CVE-2026-3549 in wofSSL
Summary
by MITRE • 03/20/2026
Heap Overflow in TLS 1.3 ECH parsing. An integer underflow existed in ECH extension parsing logic when calculating a buffer length, which resulted in writing beyond the bounds of an allocated buffer. Note that in wolfSSL, ECH is off by default, and the ECH standard is still evolving.
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Analysis
by VulDB Data Team • 03/26/2026
The vulnerability described in CVE-2026-3549 represents a critical heap overflow condition within the TLS 1.3 Encrypted Client Hello (ECH) extension parsing implementation in wolfSSL. This flaw manifests as an integer underflow during buffer length calculation operations when processing ECH extensions, creating a scenario where memory operations extend beyond allocated buffer boundaries. The vulnerability specifically targets the parsing logic of the ECH extension which is designed to provide privacy for client hello messages in TLS 1.3 connections. The integer underflow occurs when the implementation fails to properly validate or handle edge cases in the buffer size calculations, leading to potentially exploitable memory corruption conditions. This issue is particularly significant because ECH represents a relatively new and evolving standard within the TLS ecosystem, with wolfSSL having ECH functionality disabled by default, which may lead to reduced awareness and testing of this particular code path.
The technical implementation of this vulnerability stems from improper input validation within the ECH extension parser where calculations for buffer sizing do not account for potential underflow conditions. When an attacker crafts a malicious ECH extension with carefully constructed parameters, the parsing logic can produce a negative buffer length value or an unexpectedly small buffer size that is insufficient for the data being processed. This results in memory writes occurring at arbitrary locations within the heap memory space, potentially allowing for arbitrary code execution or denial of service conditions. The vulnerability falls under the CWE-129 weakness category, which specifically addresses improper validation of buffer length inputs, and more broadly relates to CWE-190, which covers integer overflow and underflow conditions. From an attack perspective, this vulnerability aligns with the ATT&CK technique T1059.007 for command and scripting interpreter, as successful exploitation could enable attackers to execute arbitrary code within the context of the affected application.
The operational impact of this vulnerability extends beyond simple memory corruption, as it represents a potential path for remote code execution within applications that utilize wolfSSL with ECH functionality enabled. The fact that ECH is disabled by default in wolfSSL means that the attack surface is initially limited, but organizations using wolfSSL in environments where ECH is enabled or configured for use are at risk. The evolving nature of the ECH standard creates additional complexity as implementations may not have been thoroughly tested against edge cases or malicious inputs that could trigger the integer underflow condition. This vulnerability demonstrates the challenges inherent in implementing new cryptographic standards where the full attack surface may not be immediately apparent. Organizations should consider the potential for exploitation through network-based attacks where an adversary can inject malicious ECH extensions into TLS handshakes, particularly in scenarios where ECH functionality has been enabled for privacy or performance optimization purposes.
Mitigation strategies for this vulnerability should focus on immediate code patches that address the integer underflow condition in the ECH extension parsing logic. The fix should implement proper bounds checking and input validation to prevent negative buffer size calculations, ensuring that all buffer length calculations are validated against maximum allowable values. Organizations should also consider disabling ECH functionality in wolfSSL unless it is explicitly required for their operational needs, particularly given that the standard is still evolving. Security monitoring should be enhanced to detect unusual TLS handshake patterns or malformed ECH extensions that might indicate exploitation attempts. The vulnerability highlights the importance of comprehensive testing for cryptographic implementations, particularly for emerging standards where edge cases may not have been fully explored during initial development phases. Additionally, regular security assessments and code reviews should be conducted to identify similar integer overflow or underflow conditions in other cryptographic libraries or custom implementations that may be susceptible to similar issues.