CVE-2015-6925 in wolfSSL
Summary
by MITRE
wolfSSL (formerly CyaSSL) before 3.6.8 allows remote attackers to cause a denial of service (resource consumption or traffic amplification) via a crafted DTLS cookie in a ClientHello message.
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Analysis
by VulDB Data Team • 06/28/2018
The vulnerability identified as CVE-2015-6925 affects wolfSSL, formerly known as CyaSSL, versions prior to 3.6.8 and represents a significant denial of service weakness in the Datagram Transport Layer Security implementation. This flaw specifically targets the DTLS cookie handling mechanism during the ClientHello message exchange, creating opportunities for remote attackers to consume excessive system resources or amplify network traffic. The vulnerability resides in the protocol's failure to properly validate and limit the size of DTLS cookies generated during the handshake process, allowing malicious actors to craft specially formatted cookie values that can trigger resource exhaustion conditions.
The technical implementation of this vulnerability stems from insufficient input validation within the DTLS cookie processing logic. When a client sends a ClientHello message containing a malformed or oversized DTLS cookie, the wolfSSL library fails to properly constrain the memory allocation or processing resources dedicated to handling such cookie data. This weakness enables attackers to send crafted cookie values that require disproportionate computational or memory resources to process, effectively creating a resource consumption attack vector. The flaw operates at the protocol level where the DTLS handshake mechanism should validate incoming cookie values against established size and format constraints before proceeding with any resource-intensive processing operations.
From an operational perspective, this vulnerability presents a substantial risk to systems relying on wolfSSL for secure DTLS communications, particularly in environments where network resources are constrained or where the library is deployed in high-traffic scenarios. Attackers can leverage this weakness to perform traffic amplification attacks by sending carefully crafted ClientHello messages that cause the target system to allocate excessive memory or processing cycles, potentially leading to system instability or complete service disruption. The impact extends beyond simple resource exhaustion as the vulnerability can be exploited to create sustained denial of service conditions that are difficult to distinguish from legitimate network congestion, making detection and mitigation challenging. This vulnerability directly maps to CWE-770, which addresses allocation of resources without limits or with inadequate limits, and aligns with ATT&CK technique T1499.004 for network denial of service attacks.
Mitigation strategies for CVE-2015-6925 primarily focus on upgrading to wolfSSL version 3.6.8 or later, which includes proper cookie validation and resource limiting mechanisms. Organizations should implement network-level protections such as rate limiting and connection tracking to prevent abuse of the DTLS handshake process, while also monitoring for unusual patterns in DTLS traffic that might indicate exploitation attempts. Additional defensive measures include configuring firewalls to limit the size of incoming DTLS packets and implementing proper logging to detect anomalous cookie values. The vulnerability highlights the importance of proper resource management in cryptographic libraries and emphasizes the need for thorough input validation in protocol implementations. Security teams should also consider implementing intrusion detection systems that can identify and alert on suspicious DTLS cookie patterns, as well as conducting regular security assessments of all systems utilizing wolfSSL or similar cryptographic libraries to identify similar implementation weaknesses that might exist in other components of the security infrastructure.