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CVE-2026-47180 — Zeroconf: Unbounded recursion in DNS compression-pointer decoder allows LAN-local denial of service

2026-09-30T02:23:15.866431+00:00

Affected: python-zeroconf

Zeroconf is a pure Python implementation of multicast DNS service discovery. Prior to 0.149.5, DNSIncoming._decode_labels_at_offset recurses once per DNS-name compression pointer, and a single mDNS packet carrying chained pointers can trigger a RecursionError that escapes DNSIncoming.__init__, causing sustained CPU burn, log flooding, and degraded mDNS-dependent features for unauthenticated hosts on the local link over UDP/5353 (224.0.0.251 / ff02::fb). This issue is fixed in version 0.149.5.

GHSA-9pgc-3ccv-5297 — zeroconf has unbounded recursion in DNS compression-pointer decoder that allows LAN-local denial of service

2026-09-30T02:23:15.866550+00:00

Affected: PyPI: zeroconf

### 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#sect…

PYSEC-2026-3436 — zeroconf has unbounded recursion in DNS compression-pointer decoder that allows LAN-local denial of service

2026-09-30T02:23:15.866634+00:00

Affected: PyPI: zeroconf

### Impact

### Patches Fixed in `zeroconf` 0.149.5 ([PR #1719](https://github.com/python-zeroconf/python-zeroconf/pull/1719)). Upgrade to `>= 0.149.5`.