OpenSSL up to 3.5.8/3.6.4/4.0.2 QUIC expected behavior violation

Summaryinfo

A vulnerability was found in OpenSSL up to 3.5.8/3.6.4/4.0.2. It has been rated as problematic. This vulnerability affects unknown code of the component QUIC. The manipulation leads to expected behavior violation. This vulnerability is referenced as CVE-2026-35191. Remote exploitation of the attack is possible. No exploit is available. Upgrading the affected component is advised.

Detailsinfo

A vulnerability was found in OpenSSL up to 3.5.8/3.6.4/4.0.2. It has been rated as problematic. This issue affects an unknown function of the component QUIC. The manipulation with an unknown input leads to a expected behavior violation vulnerability. Using CWE to declare the problem leads to CWE-440. A feature, API, or function does not perform according to its specification. Impacted is availability. The summary by CVE is:

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.

The advisory is shared at openssl-library.org. The identification of this vulnerability is CVE-2026-35191 since 04/01/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Neither technical details nor an exploit are publicly available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 09/29/2026).

Upgrading to version 3.5.9, 3.6.5 or 4.0.3 eliminates this vulnerability.

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Type

• Network Encryption Software

Name

• OpenSSL

Version

• 3.5.0
• 3.5.1
• 3.5.2
• 3.5.3
• 3.5.4
• 3.5.5
• 3.5.6
• 3.5.7
• 3.5.8
• 3.6.0
• 3.6.1
• 3.6.2
• 3.6.3
• 3.6.4
• 4.0.0
• 4.0.1
• 4.0.2

License

• open-source

Website

• Product: https://www.openssl.org/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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