CVE-2021-41253 in Zydis
Summary
by MITRE • 11/09/2021
Zydis is an x86/x86-64 disassembler library. Users of Zydis versions v3.2.0 and older that use the string functions provided in `zycore` in order to append untrusted user data to the formatter buffer within their custom formatter hooks can run into heap buffer overflows. Older versions of Zydis failed to properly initialize the string object within the formatter buffer, forgetting to initialize a few fields, leaving their value to chance. This could then in turn cause zycore functions like `ZyanStringAppend` to make incorrect calculations for the new target size, resulting in heap memory corruption. This does not affect the regular uncustomized Zydis formatter, because Zydis internally doesn't use the string functions in zycore that act upon these fields. However, because the zycore string functions are the intended way to work with the formatter buffer for users of the library that wish to extend the formatter, we still consider this to be a vulnerability in Zydis. This bug is patched starting in version 3.2.1. As a workaround, users may refrain from using zycore string functions in their formatter hooks until updating to a patched version.
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Analysis
by VulDB Data Team • 11/11/2021
The vulnerability CVE-2021-41253 affects Zydis, an x86/x86-64 disassembler library that serves as a critical component in binary analysis and reverse engineering tools. This issue specifically targets users who implement custom formatter hooks and leverage the zycore string functions to append untrusted user data to the formatter buffer. The flaw exists in versions v3.2.0 and earlier, where the library fails to properly initialize certain fields within the string object contained in the formatter buffer. This improper initialization leaves critical fields with unpredictable values, creating a condition that directly enables heap buffer overflows.
The technical implementation of this vulnerability stems from the zycore library's string handling functions, particularly `ZyanStringAppend`, which performs incorrect size calculations when the internal string object fields remain uninitialized. These fields typically contain essential metadata about string capacity, length, and allocation status that the string manipulation functions rely upon for safe memory operations. When these fields contain garbage values due to improper initialization, the `ZyanStringAppend` function computes erroneous target buffer sizes, leading to memory corruption when attempting to append data beyond allocated heap boundaries. This represents a classic heap-based buffer overflow vulnerability that can be exploited to achieve arbitrary code execution or system instability.
The operational impact of this vulnerability is significant for security tools and applications that depend on Zydis for custom disassembly formatting. Attackers could exploit this flaw by crafting malicious input data that, when processed through custom formatter hooks, triggers the buffer overflow condition. The vulnerability specifically targets users implementing custom formatters rather than those using the default Zydis formatting capabilities, as the internal formatter does not utilize the affected zycore string functions. However, this limitation does not reduce the severity since custom formatters are a common extension pattern in disassembler libraries. The vulnerability affects the broader security ecosystem where disassemblers are used for malware analysis, penetration testing, and binary code inspection, potentially compromising the integrity of these critical security tools.
Security mitigations for this vulnerability include updating to Zydis version 3.2.1 or later, which properly initializes the required string object fields. Users unable to immediately upgrade should avoid employing zycore string functions within their custom formatter hooks until they can apply the patch. From a cybersecurity perspective, this vulnerability aligns with CWE-121, heap-based buffer overflow, and represents a typical example of improper initialization flaws that can be exploited through data handling functions. The ATT&CK framework categorizes this under defensive evasion techniques, as attackers could leverage such vulnerabilities to bypass security controls in binary analysis environments. Organizations should prioritize patching this vulnerability in any systems where Zydis is used for security analysis, particularly in environments handling untrusted binary data or where custom formatter extensions are implemented. The vulnerability demonstrates the importance of proper initialization practices in memory-safe programming and highlights the risks associated with complex library interactions where multiple components must maintain consistent internal state.