CVE-2026-35191
Received Received - Intake

QUIC Server Packet Counting Vulnerability in OpenSSL

Publication date: 2026-09-29

Last updated on: 2026-09-29

Assigner: OpenSSL Software Foundation

Description
Issue summary: The OpenSSL QUIC server, when configured to not preform address validation, can be forced to count incoming packets multiple times in its unvalidated credit computation, leading to a violation of the RFC 9000 unvalidated connection amplification limit of 3 times the amount of data received. Impact summary: A remote attacker able to spoof packets to a server using the OpenSSL QUIC implementation might use the server for an amplification of a DDoS attack. CWE: CWE-440: Expected Behavior Violation Description: OpenSSL's QUIC stack, when operating as a server, enforces client address validation (RFC 9000, Section 8), to confirm the peer address is not used for a traffic amplification attack. If this feature is disabled on the server, the QUIC stack limits the amount of server data that can be sent to 3 times the amount of data received from the peer address, until such time as the TLS handshake is completed. The OpenSSL QUIC server, when operating in non-validation mode, adds the length of the whole datagram received to the unvalidated credit limit when processing each QUIC packet in the datagram. A remote peer may, after establishing a connection with an initial client hello frame, send a subsequent datagram containing multiple QUIC packets, leading the server to account the entire datagram length for each packet in the datagram, resulting in the server believing that the peer has sent more data than it actually has, thereby violating the 3x amplification limit mandated by the RFC. FIPS impact: no As the QUIC stack lives outside the FIPS module boundary, no FIPS modules are affected by this CVE.

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Meta Information

Published
2026-09-29
Last Modified
2026-09-29
Generated
2026-09-29
AI Q&A
2026-09-29
EPSS Evaluated
N/A
NVD
CVE-2026-35191
EUVD
EUVD-2026-89002

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
openssl openssl *

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-440 A feature, API, or function does not perform according to its specification.

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Executive Summary

This vulnerability affects OpenSSL's QUIC server implementation. When address validation is disabled, the server incorrectly counts incoming packets multiple times in its credit computation. This violates RFC 9000's limit on unvalidated connection amplification, allowing attackers to exploit the server for DDoS amplification attacks.

Detection Guidance

Detection involves monitoring QUIC traffic for unusual packet amplification. Check OpenSSL QUIC server logs for violations of the 3x amplification limit. Use network tools like tcpdump or Wireshark to inspect QUIC packets for multiple packets within a single datagram sent to the server.

Impact Analysis

If you operate an OpenSSL QUIC server with address validation disabled, a remote attacker could use your server to amplify DDoS attacks. This means your server might send more data to victims than it receives, potentially overwhelming their systems.

Mitigation Strategies

Enable client address validation on the OpenSSL QUIC server to enforce RFC 9000 compliance. Update OpenSSL to the latest version that patches this issue. If using a vulnerable version, restrict QUIC server access to trusted networks until patched.

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