CVE-2026-97031 in crypto tls
Summary
by MITRE • 10/09/2026
Multiple ECH outer extension references are not permitted under RFC 9849; previously, a client could send a well-crafted packet that could trigger memory exhaustion in the server process by specifying multiple references. We now reject these as malformed and curb the memory amplification vector as a result.
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Analysis
by VulDB Data Team • 10/09/2026
The Transport Layer Security protocol has evolved to include Encrypted Client Hello extensions which allow for greater privacy by encrypting parts of the handshake that were previously sent in plaintext. RFC 9849 defines strict rules regarding how these outer extension references should be handled within TLS packets to ensure interoperability and security. A critical flaw existed where a client could exploit this protocol implementation by sending multiple ECH outer extension references in a single packet, which violates the specification requirements that only one such reference is permitted per handshake message. This deviation from the standard allowed for an attack vector that targeted server resource management mechanisms rather than cryptographic weaknesses directly.
When a server receives these malformed packets containing multiple ECH outer extension references, it attempts to process each reference sequentially without adequate validation or bounds checking against expected limits. This behavior leads to excessive memory allocation as the server allocates buffers and structures associated with each additional reference encountered in the packet stream. An attacker can leverage this by sending a continuous stream of such crafted packets, causing the server process to consume increasing amounts of RAM until system resources are exhausted. This results in a denial of service condition where legitimate users cannot establish connections or maintain existing sessions due to resource starvation on the affected host infrastructure.
The operational impact of this vulnerability is significant for any organization relying on TLS termination services that support Encrypted Client Hello functionality. The memory exhaustion vector effectively allows remote unauthenticated attackers to disrupt availability by triggering out-of-memory conditions in server processes handling incoming connection requests. This type of attack does not require authentication or prior knowledge of internal network structures, making it particularly dangerous when exposed to the public internet. Service providers may experience cascading failures if multiple backend servers are impacted simultaneously, leading to widespread service degradation across their infrastructure layers until manual intervention or automatic restarts occur.
Mitigation strategies involve updating TLS implementations to strictly enforce RFC 9849 compliance by rejecting any packet that contains more than one ECH outer extension reference in the handshake phase. Security teams should verify that their web servers and load balancers are running patched versions of software such as OpenSSL, Nginx, or Apache HTTP Server depending on their specific deployment architecture. Additionally, implementing rate limiting at network perimeter devices can help mitigate the impact by detecting and blocking high volumes of malformed TLS handshakes before they reach application layer processing engines. Monitoring for unusual spikes in memory usage correlated with incoming connection attempts serves as an effective detection mechanism for ongoing exploitation attempts while patches are being deployed across affected systems.
This vulnerability aligns with CWE-400 which describes uncontrolled resource consumption leading to denial of service conditions through improper validation of input parameters. From a threat intelligence perspective, the attack technique corresponds to ATT&CK T1498 network denial of service where adversaries leverage protocol ambiguities or implementation flaws to exhaust system resources rather than exploiting logical errors in authentication or authorization mechanisms. Understanding this classification helps security operations centers prioritize remediation efforts based on both technical severity and strategic risk profiles associated with availability-focused attacks against critical infrastructure components supporting encrypted communications protocols.