CVE-2020-28928 in GraalVM Enterprise Edition
Summary
by MITRE • 11/25/2020
In musl libc through 1.2.1, wcsnrtombs mishandles particular combinations of destination buffer size and source character limit, as demonstrated by an invalid write access (buffer overflow).
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Analysis
by VulDB Data Team • 07/24/2021
The vulnerability identified as CVE-2020-28928 affects the musl libc library version 1.2.1 and earlier, representing a critical buffer overflow condition within the wcsnrtombs function. This function is responsible for converting wide character strings to multibyte character strings, a fundamental operation in internationalized applications that handle Unicode text. The flaw manifests when the destination buffer size and source character limit combinations create a scenario where the function fails to properly bounds-check memory operations, leading to memory corruption that can be exploited by malicious actors. The issue stems from improper handling of edge cases in the wide character to multibyte conversion process, specifically when the function attempts to write beyond the allocated buffer boundaries.
The technical implementation of this vulnerability involves the wcsnrtombs function in musl libc, which is designed to convert wide character strings to multibyte strings while respecting both the destination buffer size and source character limit parameters. When these parameters interact in specific ways, the function's internal loop logic fails to account for all possible boundary conditions, resulting in a situation where the function writes data beyond the intended buffer limits. This creates an invalid write access pattern that can overwrite adjacent memory regions, potentially corrupting program state or enabling arbitrary code execution. The vulnerability is classified under CWE-121 as a stack-based buffer overflow, though it manifests in a heap or data segment context due to the nature of the memory management in the affected library. The flaw represents a classic case of inadequate input validation and memory boundary checking that violates fundamental security principles.
The operational impact of CVE-2020-28928 extends across numerous systems and applications that rely on musl libc for their text processing capabilities, particularly in embedded systems, containerized environments, and any software stack that depends on this lightweight C library implementation. The vulnerability can be exploited by attackers who craft malicious wide character input that triggers the specific buffer overflow condition, potentially leading to privilege escalation, denial of service, or remote code execution depending on the target system's security context. Given that musl libc is widely used in Linux distributions, embedded devices, and container runtimes, the potential attack surface is extensive. The vulnerability affects any application that processes wide character input through the wcsnrtombs function, including web servers, database systems, and network applications that handle internationalized text processing. The exploitability of this vulnerability is particularly concerning in environments where applications process untrusted input from external sources, as the buffer overflow can be triggered through crafted Unicode text sequences that manipulate the destination buffer size and source character limit parameters in the affected function calls.
Mitigation strategies for CVE-2020-28928 primarily focus on updating to musl libc version 1.2.2 or later, where the buffer overflow has been corrected through improved bounds checking and parameter validation. System administrators should prioritize patching affected systems, particularly those running embedded applications or containerized workloads that depend on musl libc. Additionally, developers should review their code to identify any custom implementations or wrapper functions that might interact with wcsnrtombs and ensure proper input validation is implemented at higher layers of the application stack. The vulnerability highlights the importance of thorough testing for edge cases in internationalization functions and demonstrates the need for comprehensive security reviews of core library functions that handle string manipulation. Organizations should also implement runtime protections such as stack canaries, address space layout randomization, and memory protection mechanisms to reduce the impact of potential exploitation attempts. Security monitoring should include detection of unusual memory access patterns and buffer overflow indicators that might suggest exploitation of this or similar vulnerabilities in the broader system landscape.