CVE-2026-3849 in wolfSSL
Summary
by MITRE • 03/19/2026
Stack Buffer Overflow in wc_HpkeLabeledExtract via Oversized ECH Config. A vulnerability existed in wolfSSL 5.8.4 ECH (Encrypted Client Hello) support, where a maliciously crafted ECH config could cause a stack buffer overflow on the client side, leading to potential remote execution and client program crash. This could be exploited by a malicious TLS server supporting ECH. Note that ECH is off by default, and is only enabled with enable-ech.
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Analysis
by VulDB Data Team • 03/26/2026
The vulnerability identified as CVE-2026-3849 represents a critical stack buffer overflow flaw within wolfSSL version 5.8.4 that specifically affects the implementation of Encrypted Client Hello (ECH) functionality. This vulnerability manifests when the wolfSSL client processes an oversized ECH configuration provided by a malicious TLS server, creating a condition where memory corruption can occur in the stack-based buffer allocated for handling ECH data structures. The flaw exists within the wc_HpkeLabeledExtract function which serves as a critical component in the ECH implementation, responsible for handling key derivation operations during the encrypted handshake process. The buffer overflow occurs due to insufficient input validation and bounds checking when processing the ECH configuration parameters, particularly when these parameters exceed the expected buffer size limits.
The technical execution of this vulnerability requires a malicious TLS server to first establish a connection with a vulnerable wolfSSL client and then provide an oversized ECH configuration that exceeds the allocated stack buffer space. This scenario aligns with CWE-121 Stack-based Buffer Overflow, which describes a condition where a program writes data beyond the boundaries of a fixed-length buffer allocated on the stack. The operational impact of this vulnerability is significant as it can lead to complete client-side program termination through crashes, but more critically, it may enable remote code execution if proper memory corruption techniques are employed by an attacker. The attack vector specifically targets the client-side implementation where ECH support is explicitly enabled through the enable-ech configuration flag, making the vulnerability conditional on the client having ECH functionality activated.
The security implications extend beyond simple program crashes to potential privilege escalation and system compromise scenarios, particularly when the vulnerable wolfSSL client is running with elevated privileges or is part of critical infrastructure. This vulnerability is particularly concerning within the context of the ATT&CK framework under the T1059 Execution Techniques category, as it could enable adversaries to execute arbitrary code on client systems. The ECH feature itself is designed to enhance privacy by encrypting the client hello message to prevent traffic analysis, but this implementation flaw creates a security regression where an attacker can exploit the very mechanism meant to protect privacy. The vulnerability demonstrates a classic example of how advanced cryptographic features can introduce new attack surfaces when not properly validated, as the oversized parameter handling does not account for potential malicious inputs that could exceed normal operational bounds.
Mitigation strategies for CVE-2026-3849 focus primarily on immediate software updates to wolfSSL versions that address the buffer overflow in wc_HpkeLabeledExtract function. Organizations should disable ECH functionality on affected systems until the patch is applied, as this provides the most effective immediate protection. The recommended approach includes implementing proper input validation and bounds checking for all ECH configuration parameters, ensuring that buffer allocations are sufficient to handle maximum expected input sizes while maintaining defensive programming practices. Additionally, network monitoring should be enhanced to detect potential exploitation attempts through unusual ECH configuration patterns, and system administrators should consider implementing runtime protections such as stack canaries or address space layout randomization to complicate exploitation attempts. The vulnerability highlights the importance of thorough security testing for cryptographic implementations and proper adherence to secure coding practices, particularly when handling user-provided data in security-critical components.