CVE-2026-1678: DNS Resolver Buffer Overflow Vulnerability

CVE-2026-1678 Overview

A critical out-of-bounds write vulnerability exists in the Zephyr RTOS dns_unpack_name() function. The function caches the buffer tailroom once and reuses it while appending DNS labels. As the buffer grows during DNS name unpacking, the cached size becomes incorrect, allowing the final null terminator to be written past the buffer boundary. When assertions are disabled (the default configuration), a malicious DNS response can trigger this out-of-bounds write condition when CONFIG_DNS_RESOLVER is enabled.

Critical Impact

This vulnerability allows remote attackers to trigger an out-of-bounds write via specially crafted DNS responses, potentially leading to code execution or system compromise on affected Zephyr RTOS devices.

Affected Products

• Zephyr RTOS with CONFIG_DNS_RESOLVER enabled
• Systems with assertions disabled (default configuration)
• IoT and embedded devices running vulnerable Zephyr versions

Discovery Timeline

• 2026-03-05 - CVE CVE-2026-1678 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-1678

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-Bounds Write), a memory corruption flaw that occurs when data is written beyond the boundaries of allocated memory. The root issue lies in how dns_unpack_name() manages buffer size calculations during DNS label processing.

When parsing DNS responses, the function calculates available buffer space (tailroom) at initialization and caches this value for subsequent operations. However, as DNS labels are appended to the buffer, the actual available space decreases while the cached value remains static. This discrepancy creates a classic memory safety issue where the function believes more space is available than actually exists.

The vulnerability can be exploited remotely through network-delivered DNS responses, requiring no authentication or user interaction. An attacker controlling or spoofing DNS responses can craft malicious payloads that trigger the out-of-bounds write condition, potentially corrupting adjacent memory structures or achieving code execution.

Root Cause

The fundamental flaw stems from improper buffer size tracking in the dns_unpack_name() function. The function caches the buffer's available space (tailroom) at the beginning of the operation and fails to update this cached value as data is appended. This stale cache results in incorrect boundary checks when writing the final null terminator, allowing writes beyond the allocated buffer. The issue is exacerbated by the default configuration having assertions disabled, which would otherwise catch this boundary violation during development or testing.

Attack Vector

The attack vector is network-based, targeting the DNS resolver functionality in Zephyr RTOS. An attacker can exploit this vulnerability by:

1. Positioning themselves to respond to DNS queries from the target device (via DNS spoofing, man-in-the-middle, or compromised DNS infrastructure)
2. Crafting a malicious DNS response with carefully constructed label sequences that maximize buffer growth
3. Sending the response to trigger the out-of-bounds write when the null terminator is written

The vulnerability manifests during DNS name unpacking when processing responses with multiple labels. As each label is appended to the output buffer, the cached tailroom value becomes increasingly inaccurate, until the final null terminator write exceeds the actual buffer boundary. For detailed technical information, see the GitHub Security Advisory.

Detection Methods for CVE-2026-1678

Indicators of Compromise

• Unexpected device crashes or reboots when processing DNS responses
• Memory corruption errors or fault conditions in DNS resolver components
• Abnormal DNS response patterns with unusually long or numerous labels
• Stack canary violations or memory protection faults in embedded device logs

Detection Strategies

• Monitor for malformed or oversized DNS responses targeting embedded devices
• Implement network-based detection for DNS responses with excessive label counts or unusual label lengths
• Deploy memory safety monitoring on Zephyr RTOS devices where supported
• Analyze DNS traffic patterns for anomalous response sizes relative to query complexity

Monitoring Recommendations

• Enable verbose logging for DNS resolver operations on Zephyr devices during security assessments
• Implement network intrusion detection rules for suspicious DNS response characteristics
• Monitor device stability metrics for unexplained crashes correlating with DNS activity
• Consider enabling assertions in development and testing environments to catch boundary violations

How to Mitigate CVE-2026-1678

Immediate Actions Required

• Review deployment configurations for CONFIG_DNS_RESOLVER usage and assess exposure
• Enable assertions (CONFIG_ASSERT=y) in test environments to detect exploitation attempts
• Implement network segmentation to limit DNS traffic sources for critical IoT devices
• Apply vendor patches as soon as available from the Zephyr project

Patch Information

Security updates addressing this vulnerability are tracked through the Zephyr Project GitHub Security Advisory. Organizations should monitor this advisory for patch availability and apply updates according to their firmware update procedures. Ensure all affected devices are updated to patched Zephyr RTOS versions once available.

Workarounds

• Disable CONFIG_DNS_RESOLVER if DNS resolution is not required for device functionality
• Enable CONFIG_ASSERT=y to activate runtime assertions that can detect buffer overflows
• Implement upstream DNS filtering to validate response sizes before they reach vulnerable devices
• Consider using static IP configurations or alternative name resolution mechanisms where feasible
• Deploy network-level protections such as DNS response validation proxies

# Configuration example - Enable assertions for buffer overflow detection
# Add to Zephyr project configuration (prj.conf)
CONFIG_ASSERT=y
CONFIG_ASSERT_LEVEL=2

# Alternative: Disable DNS resolver if not required
CONFIG_DNS_RESOLVER=n