CVE-2026-6507: dnsmasq Out-of-Bounds Write DoS Vulnerability

CVE-2026-6507 Overview

A critical memory corruption vulnerability has been discovered in dnsmasq, a lightweight DNS and DHCP server widely used in network infrastructure. A remote attacker can exploit an out-of-bounds write vulnerability by sending a specially crafted BOOTREPLY (Bootstrap Protocol Reply) packet to a dnsmasq server configured with the --dhcp-split-relay option. This flaw leads to memory corruption, causing the dnsmasq daemon to crash and resulting in a denial of service (DoS) condition.

Critical Impact

Remote attackers can crash dnsmasq services without authentication, disrupting DNS and DHCP services across affected networks. Systems using the --dhcp-split-relay configuration option are vulnerable to network-based attacks.

Affected Products

• dnsmasq instances configured with --dhcp-split-relay option
• Linux distributions and network appliances running vulnerable dnsmasq versions
• Embedded systems and routers utilizing dnsmasq for DHCP relay functionality

Discovery Timeline

• 2026-04-17 - CVE-2026-6507 published to NVD
• 2026-04-17 - Last updated in NVD database

Technical Details for CVE-2026-6507

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-Bounds Write), a dangerous memory corruption flaw that occurs when a program writes data beyond the boundaries of allocated memory. In the context of dnsmasq, the vulnerability manifests when processing BOOTREPLY packets while the --dhcp-split-relay option is enabled.

The flaw allows remote attackers to trigger memory corruption without requiring any authentication or user interaction. The attack can be launched over the network, making it particularly dangerous for internet-facing dnsmasq deployments or internal networks where an attacker has gained initial access.

While the vulnerability does not allow data exfiltration or arbitrary code execution based on current analysis, the availability impact is significant—successful exploitation causes the dnsmasq daemon to crash, disrupting all DNS and DHCP services provided by the affected server.

Root Cause

The root cause of CVE-2026-6507 lies in improper bounds checking when handling BOOTREPLY packets in dnsmasq's DHCP relay implementation. When the --dhcp-split-relay option is configured, the server processes Bootstrap Protocol packets to facilitate DHCP relay operations. The vulnerability occurs because the code fails to properly validate the size and structure of incoming BOOTREPLY packets before writing data to memory buffers.

This inadequate input validation allows a maliciously crafted packet to cause writes beyond the allocated buffer boundaries, corrupting adjacent memory regions and ultimately causing the daemon to crash.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Identifying a dnsmasq server configured with the --dhcp-split-relay option
2. Crafting a malicious BOOTREPLY packet with carefully constructed fields designed to trigger the out-of-bounds write condition
3. Sending the malicious packet to the target dnsmasq server
4. The server processes the packet, triggering memory corruption and crashing the daemon

The vulnerability mechanism involves sending malformed BOOTREPLY packets that exploit insufficient bounds checking in dnsmasq's DHCP relay code path. When the --dhcp-split-relay option is enabled, the server expects to receive and process Bootstrap Protocol replies, but fails to properly validate packet contents before writing them to internal buffers. For detailed technical analysis, refer to the Red Hat Bug Report #2459181.

Detection Methods for CVE-2026-6507

Indicators of Compromise

• Unexpected dnsmasq process crashes or service restarts
• Log entries indicating segmentation faults or memory access violations in dnsmasq
• Unusual BOOTREPLY packets from external or unexpected sources
• Repeated DHCP service disruptions without apparent cause

Detection Strategies

• Monitor dnsmasq process stability and implement alerting for unexpected crashes
• Configure network intrusion detection systems (IDS) to flag anomalous BOOTP/DHCP traffic patterns
• Review system logs for segfault or memory corruption errors related to dnsmasq
• Implement network flow monitoring to detect unusual DHCP relay traffic

Monitoring Recommendations

• Enable verbose logging for dnsmasq to capture packet processing errors
• Deploy network monitoring to identify malformed BOOTP packets targeting dnsmasq servers
• Implement automated service health checks with rapid alerting capabilities
• Use SentinelOne Singularity Platform for real-time endpoint monitoring and crash detection

How to Mitigate CVE-2026-6507

Immediate Actions Required

• Audit all dnsmasq configurations to identify instances using the --dhcp-split-relay option
• Consider temporarily disabling the --dhcp-split-relay option if not operationally critical
• Implement network segmentation to restrict access to dnsmasq servers from untrusted networks
• Deploy firewall rules to limit BOOTP/DHCP traffic to trusted relay agents only

Patch Information

Organizations should monitor vendor security advisories for patch availability. Refer to the Red Hat CVE-2026-6507 Advisory for distribution-specific patch information and updates. Apply vendor-provided patches as soon as they become available.

Workarounds

• Disable the --dhcp-split-relay option if DHCP relay functionality is not required
• Implement firewall rules to block BOOTP traffic (UDP ports 67/68) from untrusted sources
• Deploy dnsmasq behind a reverse proxy or load balancer with packet inspection capabilities
• Consider using alternative DHCP relay implementations until patches are available

# Configuration example - Restrict DHCP traffic with iptables
# Allow BOOTP/DHCP only from trusted relay agents
iptables -A INPUT -p udp --dport 67 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 68 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 67 -j DROP
iptables -A INPUT -p udp --dport 68 -j DROP

# Verify dnsmasq configuration for vulnerable option
grep -r "dhcp-split-relay" /etc/dnsmasq.conf /etc/dnsmasq.d/