CVE-2016-4356 in Libksba
Summary
by MITRE
The append_utf8_value function in the DN decoder (dn.c) in Libksba before 1.3.3 allows remote attackers to cause a denial of service (out-of-bounds read) by clearing the high bit of the byte after invalid utf-8 encoded data.
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Analysis
by VulDB Data Team • 08/23/2022
The vulnerability identified as CVE-2016-4356 resides within the Libksba library version 1.3.2 and earlier, specifically within the DN decoder component that handles distinguished name parsing. This issue manifests in the append_utf8_value function located in the dn.c file, where improper handling of UTF-8 encoded data creates a critical out-of-bounds read condition. The flaw occurs when processing invalid UTF-8 sequences, particularly when the function clears the high bit of a byte that follows invalid UTF-8 encoded data, leading to memory access violations that can be exploited remotely.
The technical implementation of this vulnerability stems from insufficient input validation and boundary checking within the UTF-8 decoding logic. When malformed UTF-8 sequences are encountered, the append_utf8_value function attempts to process the subsequent byte by clearing its high bit, which effectively alters the byte value in a manner that bypasses normal validation checks. This manipulation causes the decoder to read memory locations beyond the intended buffer boundaries, potentially accessing uninitialized memory or data from adjacent memory regions. The out-of-bounds read operation can result in unpredictable behavior including program crashes, data corruption, or information disclosure depending on the memory layout and the specific data being accessed.
From an operational perspective, this vulnerability presents a significant denial of service risk to systems utilizing Libksba for certificate handling and X.509 distinguished name processing. Attackers can craft malicious certificates or data streams containing invalid UTF-8 sequences that trigger the vulnerable code path, causing applications using Libksba to crash or behave unpredictably. The remote exploitation capability means that systems processing certificates from untrusted sources, such as web servers, email servers, or certificate authorities, could be compromised without user interaction. This vulnerability particularly affects applications that parse X.509 certificates, PKCS#12 files, or other cryptographic data structures that utilize distinguished names, making it a widespread concern across cryptographic infrastructure components.
The vulnerability maps to CWE-125 out-of-bounds read within the Common Weakness Enumeration framework, specifically categorized under improper input validation and memory safety issues. From an adversary perspective, this flaw aligns with ATT&CK technique T1059.007 for execution through system services and T1566 for initial access via malicious certificates. The remediation strategy involves upgrading to Libksba version 1.3.3 or later, where the UTF-8 validation logic has been corrected to properly handle invalid sequences without clearing high bits or performing unsafe memory operations. Additionally, administrators should implement input sanitization measures and monitor for unusual certificate processing patterns that might indicate exploitation attempts. The fix typically involves strengthening boundary checking in the UTF-8 decoding routines and ensuring that invalid sequences are properly rejected rather than processed in ways that could lead to memory corruption.