Executive brief
python-zeroconf is a library used by applications like Home Assistant to discover and register devices on a local network. A vulnerability in how it stores network information allows an attacker on the same local network to flood the system with fake device records. This can lead to the application running out of memory and crashing, or becoming so slow that it can no longer find or manage smart devices, potentially disrupting home automation or office network services.
Technical details
A resource exhaustion vulnerability (CWE-770/CWE-400) exists in python-zeroconf's DNSCache component. Prior to version 0.149.7, the `DNSCache._async_add` method inserted incoming mDNS response records into internal data structures (cache, _expirations, _expire_heap, and service_cache) without any upper bound on the number of entries. An unauthenticated attacker on the same local link can multicast valid mDNS responses with unique names over UDP port 5353 to trigger unbounded memory growth. This results in memory exhaustion (OOM), linear performance degradation of cache lookups, and failure of discovery/registration callbacks. The fix introduces a hard cap of 10,000 records (`_MAX_CACHE_RECORDS`) and implements an eviction policy for the oldest records when the limit is reached.
Affected products
- python-zeroconf python-zeroconf < 0.149.7
Timeline
- 2026-05-17: patched: Fix committed to repository
- 2026-05-18: disclosed: Issue reported on GitHub
- 2026-07-17: advisory: CVE-2026-47184 published
References
- https://github.com/python-zeroconf/python-zeroconf/commit/0ad3f37b5b852b8f614d322283d148efb2cef6e4
- https://github.com/python-zeroconf/python-zeroconf/issues/1715
- https://github.com/python-zeroconf/python-zeroconf/pull/1718
- https://github.com/python-zeroconf/python-zeroconf/releases/tag/0.149.7
- https://github.com/python-zeroconf/python-zeroconf/security/advisories/GHSA-rfg2-pjw2-56x2