CVE-2024-25629: C-ares Library DoS Vulnerability

CVE-2024-25629 Overview

CVE-2024-25629 is an out-of-bounds read vulnerability in c-ares, a widely-used C library for performing asynchronous DNS requests. The vulnerability exists in the ares__read_line() function which is responsible for parsing local configuration files such as /etc/resolv.conf, /etc/nsswitch.conf, the HOSTALIASES file, and in versions prior to 1.27.0, the /etc/hosts file. When any of these configuration files contains an embedded NULL character as the first character in a new line, the library attempts to read memory prior to the start of the given buffer, potentially resulting in an application crash.

Critical Impact

Applications using c-ares for DNS resolution may crash when parsing maliciously crafted or corrupted configuration files, leading to denial of service conditions affecting DNS-dependent services and applications.

Affected Products

• c-ares versions prior to 1.27.0
• Fedora 38
• Fedora 39
• Fedora 40

Discovery Timeline

• 2024-02-23 - CVE-2024-25629 published to NVD
• 2025-02-05 - Last updated in NVD database

Technical Details for CVE-2024-25629

Vulnerability Analysis

The vulnerability stems from improper handling of embedded NULL characters during configuration file parsing in the c-ares library. The ares__read_line() function processes lines from various DNS-related configuration files but fails to properly validate the calculated string length when a NULL byte appears as the first character on a new line. This triggers a buffer underflow condition where the code attempts to access memory before the allocated buffer's starting address.

The weakness is classified under CWE-127 (Buffer Under-read) and CWE-125 (Out-of-Bounds Read). The attack requires local access to modify configuration files that c-ares parses, meaning an attacker would need the ability to write to files like /etc/resolv.conf or control the HOSTALIASES environment variable pointing to a malicious file.

Root Cause

The root cause lies in the ares__read_line() function's failure to validate that the calculated string length is greater than zero before performing buffer index operations. When a line starts with a NULL character, the length calculation using ares_strlen() returns zero, but subsequent code still attempts to access buffer indices based on this length without proper bounds checking, leading to access of memory at negative offsets from the buffer start.

Attack Vector

Exploitation requires local access to the target system with the ability to modify DNS configuration files or control environment variables that specify configuration file paths. An attacker could craft a malicious configuration file with a strategically placed NULL byte at the start of a line. When an application using c-ares parses this file during DNS operations, the out-of-bounds read occurs, causing a crash and denial of service.

The attack surface includes:

• Write access to /etc/resolv.conf
• Write access to /etc/nsswitch.conf
• Control over HOSTALIASES environment variable
• Write access to /etc/hosts (in versions prior to 1.27.0)

       return (offset != 0) ? 0 : (ferror(fp)) ? ARES_EFILE : ARES_EOF;
     }
     len = offset + ares_strlen(*buf + offset);

+    /* Probably means there was an embedded NULL as the first character in
+     * the line, throw away line */
+    if (len == 0) {
+      offset = 0;
+      continue;
+    }
+
     if ((*buf)[len - 1] == '\n') {
       (*buf)[len - 1] = 0;
       break;

Source: GitHub Commit a804c04ddc8245fc8adf0e92368709639125e183

The patch adds a validation check to detect when the line length is zero (indicating an embedded NULL as the first character) and discards the line by resetting the offset and continuing to the next iteration.

Detection Methods for CVE-2024-25629

Indicators of Compromise

• Unexpected crashes in applications using c-ares library for DNS resolution
• Presence of configuration files (/etc/resolv.conf, /etc/nsswitch.conf, HOSTALIASES file) containing embedded NULL bytes
• Core dumps from c-ares-dependent applications indicating buffer access violations
• Unusual modifications to DNS configuration files with non-printable characters

Detection Strategies

• Monitor for modifications to /etc/resolv.conf, /etc/nsswitch.conf, and /etc/hosts files, particularly for binary content or NULL bytes
• Implement file integrity monitoring on critical DNS configuration files
• Use application crash monitoring to detect repeated crashes in DNS-dependent services
• Audit HOSTALIASES environment variable usage across running processes

Monitoring Recommendations

• Enable system auditing for write operations to /etc/resolv.conf and related DNS configuration files
• Configure centralized logging to capture application crashes and segmentation faults from c-ares-dependent applications
• Implement automated scanning of configuration files for embedded NULL characters or binary content
• Monitor package versions to identify systems running vulnerable c-ares versions prior to 1.27.0

How to Mitigate CVE-2024-25629

Immediate Actions Required

• Upgrade c-ares to version 1.27.0 or later immediately
• Audit and verify integrity of DNS configuration files on all affected systems
• Restrict write permissions to /etc/resolv.conf, /etc/nsswitch.conf, and /etc/hosts to root only
• Review and sanitize any HOSTALIASES environment variable configurations in use

Patch Information

The vulnerability is fixed in c-ares version 1.27.0. The fix adds validation to check if the line length is zero after parsing, which indicates an embedded NULL character at the start of a line. When detected, the line is discarded and parsing continues with the next line. Users should update to the patched version immediately.

For detailed patch information, see the GitHub Security Advisory and the GitHub Commit.

Fedora users can apply updates through the standard package management system as updates have been released for Fedora 38, 39, and 40.

Workarounds

• No official workarounds exist for this vulnerability according to the vendor advisory
• As a defense-in-depth measure, ensure strict file permissions on DNS configuration files
• Implement file integrity monitoring to detect unauthorized modifications to configuration files
• Consider running DNS-dependent applications with reduced privileges to limit the impact of potential crashes

# Verify c-ares version on your system
pkg-config --modversion libcares

# Update c-ares on Fedora systems
sudo dnf update c-ares

# Verify configuration file permissions
ls -la /etc/resolv.conf /etc/nsswitch.conf /etc/hosts
chmod 644 /etc/resolv.conf /etc/nsswitch.conf /etc/hosts
chown root:root /etc/resolv.conf /etc/nsswitch.conf /etc/hosts