CVE-2026-3849 is a stack buffer overflow vulnerability classified under CWE-787 found in wolfSSL version 5.6.0-stable, specifically in the wc_HpkeLabeledExtract function that processes Encrypted Client Hello (ECH) configurations. ECH is a TLS extension designed to encrypt the ClientHello message to improve privacy. The vulnerability arises when a malicious TLS server sends an oversized ECH configuration, which is not properly bounds-checked, leading to a stack buffer overflow on the client side. This overflow can corrupt the stack, potentially allowing an attacker to execute arbitrary code remotely or cause a denial-of-service via client crashes. The vulnerability does not require any authentication or user interaction and can be triggered simply by establishing a TLS connection with a malicious server that supports ECH. Since ECH is off by default and must be enabled explicitly with the enable-ech option, only clients with this feature enabled are vulnerable. The CVSS 4.0 base score is 6.9, reflecting a medium severity with network attack vector, low attack complexity, no privileges or user interaction required, and impacts on confidentiality, integrity, and availability. No patches have been linked yet, and no known exploits are reported in the wild. The vulnerability highlights the risks of implementing new TLS extensions without thorough input validation and bounds checking.

The vulnerability could allow malicious TLS servers to compromise client systems running vulnerable wolfSSL versions with ECH enabled. Successful exploitation may lead to remote code execution, allowing attackers to execute arbitrary code in the context of the client application, potentially leading to full system compromise. Even if code execution is not achieved, the overflow can cause client crashes, resulting in denial-of-service conditions. This impacts confidentiality by potentially exposing sensitive client data, integrity by allowing unauthorized code execution, and availability by causing application failures. Organizations relying on wolfSSL for secure communications, especially those enabling ECH for privacy enhancements, face risks of targeted attacks from malicious or compromised TLS servers. This is particularly concerning for embedded devices, IoT, and security appliances using wolfSSL. The absence of known exploits reduces immediate risk but does not eliminate the threat, especially as ECH adoption grows.

Organizations should immediately audit their use of wolfSSL to determine if ECH is enabled. If ECH is enabled, disable it until a patched version is available. Monitor wolfSSL vendor advisories for patches addressing CVE-2026-3849 and apply updates promptly once released. Implement network-level controls to restrict connections to trusted TLS servers where feasible. Employ runtime protections such as stack canaries and address space layout randomization (ASLR) to mitigate exploitation impact. Conduct thorough testing of wolfSSL integrations with ECH enabled in controlled environments to detect abnormal behavior. For embedded and IoT devices, ensure firmware updates can be deployed quickly to remediate this vulnerability. Additionally, consider using alternative TLS libraries without this vulnerability if immediate patching is not possible. Maintain vigilant monitoring for suspicious TLS handshake anomalies that could indicate exploitation attempts.