CVE-2016-6559 in C Library
Summary
by MITRE
Improper bounds checking of the obuf variable in the link_ntoa() function in linkaddr.c of the BSD libc library may allow an attacker to read or write from memory. The full impact and severity depends on the method of exploit and how the library is used by applications. According to analysis by FreeBSD developers, it is very unlikely that applications exist that utilize link_ntoa() in an exploitable manner, and the CERT/CC is not aware of any proof of concept. A blog post describes the functionality of link_ntoa() and points out that none of the base utilities use this function in an exploitable manner. For more information, please see FreeBSD Security Advisory SA-16:37.
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Analysis
by VulDB Data Team • 12/27/2024
The vulnerability identified as CVE-2016-6559 represents a classic buffer overread condition within the BSD libc library's link_ntoa() function located in linkaddr.c. This flaw stems from inadequate bounds checking of the obuf variable, creating a potential pathway for memory access violations that could enable either information disclosure or arbitrary code execution depending on the exploitation method. The vulnerability operates at the intersection of memory safety and network address resolution, where the function processes link-level network addresses and converts them to string representations. The technical nature of this issue aligns with CWE-129, which specifically addresses improper validation of length bounds, and more broadly with CWE-787, concerning out-of-bounds write operations that can lead to memory corruption. The vulnerability exists within the core networking libraries that are fundamental to BSD-based operating systems and their derivatives, making it particularly concerning due to the widespread use of these systems in production environments.
The operational impact of CVE-2016-6559 manifests through potential privilege escalation and information leakage scenarios that could compromise system integrity and confidentiality. While the FreeBSD security team conducted extensive analysis and concluded that no known applications utilize link_ntoa() in an exploitable manner, the theoretical attack surface remains significant due to the function's integration within the standard library. The vulnerability's exploitation potential depends heavily on how applications invoke the function and whether they pass untrusted input that could trigger the bounds checking failure. This aligns with ATT&CK technique T1059.007, which covers command and scripting interpreter usage, as attackers might leverage such vulnerabilities to execute malicious code through compromised network applications. The function's use in network address resolution makes it particularly dangerous because network services commonly invoke it during address parsing operations, potentially allowing attackers to craft malicious network traffic that triggers the vulnerability.
Mitigation strategies for CVE-2016-6559 focus on both immediate patching and long-term architectural improvements to prevent similar vulnerabilities in network address processing functions. The primary solution involves applying the official FreeBSD security patch that corrects the bounds checking implementation in the link_ntoa() function, ensuring that all buffer operations properly validate input parameters before processing. Organizations should also implement network monitoring to detect unusual address resolution patterns that might indicate exploitation attempts, particularly focusing on applications that handle network address conversions. The vulnerability's low likelihood of exploitation in practice, as noted by FreeBSD developers and CERT/CC, suggests that immediate patching may be prioritized based on system criticality rather than urgent response. Security teams should conduct thorough application audits to identify any custom implementations that might indirectly invoke the vulnerable function, as well as review network service configurations that might be susceptible to crafted address inputs. Additionally, implementing runtime protections such as stack canaries and address space layout randomization can provide defense-in-depth measures against potential exploitation attempts, though these are not substitutes for the core patching solution. The vulnerability serves as a reminder of the critical importance of proper input validation in system libraries, particularly those handling network communications where malformed inputs can lead to serious security consequences.