CVE-2026-39849: ftldns — Exploit & Fix

Description

Pi-hole FTL is the core engine of the Pi-hole network-level advertisement and tracker blocker. In versions before 6.6.1, the `dns.interface` configuration field in Pi-hole FTL accepted newline characters without validation, allowing an attacker to inject arbitrary directives into the generated dnsmasq configuration file. On installations with no admin password set (the default for many deployments), the configuration API is fully accessible without credentials, allowing a network-adjacent attacker to inject the payload, enable the built-in DHCP server, and achieve arbitrary command execution on the host the next time any device on the network requests a DHCP lease. The injected value is persisted to /etc/pihole/pihole.toml and survives restarts. The strncpy in the code path limits the total interface field to 31 bytes, but payloads such as wlan0\ndhcp-script=/tmp/p fit within this constraint. The dnsmasq config validation introduced in FTL 6.6 only checks syntactic validity, so valid directives injected via newline pass validation successfully. This issue has been fixed in version 6.6.1.

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CVSS v3.1 Score

8.8

HIGH

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

EPSS — Exploit Prediction

0.0009

Probability of exploitation

0.25%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-93 CWE-93

Affected Products

Vendor	Product	Versions
pi-hole	ftldns	All

References

Advisories & Patches

https://github.com/pi-hole/FTL/commit/0c46e4ec7fe57f762fce261625f2cf5d43806e6d https://github.com/pi-hole/FTL/security/advisories/GHSA-9cqv-839p-gpq2 https://github.com/pi-hole/FTL/security/advisories/GHSA-9cqv-839p-gpq2

Exploits

https://github.com/pi-hole/FTL/security/advisories/GHSA-9cqv-839p-gpq2 https://github.com/pi-hole/FTL/security/advisories/GHSA-9cqv-839p-gpq2

Other References

https://github.com/pi-hole/FTL/releases/tag/v6.6.1

Frequently Asked Questions

What is CVE-2026-39849? +

Pi-hole FTL is the core engine of the Pi-hole network-level advertisement and tracker blocker. In versions before 6.6.1, the `dns.interface` configuration field in Pi-hole FTL accepted newline characters without validation, allowing an attacker to inject arbitrary directives into the generated dnsmasq configuration file. On installations with no admin password set (the default for many deployments), the configuration API is fully accessible without credentials, allowing a network-adjacent attacker to inject the payload, enable the built-in DHCP server, and achieve arbitrary command execution on the host the next time any device on the network requests a DHCP lease. The injected value is persisted to /etc/pihole/pihole.toml and survives restarts. The strncpy in the code path limits the total interface field to 31 bytes, but payloads such as wlan0\ndhcp-script=/tmp/p fit within this constraint. The dnsmasq config validation introduced in FTL 6.6 only checks syntactic validity, so valid directives injected via newline pass validation successfully. This issue has been fixed in version 6.6.1. It has a CVSS v3.1 base score of 8.8 (HIGH).

How severe is CVE-2026-39849? +

CVE-2026-39849 has a CVSS v3.1 score of 8.8 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0009, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-39849? +

CVE-2026-39849 affects products from pi-hole, specifically: ftldns. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-39849? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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