Description

Avahi is a system which facilitates service discovery on a local network via the mDNS/DNS-SD protocol suite. In versions up to and including 0.9-rc2, the simple protocol server ignores the documented client limit and accepts unlimited connections, allowing for easy local DoS. Although `CLIENTS_MAX` is defined, `server_work()` unconditionally `accept()`s and `client_new()` always appends the new client and increments `n_clients`. There is no check against the limit. When client cannot be accepted as a result of maximal socket number of avahi-daemon, it logs unconditionally error per each connection. Unprivileged local users can exhaust daemon memory and file descriptors, causing a denial of service system-wide for mDNS/DNS-SD. Exhausting local file descriptors causes increased system load caused by logging errors of each of request. Overloading prevents glibc calls using nss-mdns plugins to resolve `*.local.` names and link-local addresses. As of time of publication, no known patched versions are available, but a candidate fix is available in pull request 808, and some workarounds are available. Simple clients are offered for nss-mdns package functionality. It is not possible to disable the unix socket `/run/avahi-daemon/socket`, but resolution requests received via DBus are not affected directly. Tools avahi-resolve, avahi-resolve-address and avahi-resolve-host-name are not affected, they use DBus interface. It is possible to change permissions of unix socket after avahi-daemon is started. But avahi-daemon does not provide any configuration for it. Additional access restrictions like SELinux can also prevent unwanted tools to access the socket and keep resolution working for trusted users.

Weaknesses 1

Affected Products 2

References 4

• openwall.com http://www.openwall.com/lists/oss-security/2025/12/19/1
  ExploitMailing List
• github.com https://github.com/avahi/avahi/pull/808
  Issue TrackingPatch
• github.com https://github.com/avahi/avahi/security/advisories/GHSA-73wf-3xmj-x82q
  ExploitVendor Advisory
• zeropath.com https://zeropath.com/blog/avahi-simple-protocol-server-dos-cve-2025-59529
  ExploitThird Party Advisory

Remediation

• github.com https://github.com/avahi/avahi/pull/808
  Issue TrackingPatch