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3.7

OpenSSL server can be used to amplify DDoS attacks

CVE-2026-35191 · published 20 days ago
Summary

When OpenSSL’s QUIC server is set to skip address checks, it can mistakenly count incoming data multiple times, allowing an attacker to send small spoofed packets and cause the server to reply with much larger responses. This lets the server be turned into a tool for amplifying denial‑of‑service attacks. Fix the issue by ensuring address validation is enabled or by applying the latest OpenSSL updates that correct the counting logic.

What to do
  • Update canonical openssl to version 3.5.5-1ubuntu3.6.
  • Update debian openssl to version 3.5.7-1~deb13u3.
  • Update debian openssl to version 3.6.5-1.
  • Update bellsoft openssl to version 3.5.9-r0.
  • Update openssl to version 3.5.9-r0.
Affected software
Ecosystem VendorProductAffected versions
– openssl openssl < 4.0.3
>= 3.5.0, < 3.5.9
>= 3.6.0, < 3.6.5
>= 4.0.0, < 4.0.3
Debian:13 debian openssl < 3.5.7-1~deb13u3
Fix: upgrade to 3.5.7-1~deb13u3
Ubuntu:Pro:22.04:LTS canonical nodejs All versions
Ubuntu:26.04:LTS canonical edk2 All versions
Ubuntu:26.04:LTS canonical edk2-hwe All versions
Ubuntu:26.04:LTS canonical openssl < 3.5.5-1ubuntu3.6
Fix: upgrade to 3.5.5-1ubuntu3.6
Debian:14 debian openssl < 3.6.5-1
Fix: upgrade to 3.6.5-1
Alpaquita:25 bellsoft openssl >= 3.5.0-r0, < 3.5.9-r0
Fix: upgrade to 3.5.9-r0
BellSoft Hardened Containers:25 – openssl >= 3.5.0-r0, < 3.5.9-r0
Fix: upgrade to 3.5.9-r0
Original advisory text
BELL-CVE-2026-35191
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.
References
Fix within
Internet-facing 60 days
Internal At next upgrade
  • Not known to be exploited
  • Needs hands-on effort to exploit
  • Gives an attacker partial control
Severity
3.7 Low
Exploitation
<1% chance of attack within 30 days
Type
CWE-440Expected Behavior Violation
Timeline
Published29 Sep 2026
Updated19 Oct 2026
First seen29 Sep 2026
Sources
CVE-2026-35191 · NVD
CVE-2026-35191 · MITRE
DEBIAN-CVE-2026-35191 · OSV
UBUNTU-CVE-2026-35191 · OSV
CVE-2026-35191 · OSV
BELL-CVE-2026-35191 · OSV
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