CVE-2025-26519: Musl-libc Buffer Overflow Vulnerability

CVE-2025-26519 Overview

CVE-2025-26519 is an out-of-bounds write vulnerability affecting musl libc versions 0.9.13 through 1.2.5. The vulnerability exists in the iconv character conversion implementation and can be triggered when processing untrusted EUC-KR encoded text during conversion to UTF-8. This memory corruption flaw could allow attackers to write data beyond allocated buffer boundaries, potentially leading to code execution or system compromise.

Critical Impact

Successful exploitation of this out-of-bounds write vulnerability could allow attackers to corrupt memory, crash applications, or achieve arbitrary code execution on systems processing untrusted character-encoded data through musl's iconv implementation.

Affected Products

• musl libc versions 0.9.13 through 1.2.5
• Systems and applications built against vulnerable musl libc versions
• Alpine Linux and other musl-based distributions using affected versions

Discovery Timeline

• February 14, 2025 - CVE-2025-26519 published to NVD
• December 10, 2025 - Last updated in NVD database

Technical Details for CVE-2025-26519

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-Bounds Write), a memory corruption issue that occurs when a program writes data past the end or before the beginning of an intended buffer. In the context of CVE-2025-26519, the flaw manifests within musl libc's iconv character set conversion routines, specifically when handling EUC-KR (Extended Unix Code for Korean) to UTF-8 conversions.

The attack requires local access and involves high complexity to successfully exploit. An attacker must be able to control or influence the input to iconv conversion operations, typically by providing maliciously crafted EUC-KR encoded text that triggers the boundary violation during the conversion process. When exploited, this vulnerability can impact confidentiality, integrity, and availability of affected systems.

Root Cause

The root cause of this vulnerability lies in improper bounds checking within the EUC-KR character encoding handler in musl's iconv implementation. During the conversion process from EUC-KR to UTF-8, certain malformed or specially crafted input sequences can cause the conversion routine to write beyond the allocated output buffer boundaries. This occurs because the code fails to properly validate the relationship between input byte sequences and the required output buffer space before performing write operations.

Attack Vector

The attack vector for CVE-2025-26519 is local, requiring an attacker to have the ability to supply untrusted EUC-KR encoded text to an application that uses musl's iconv for character conversion. Exploitation scenarios include:

• Applications that accept user-supplied text files with EUC-KR encoding
• Web applications processing Korean text through iconv conversions
• Mail servers or text processing utilities handling international character sets
• Any service that performs character encoding conversion on untrusted input using musl libc

The vulnerability is exploited by crafting specific EUC-KR byte sequences that, when processed by the vulnerable iconv implementation, cause writes beyond buffer boundaries. The specific exploitation technique involves understanding the internal state machine of the EUC-KR decoder and providing input that confuses the boundary calculations. Detailed technical analysis is available in the Openwall OSS Security Discussion.

Detection Methods for CVE-2025-26519

Indicators of Compromise

• Unexpected application crashes or segmentation faults in programs performing character encoding conversions
• Memory corruption signatures in core dumps from musl-linked applications using iconv
• Anomalous behavior in text processing applications handling Korean-encoded content
• Address Sanitizer (ASan) violations indicating out-of-bounds writes in iconv-related functions

Detection Strategies

• Monitor for abnormal termination of applications that process international text encodings
• Implement runtime memory safety checks using tools like AddressSanitizer during development and testing
• Deploy file integrity monitoring on musl libc shared libraries to detect unauthorized modifications
• Use system call monitoring to identify unusual memory access patterns in iconv operations

Monitoring Recommendations

• Enable verbose logging for applications that handle character encoding conversions
• Monitor system logs for repeated crashes in musl-linked applications processing EUC-KR text
• Implement network monitoring for suspicious payloads containing malformed Korean-encoded content
• Track library version information across containerized environments to identify vulnerable musl versions

How to Mitigate CVE-2025-26519

Immediate Actions Required

• Upgrade musl libc to version 1.2.6 or later immediately on all affected systems
• Identify all applications and containers using vulnerable musl libc versions through software composition analysis
• Implement input validation and sanitization for applications processing untrusted character-encoded text
• Consider temporarily disabling or restricting EUC-KR encoding support in critical applications until patching is complete

Patch Information

The musl project has released fixes addressing this vulnerability. The patches are available in the official musl git repository:

• musl Commit c47ad25e - Primary fix for the out-of-bounds write
• musl Commit e5adcd97 - Additional hardening

Users should upgrade to musl libc version 1.2.6 which incorporates these security fixes. For distributions based on musl (such as Alpine Linux), apply the latest security updates from your distribution's package manager.

Workarounds

• Restrict iconv operations to trusted input sources only until the patch can be applied
• Implement application-level input validation to reject or sanitize potentially malicious EUC-KR encoded data before processing
• Use alternative character encoding libraries for critical applications if immediate patching is not feasible
• Deploy network-level filtering to block or quarantine suspicious EUC-KR encoded content from untrusted sources

# Verify musl libc version on Alpine Linux
apk info musl

# Update musl to patched version
apk update && apk upgrade musl

# For systems building from source, update to 1.2.6 or later
git clone git://git.musl-libc.org/musl
cd musl
git checkout v1.2.6
./configure && make && make install