CVE-2016-10708 in OpenSSH
Summary
by MITRE
sshd in OpenSSH before 7.4 allows remote attackers to cause a denial of service (NULL pointer dereference and daemon crash) via an out-of-sequence NEWKEYS message, as demonstrated by Honggfuzz, related to kex.c and packet.c.
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Analysis
by VulDB Data Team • 05/05/2026
The vulnerability identified as CVE-2016-10708 represents a critical denial of service flaw within the OpenSSH implementation that affects versions prior to 7.4. This weakness specifically targets the sshd daemon component which handles secure shell connections and authentication processes. The vulnerability manifests when the daemon receives an out-of-sequence NEWKEYS message during the key exchange process, creating a condition where a NULL pointer dereference occurs, ultimately leading to daemon crash and complete service unavailability. The issue was discovered through analysis using the Honggfuzz fuzzer, demonstrating how sophisticated testing tools can reveal critical flaws in widely deployed security infrastructure.
The technical root cause of this vulnerability lies within the key exchange handling code located in the kex.c and packet.c source files of the OpenSSH implementation. During the secure shell key exchange protocol, multiple messages are exchanged between client and server to establish encryption keys for secure communication. The NEWKEYS message is a crucial component in this process, signaling that both parties have completed key exchange and are ready to switch to the newly negotiated encryption keys. When sshd receives this message at an inappropriate time or in an unexpected sequence, the code fails to properly validate the message state, leading to a NULL pointer dereference that terminates the daemon process.
This vulnerability presents significant operational impact for systems relying on OpenSSH for secure remote access and administrative functions. The denial of service condition can be exploited by remote attackers without requiring authentication, making it particularly dangerous in environments where sshd is exposed to untrusted networks. The crash of the sshd daemon results in complete service interruption, preventing legitimate users from establishing secure connections and potentially creating windows for further attacks. Organizations with critical infrastructure relying on SSH services face substantial risk from this vulnerability, as it can be leveraged to disrupt business operations and potentially mask more sophisticated attacks.
The vulnerability maps to CWE-476 which describes NULL pointer dereference conditions, and aligns with ATT&CK technique T1499.004 for network denial of service attacks. Organizations should immediately implement the official patch released by OpenSSH version 7.4, which addresses the improper handling of out-of-sequence key exchange messages. Additional mitigations include implementing network segmentation to limit exposure of sshd services, configuring intrusion detection systems to monitor for anomalous key exchange patterns, and ensuring comprehensive monitoring of sshd daemon processes for unexpected termination events. System administrators should also consider implementing rate limiting and connection throttling mechanisms to reduce the effectiveness of potential exploitation attempts while maintaining service availability for legitimate users.