CVE-2016-7406 in Dropbear SSH
Summary
by MITRE
Format string vulnerability in Dropbear SSH before 2016.74 allows remote attackers to execute arbitrary code via format string specifiers in the (1) username or (2) host argument.
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Analysis
by VulDB Data Team • 11/04/2025
The CVE-2016-7406 vulnerability represents a critical format string flaw in the Dropbear SSH implementation that affected versions prior to 2016.74. This vulnerability resides in the authentication handling mechanism of the SSH server, specifically in how it processes user input during the connection establishment phase. The flaw manifests when the server receives specially crafted username or host arguments that contain format string specifiers, creating an exploitable condition that can be leveraged by remote attackers to execute arbitrary code on the affected system.
The technical nature of this vulnerability stems from improper input validation and handling within the Dropbear SSH server implementation. When the server processes authentication requests, it fails to properly sanitize user-supplied data before using it in printf-style functions. This allows attackers to inject format specifiers such as %s, %d, or %x into the username or host arguments, which are then interpreted by the vulnerable printf function. The exploitation occurs because the server does not employ proper input sanitization techniques or use safe string formatting functions that would prevent these specifiers from being executed as code.
The operational impact of CVE-2016-7406 is severe and multifaceted, as it provides remote attackers with full code execution capabilities on systems running vulnerable Dropbear SSH implementations. This vulnerability can be exploited without authentication, making it particularly dangerous as attackers can leverage it to gain unauthorized access to systems. The attack surface includes any system running Dropbear SSH version 2016.73 or earlier, which was widely deployed in embedded systems, routers, and network appliances. Successful exploitation can lead to complete system compromise, data exfiltration, and potential lateral movement within network environments where these systems are deployed.
This vulnerability aligns with CWE-134, which specifically addresses the use of format strings inappropriately, and maps to several ATT&CK techniques including T1078 for valid accounts and T1059 for command and scripting interpreter. The attack chain typically involves an initial reconnaissance phase where attackers identify vulnerable systems, followed by exploitation using crafted format string specifiers that can be used to leak memory addresses, overwrite function pointers, or directly execute shellcode. The vulnerability also demonstrates characteristics of T1566 related to credential access through remote services, as it can be leveraged to bypass authentication mechanisms and gain persistent access to target systems.
Mitigation strategies for CVE-2016-7406 primarily focus on immediate remediation through software updates, with organizations required to upgrade to Dropbear SSH version 2016.74 or later where the vulnerability has been patched. System administrators should also implement network segmentation and access controls to limit exposure of SSH services to untrusted networks. Additional protective measures include monitoring for suspicious authentication attempts and implementing intrusion detection systems that can identify potential exploitation attempts. Organizations should also conduct comprehensive vulnerability assessments to identify all systems running vulnerable Dropbear versions and ensure proper patch management procedures are in place to prevent similar vulnerabilities from occurring in the future.