/openssl/commit/2e8f54666b3fb7b05ff5f58aa6cac9285163654e
   • https://github.com/openssl/openssl/commit/4533ee8a5686c953ed3b644738ac4bdf20806538
   • https://github.com/openssl/openssl/commit/64d3102fb5b54311e92517f26ba00169d719e74a
   • https://github.com/openssl/openssl/commit/f9eaecf5bdd6692da052bc65b0332af2a938ac03

   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: OpenSSL QUIC stack does not enforce connection level flow control for streams. Remote peers may send more bytes as long as they fit within the stream flow control limits. Impact summary: A malicious remote peer may exploit the lack of connection flow control for streams to make the QUIC stack receive ~100MB of memory instead of 768 KiB (default flow control window size). CWE: CWE-770: Allocation of Resources Without Limits or Throttling Description: The local QUIC stack advertises two flow control limits to its remote peer: stream flow control limit and connection flow control limit. The remote peer must follow both limits when transmitting stream data. Whenever the local QUIC stack receives a stream frame, it validates that the size of the received stream frame stays within flow control limits. If either limit is exceeded (stream level or connection level), then the QUIC stack must close the connection with a flow control error. The vulnerable OpenSSL QUIC stack enforces the stream-level but not the connection-level limit. To exploit the issue, three conditions must be met: - the remote peer opens several streams - each stream must stay within the stream-level flow control limit - there must be no zero-offset byte sent on any of the streams (to prevent the vulnerable QUIC stack from consuming data). By meeting the conditions above, the remote peer may make the local stack allocate 2 x MAX_STREAMS x (stream flow control limit) bytes of memory. MAX_STREAMS defaults to 100, and the limit applies to both bidirectional and unidirectional streams, making it 200 in total. The default flow control window for a stream is 512kB. The remote peer may force the vulnerable QUIC stack to allocate 100MB of heap per connection. 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-770
   Affected versions (publisher)	added	—	New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/75xxx/CVE-2026-75804.json">CVE-2026-75804</a>
   Reference	added	—	https://github.com/openssl/openssl/commit/2e8f54666b3fb7b05ff5f58aa6cac9285163654e
   Reference	added	—	https://github.com/openssl/openssl/commit/4533ee8a5686c953ed3b644738ac4bdf20806538
   Reference	added	—	https://github.com/openssl/openssl/commit/64d3102fb5b54311e92517f26ba00169d719e74a
   Reference	added	—	https://github.com/openssl/openssl/commit/f9eaecf5bdd6692da052bc65b0332af2a938ac03
   Reference	added	—	https://openssl-library.org/news/secadv/20260929.txt

   Source identifier: openssl-security@openssl.org