CVE-2026-47180
MEDIUM6.5zeroconf has unbounded recursion in DNS compression-pointer decoder that allows LAN-local denial of service
Description
### Impact `DNSIncoming._decode_labels_at_offset` recurses once per DNS-name compression pointer (RFC 1035 §4.1.4). Pointer cycles and label counts were capped, but the chain length of unique forward pointers was not. A single ~3 kB mDNS packet carrying ~1500 chained pointers drives the recursion past CPython's default limit, and `RecursionError` was not listed in `DECODE_EXCEPTIONS`, so it escaped `DNSIncoming.__init__` and was logged by asyncio's default exception handler. Any unauthenticated host on the local link (UDP/5353, `224.0.0.251` / `ff02::fb`) can degrade the mDNS listener; that includes a guest on the same Wi-Fi, a compromised IoT device, or a container on a shared bridge. Replaying at a few hertz produces sustained CPU burn and log flooding, and mDNS-dependent features (HomeKit, Chromecast/Matter, AirPlay, printers) degrade while the attack is in flight. ### Patches Fixed in `zeroconf` 0.149.5 ([PR #1719](https://github.com/python-zeroconf/python-zeroconf/pull/1719)). Upgrade to `>= 0.149.5`. ### Workarounds There is no in-process workaround; upgrading is the fix. Otherwise, restrict mDNS (UDP/5353) to trusted Layer-2 segments via AP client isolation, guest-network separation, or host firewall rules. ### Resources - [PR #1719](https://github.com/python-zeroconf/python-zeroconf/pull/1719), fix - [Issue #1713](https://github.com/python-zeroconf/python-zeroconf/issues/1713), public tracking issue - [RFC 1035 §4.1.4](https://www.rfc-editor.org/rfc/rfc1035#section-4.1.4), [RFC 6762](https://www.rfc-editor.org/rfc/rfc6762), [CWE-674](https://cwe.mitre.org/data/definitions/674.html)
Affected packages (1)
- PyPI/zeroconffrom 0, < 0.149.5
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 3.1 | MEDIUM6.5 | CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |