CVE-2026-3849 is a stack buffer overflow vulnerability identified in the wolfSSL library, specifically within the wc_HpkeLabeledExtract function that processes Encrypted Client Hello (ECH) configurations. ECH is a TLS extension designed to encrypt the Client Hello message to improve privacy. In wolfSSL version 5.6.0-stable, a maliciously crafted oversized ECH config can trigger a stack buffer overflow on the client side. This overflow may lead to a client program crash or potentially allow remote code execution, as the vulnerability enables overwriting of stack memory. Exploitation requires a malicious TLS server that supports ECH and sends a specially crafted ECH config to the client. Since ECH is disabled by default and must be explicitly enabled with the enable-ech option, the attack surface is limited to clients with ECH enabled. The vulnerability does not require authentication or user interaction, and the attacker can exploit it remotely during the TLS handshake. The CVSS 4.0 base score is 6.9, indicating a medium severity level due to the potential impact on confidentiality and availability, ease of exploitation, and lack of required privileges. No public exploits or active exploitation have been reported as of the publication date. The vulnerability is tracked under CWE-787 (Out-of-bounds Write). No official patches were linked in the provided data, so mitigation currently relies on configuration changes or updates from wolfSSL.

The primary impact of CVE-2026-3849 is on the confidentiality, integrity, and availability of client applications using wolfSSL with ECH enabled. A successful exploit could cause client crashes, resulting in denial of service. More critically, the stack buffer overflow could be leveraged for remote code execution, allowing an attacker to execute arbitrary code on the client system. This could lead to data theft, system compromise, or further lateral movement within a network. Since the vulnerability is triggered during the TLS handshake, it affects all applications relying on wolfSSL for secure communications with ECH enabled, including embedded devices, IoT products, and security-sensitive applications. The lack of authentication or user interaction requirements increases the risk. However, the limited adoption of ECH and its disabled-by-default status reduce the overall exposure. Organizations with wolfSSL-enabled clients that have ECH activated face a moderate risk of targeted attacks, especially if they communicate with untrusted or potentially malicious servers.

To mitigate CVE-2026-3849, organizations should first verify if wolfSSL is used in their environments and whether ECH is enabled. Since ECH is off by default, disabling it (not enabling the enable-ech option) is an immediate mitigation step to eliminate the attack vector. If ECH functionality is required, organizations should monitor wolfSSL vendor advisories for patches addressing this vulnerability and apply updates promptly once available. Additionally, implementing strict TLS server validation and restricting connections to trusted servers can reduce exposure to malicious ECH configs. Employing runtime protections such as stack canaries, address space layout randomization (ASLR), and control flow integrity (CFI) can help mitigate exploitation impact. Network-level controls like TLS inspection and anomaly detection may identify suspicious handshake behaviors. Finally, conducting thorough testing of wolfSSL integrations and monitoring client application crashes or unusual behavior can help detect exploitation attempts early.