CVE-2016-6263 in libidn
Summary
by MITRE
The stringprep_utf8_nfkc_normalize function in lib/nfkc.c in libidn before 1.33 allows context-dependent attackers to cause a denial of service (out-of-bounds read and crash) via crafted UTF-8 data.
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Analysis
by VulDB Data Team • 09/15/2022
The vulnerability identified as CVE-2016-6263 resides within the libidn library's stringprep_utf8_nfkc_normalize function located in lib/nfkc.c. This flaw represents a critical security issue affecting versions of libidn prior to 1.33, where the function fails to properly validate UTF-8 input data during normalization processes. The vulnerability manifests as a context-dependent out-of-bounds read condition that can be exploited by malicious actors to trigger application crashes and potentially achieve denial of service outcomes. The root cause stems from inadequate input sanitization mechanisms that do not account for malformed UTF-8 sequences, particularly those containing non-ASCII characters that may be processed through the normalization algorithm.
The technical exploitation of this vulnerability occurs when crafted UTF-8 data is passed to the stringprep_utf8_nfkc_normalize function, which then attempts to process the input without proper bounds checking. This deficiency allows attackers to manipulate memory access patterns that result in reading beyond allocated buffer boundaries. The flaw specifically affects the normalization process that converts UTF-8 strings to their canonical form using NFKC (Normalization Form Compatibility Decomposition followed by Canonical Composition) rules. When encountering malformed UTF-8 sequences, the function's internal state management becomes compromised, leading to unpredictable memory access patterns that ultimately result in application crashes. This behavior aligns with CWE-125, which describes out-of-bounds read vulnerabilities, and represents a classic example of memory safety issues in string processing libraries. The vulnerability demonstrates characteristics consistent with ATT&CK technique T1499.004, which involves network denial of service attacks through application flaws.
The operational impact of this vulnerability extends beyond simple service disruption to potentially enable more sophisticated attack vectors. An attacker could leverage this flaw to cause repeated application crashes, leading to sustained denial of service conditions that affect legitimate users and systems relying on libidn for internationalized domain name processing. The vulnerability is particularly concerning in network services that handle user-supplied data, such as web servers, mail systems, and DNS resolvers that utilize libidn for domain name normalization. The out-of-bounds read condition can also potentially be chained with other vulnerabilities to achieve more severe outcomes including arbitrary code execution, though the immediate impact is primarily denial of service. Organizations using affected versions of libidn should consider the broader implications for their security posture, as this vulnerability can be exploited by attackers without requiring elevated privileges or specialized knowledge of the target system architecture.
Mitigation strategies for CVE-2016-6263 focus primarily on upgrading to libidn version 1.33 or later, which contains the necessary patches to address the normalization function's input validation issues. System administrators should implement comprehensive patch management procedures to ensure all affected systems receive the security updates promptly. Additionally, organizations can implement input validation measures at application layers that interface with libidn to provide additional defense in depth. Monitoring and logging mechanisms should be enhanced to detect potential exploitation attempts through unusual patterns of malformed UTF-8 data. The vulnerability highlights the importance of proper memory safety practices in cryptographic and internationalization libraries, where input validation becomes critical for preventing exploitation. Security teams should also consider implementing network segmentation and access controls to limit potential attack surfaces where libidn functionality is utilized, particularly in internet-facing services that process user-generated content.