CVE-2026-1425: SmartDNS Buffer Overflow Vulnerability

CVE-2026-1425 Overview

A stack-based buffer overflow vulnerability has been discovered in pymumu SmartDNS up to version 47.1. This security flaw affects the _dns_decode_rr_head and _dns_decode_SVCB_HTTPS functions within the src/dns.c file, specifically in the SVCB Record Parser component. When exploited, malicious actors can manipulate DNS records to trigger a buffer overflow condition, potentially leading to code execution or denial of service.

Critical Impact

Remote attackers can exploit this stack-based buffer overflow via network-based attacks, though the high complexity requirement makes successful exploitation difficult.

Affected Products

• pymumu SmartDNS up to version 47.1
• SmartDNS installations using SVCB/HTTPS record parsing functionality
• DNS resolver deployments utilizing vulnerable SmartDNS versions

Discovery Timeline

• 2026-01-26 - CVE-2026-1425 published to NVD
• 2026-01-26 - Last updated in NVD database

Technical Details for CVE-2026-1425

Vulnerability Analysis

This vulnerability resides in the DNS record parsing logic of SmartDNS, specifically within functions responsible for decoding resource record headers and SVCB/HTTPS records. The SVCB (Service Binding) and HTTPS DNS record types are relatively newer additions to the DNS ecosystem, designed to facilitate service discovery and connection optimization.

The vulnerable code path fails to properly validate buffer boundaries when processing malformed or crafted DNS response data. When the parser encounters specially constructed SVCB/HTTPS records, it writes beyond the allocated stack buffer, creating a classic stack-based buffer overflow condition (CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer).

The network-based attack vector means that any SmartDNS instance processing DNS traffic could potentially be targeted, though the high attack complexity indicates that specific conditions must be met for successful exploitation.

Root Cause

The root cause is improper bounds checking in the _dns_decode_rr_head and _dns_decode_SVCB_HTTPS functions within src/dns.c. When parsing DNS records, the code fails to validate that incoming data lengths fit within the allocated stack buffer before copying data, allowing an attacker to overflow the buffer with controlled input.

Attack Vector

The attack is conducted remotely over the network by sending malicious DNS responses containing specially crafted SVCB or HTTPS records to a vulnerable SmartDNS instance. The attacker would need to position themselves to deliver malformed DNS responses to the target, either through DNS cache poisoning, man-in-the-middle positioning, or by controlling a malicious authoritative DNS server.

The exploitation is documented as difficult due to the high complexity requirement. Attackers would need to craft precise payloads that account for stack layout, memory protections, and specific parsing behavior to achieve reliable exploitation.

The security patch identified as commit 2d57c4b4e1add9b4537aeb403f794a084727e1c8 addresses this issue by implementing proper bounds validation. See the GitHub Commit Update for technical details on the fix implementation.

Detection Methods for CVE-2026-1425

Indicators of Compromise

• Unexpected SmartDNS service crashes or restarts, particularly when processing SVCB/HTTPS DNS records
• Anomalous DNS traffic containing malformed or unusually large SVCB/HTTPS record responses
• Core dumps or segmentation fault logs from the SmartDNS process
• Unusual outbound network connections from systems running SmartDNS following DNS query processing

Detection Strategies

• Monitor SmartDNS process stability and implement alerting for unexpected service terminations
• Deploy network intrusion detection signatures for malformed DNS SVCB/HTTPS records with suspicious payload sizes
• Enable verbose logging for DNS record parsing and monitor for error conditions in SVCB/HTTPS processing
• Use memory sanitizers in development/testing environments to detect buffer overflow conditions

Monitoring Recommendations

• Implement DNS traffic inspection at network boundaries to identify anomalous SVCB/HTTPS record responses
• Configure system monitoring to alert on SmartDNS process memory consumption anomalies
• Enable and retain DNS query/response logs for forensic analysis capabilities
• Monitor for exploitation attempts through correlation of DNS traffic patterns with SmartDNS service health metrics

How to Mitigate CVE-2026-1425

Immediate Actions Required

• Update SmartDNS to a version containing patch commit 2d57c4b4e1add9b4537aeb403f794a084727e1c8 or later
• Review network architecture to minimize exposure of SmartDNS instances to untrusted DNS sources
• Implement network-level filtering to block DNS responses from unauthorized sources
• Consider temporarily disabling SVCB/HTTPS record processing if the feature is not required

Patch Information

The vulnerability has been addressed in the SmartDNS repository through commit 2d57c4b4e1add9b4537aeb403f794a084727e1c8. Organizations should update their SmartDNS installations by pulling the latest source code and rebuilding, or by installing updated packages when available from their distribution.

Additional technical details and vulnerability tracking information is available via VulDB #342841.

Workarounds

• Restrict DNS resolution to trusted upstream resolvers only, limiting exposure to attacker-controlled DNS responses
• Deploy SmartDNS behind a validating DNS proxy that can filter malformed records
• Implement network segmentation to isolate SmartDNS instances from untrusted network segments
• Use containerization or sandboxing to limit the impact of potential exploitation

# Configuration example - Restrict upstream resolvers to trusted sources only
# In smartdns.conf, specify only trusted upstream DNS servers
server 8.8.8.8 -group trusted
server 8.8.4.4 -group trusted
server-group trusted

# Optionally disable HTTPS record type if not needed
# (Check SmartDNS documentation for specific configuration options)