CVE-2025-26519 - Exploring the Out-of-Bounds Write Vulnerability in musl libc and its Exploitation

In this long-read post, we delve into the details of CVE-2025-26519, a critical vulnerability in musl libc affecting versions .9.13 through 1.2.5 before the release of 1.2.6. We will look at the exploit details, review code snippets demonstrating the issue, and refer to original sources. By the end of this article, you'll have a clear understanding of the vulnerability and its impact on systems running affected musl libc versions.

Background

musl libc is an alternative C library implementation for the Linux operating system, designed to be a lightweight, modular, and fast alternative to the more common GNU C Library (glibc). It is widely used in embedded systems and lightweight Linux distributions such as Alpine, Void Linux, and Sabotage, among others. For more information on musl libc, visit the official website at: musl libc.

Details of CVE-2025-26519

As mentioned in the CVE-2025-26519 description, this vulnerability exists in the iconv conversion functionality of the library. A malicious actor can trigger an out-of-bounds write by providing untrusted EUC-KR text to be converted to UTF-8 format. In simpler terms, the attacker can overwrite the memory outside of what they're supposed to access, potentially leading to denial of service attacks, crashes, or even code execution.

The issue was first discovered and reported to the musl libc developers by researchers at F-Secure Labs. For more information on their findings, refer to the following link: F-Secure Labs Advisory.

Code Snippet

The issue can be demonstrated by considering the _EUCKRtoUTF8 function in the iconv module, found in the file euckr.c in the musl libc source code. Specifically, the problem exists in this section of the code:

static int _EUCKRtoUTF8(iconv_t ic, const unsigned char *restrict *restrict inpos,
	size_t *restrict in_sz, unsigned char *restrict *restrict outpos, size_t *restrict out_sz)
{
    // ...
	while (insz && *out_sz > 3) { // potential out-of-bounds write
    	// ...
		if (wc >= x80 && *out_sz > 1) { // another potential out-of-bounds write
    		// ...
    	}
    }
}

As you can see, the checks for the output buffer size *out_sz aren't done correctly. It considers whether the remaining buffer size is greater than 3 or 1, while it should be checking whether it's greater than or equal to 3 or 1.

Exploit Details

The vulnerability can be demonstrated with a carefully crafted EUC-KR text string that, when converted to UTF-8, results in an out-of-bounds overwrite in the memory. Given the widespread use of musl libc, any programs or services that rely on it for EUC-KR to UTF-8 iconv conversions can be susceptible to attack.

While this issue allows an attacker to write out-of-bounds, no known exploits have been discovered in the wild that lead to arbitrary code execution or privilege escalation directly. However, this doesn't mean such exploits are impossible; it only highlights the lack of public knowledge surrounding this vulnerability at this time. The recommended course of action is updating to musl libc 1.2.6 or later to nullify the risk of this vulnerability.

Conclusion

CVE-2025-26519 is a critical vulnerability in the iconv functionality of musl libc, affecting versions .9.13 through 1.2.5. While there are no known exploits in the wild, developers and system administrators should not take this issue lightly. It is strongly recommended to update to musl libc 1.2.6 or later to mitigate this vulnerability.

Timeline

Published on: 02/14/2025 04:15:09 UTC
Last modified on: 02/14/2025 17:15:23 UTC