CVE-2020-28928: Musl-libc Buffer Overflow Vulnerability

CVE-2020-28928 Overview

CVE-2020-28928 is a buffer overflow vulnerability in the musl libc library through version 1.2.1. The wcsnrtombs function mishandles particular combinations of destination buffer size and source character limit, resulting in an invalid write access (buffer overflow). This vulnerability affects systems using musl libc as their standard C library implementation, which is commonly found in lightweight Linux distributions, container images, and embedded systems.

Critical Impact

Applications linked against vulnerable versions of musl libc may experience denial of service conditions through crashes when processing malformed wide character strings with specific buffer size and character limit combinations.

Affected Products

• musl-libc musl (through version 1.2.1)
• Debian Linux 9.0
• Fedora 33 and 34
• Oracle GraalVM Enterprise 20.3.2 and 21.1.0

Discovery Timeline

• 2020-11-24 - CVE-2020-28928 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-28928

Vulnerability Analysis

The vulnerability exists in the wcsnrtombs function within musl libc, which is responsible for converting a wide character string to a multibyte string. The function fails to properly validate the relationship between the destination buffer size and the source character limit parameters. When specific combinations of these values are provided, the function performs write operations beyond the allocated buffer boundaries, resulting in a classic out-of-bounds write condition (CWE-787).

The local attack vector requires an attacker to have the ability to influence the parameters passed to wcsnrtombs or supply crafted input that triggers the vulnerable code path. While exploitation requires local access and low privileges, successful exploitation can cause application crashes, leading to denial of service conditions.

Root Cause

The root cause lies in improper bounds checking within the wcsnrtombs implementation. The function inadequately handles edge cases where the destination buffer size and source character limit create a scenario that the buffer management logic does not anticipate. This results in the function writing data past the end of the destination buffer, corrupting adjacent memory.

The vulnerability is classified as CWE-787 (Out-of-bounds Write), indicating that the software writes data past the intended buffer, which can corrupt data, crash the program, or potentially lead to code execution in more severe scenarios.

Attack Vector

The attack requires local access to the system with low privileges. An attacker could exploit this vulnerability by:

1. Crafting input that causes an application to invoke wcsnrtombs with a carefully chosen destination buffer size and source character limit
2. Providing wide character strings that trigger the specific combination of parameters that expose the vulnerability
3. Causing the vulnerable application to crash or behave unexpectedly

The vulnerability does not require user interaction, and while the scope is unchanged, successful exploitation results in high availability impact through denial of service.

Detection Methods for CVE-2020-28928

Indicators of Compromise

• Unexpected application crashes with stack traces referencing wcsnrtombs or related wide character conversion functions
• Segmentation fault signals in applications using musl libc for string conversion operations
• Core dumps showing memory corruption near wide character string buffers
• Abnormal memory access patterns in applications processing internationalized text

Detection Strategies

• Monitor system logs for segmentation faults in applications known to use musl libc
• Implement crash analysis to identify patterns associated with wide character string processing
• Deploy runtime memory protection tools like AddressSanitizer in development and testing environments
• Use static analysis tools to identify potentially vulnerable code paths calling wcsnrtombs

Monitoring Recommendations

• Enable core dump collection and analysis for applications using musl libc
• Monitor container and Alpine Linux-based systems where musl libc is the default C library
• Implement application-level monitoring for unexpected terminations during string conversion operations
• Track musl libc version inventory across infrastructure to identify vulnerable deployments

How to Mitigate CVE-2020-28928

Immediate Actions Required

• Update musl libc to version 1.2.2 or later, which contains the fix for this vulnerability
• Identify all systems and containers using musl libc (common in Alpine Linux-based images)
• Rebuild container images using updated base images with patched musl libc
• Apply operating system patches from Debian, Fedora, or Oracle as applicable

Patch Information

The musl libc project has addressed this vulnerability in versions after 1.2.1. Patched versions are available from the musl Release Information page. Multiple Linux distributions have released patches addressing this vulnerability:

• Debian has issued patches via their LTS announcement as documented in the Debian LTS Announcement
• Fedora has released package updates for versions 33 and 34 via their package announcement system
• Oracle has addressed this in GraalVM through their Critical Patch Updates documented in the Oracle CPU July 2021 Alert and Oracle CPU October 2021 Alert

Workarounds

• If immediate patching is not possible, implement input validation to restrict the parameters passed to wcsnrtombs functions
• Consider using alternative wide character conversion functions with explicit bounds checking
• Deploy memory protection mechanisms such as stack canaries and ASLR to reduce exploitation risk
• Isolate vulnerable applications in sandboxed environments to limit impact of potential crashes