CVE-2026-55958: WolfSSL Renesas TSIP TLS 1.3 Out-of-Bounds Write DoS Vulnerability

Technical summary

The vulnerability resides in the tsip_StoreMessage() function within WolfSSL's Renesas TSIP TLS port. During TLS 1.3 handshake processing, accumulated transcript data is accumulated in a fixed-size buffer (MSGBAG_SIZE) with a capacity of 8 KB. The code contains a capacity check that sets an error condition when the buffer would overflow, but critically, the function fails to return after the check—control flow falls through to an XMEMCPY operation that executes the problematic copy regardless of the error state. Once the handshake transcript size exceeds 8 KB, the copy writes past the allocated buffer boundary into adjacent heap memory. This scenario can occur either through a legitimate but unusually large certificate chain or when a malicious or man-in-the-middle server transmits an oversized handshake message to a client that does not strictly validate certificate chain constraints. The issue is classified as CWE-787 (Out-of-bounds Write) with contributing factors in CWE-393 (Return of Wrong Status Code).

Business impact

Organizations deploying WolfSSL with Renesas TSIP hardware acceleration face localized denial of service risk across their embedded IoT, industrial, or automotive device fleets. An unauthenticated remote attacker positioned to intercept or control a TLS server response (whether through a compromised endpoint or network-level man-in-the-middle capability) can crash connected devices without authentication. For critical infrastructure, medical devices, or connected automotive systems relying on this configuration, unexpected device resets degrade availability and may trigger cascading safety or operational issues. The vulnerability does not enable data exfiltration or unauthorized access, but the DoS impact can be severe in time-sensitive or safety-critical deployments.

Affected systems

This vulnerability exclusively affects WolfSSL builds compiled with the WOLFSSL_RENESAS_TSIP_TLS configuration flag, deployed as a TLS 1.3 client on Renesas microcontrollers with TSIP (Trusted Secure IP) hardware enabled. Other WolfSSL configurations, including software-only TLS implementations and server-mode deployments, are unaffected. Systems using alternative TLS libraries or non-Renesas hardware acceleration are not impacted.

Exploitability

Exploitation is straightforward for a network-positioned attacker. The attack vector is network-based with low attack complexity—no user interaction or special privileges are required. An attacker must be able to present a crafted TLS server response with an oversized handshake message to the vulnerable client. This is easiest if the attacker controls a TLS endpoint (e.g., compromised or rogue server), but can also be achieved through man-in-the-middle interception of legitimate server communications. Clients that enforce strict certificate chain validation may mitigate exposure by rejecting excessively large chains before processing, but the vulnerability remains exploitable against clients with lenient chain size validation.

Remediation

Remediation requires patching WolfSSL to a version that corrects the tsip_StoreMessage() logic. The fix must ensure the function returns immediately upon detecting a buffer overflow condition, preventing the fallthrough to XMEMCPY. Verify the specific patched version against the Renesas and WolfSSL advisory releases. Additionally, configure client-side TLS validation to enforce reasonable limits on certificate chain size, and consider deploying network egress filters to limit the size of TLS handshake messages accepted from untrusted servers. Organizations should prioritize patching in production environments, especially for safety-critical or high-availability systems.

Patch guidance

Obtain and deploy the patched WolfSSL release from the official WolfSSL repository or Renesas security advisory. The patch must be verified against the vendor's official CVE advisory to confirm the specific version numbers addressing this flaw. Recompile affected firmware with the corrected WolfSSL library, ensuring the WOLFSSL_RENESAS_TSIP_TLS flag remains properly configured during the build. Test patched firmware in a staging environment against representative TLS server configurations before production rollout. For OEM customers, coordinate with Renesas to obtain updated reference implementations and firmware images.

Detection guidance

Monitor Renesas MCU devices running WolfSSL for unexpected reboots, reset codes, or heap corruption signatures in debug logs. Network-level detection is difficult because oversized handshake messages can appear superficially legitimate; however, security teams can flag TLS server responses that include unusually large certificate chains or handshake payloads by configuring network sensors or proxies to log and alert on handshake sizes exceeding reasonable thresholds (e.g., >5 MB). Correlation of device crashes with TLS connection attempts to specific remote servers may indicate exploitation attempts. Enable verbose logging in WolfSSL builds to capture transcript size errors (if logging is available in the configuration).

Why prioritize this

This vulnerability merits immediate attention for organizations deploying the specific Renesas TSIP TLS configuration, despite its narrow scope. The CVSS score of 7.5 (HIGH) reflects high exploitability and denial of service impact. Although the affected population is limited to a specialized embedded TLS configuration, the consequences—remote crash of distributed devices with no authentication required—can be severe in critical infrastructure, industrial IoT, and automotive applications. The vulnerability is readily exploitable by a network attacker, and patching is the only reliable remediation path.

Risk score, explained

The CVSS 3.1 score of 7.5 (HIGH) is derived from: Network attack vector (AV:N), low attack complexity (AC:L), no privilege requirement (PR:N), no user interaction (UI:N), unchanged scope (S:U), and high availability impact (A:H). The absence of confidentiality and integrity impact (C:N/I:N) reflects that the vulnerability causes denial of service rather than data compromise. The HIGH severity is appropriate for the affected Renesas TSIP TLS builds, where remote unauthenticated denial of service is a serious operational risk. However, the overall population impact is tempered by the requirement for the specific WOLFSSL_RENESAS_TSIP_TLS configuration; organizations using standard WolfSSL or other TLS libraries face no risk.

Frequently asked questions