CVE-2026-89147

Net-SNMP through 5.9.5.2 contains an unauthenticated denial-of-service flaw in its SMUX subagent module: smux_accept() performs a blocking read with no timeout on each newly accepted connection. An unauthenticated remote client that connects to the SMUX listener (standard TCP port 199) and simply sends no data causes the single-threaded snmpd main loop to block indefinitely, suspending all SNMP processing on the host. The attacker gains a sustained, trivially repeatable denial of service - SNMP polling and monitoring fail for as long as the idle connection is held - with no confidentiality or integrity impact (CVSS 4.0: 8.7, high availability impact). Any deployment running snmpd with the SMUX listener enabled is affected, with risk concentrated on hosts where the SMUX port is reachable from untrusted networks or the internet. There is no public proof-of-concept, the issue is not in CISA KEV, and no in-the-wild exploitation is known; the CVE was assigned by VulnCheck.

What to do: Upgrade snmpd to a patched Net-SNMP release beyond 5.9.5.2 as soon as your distribution or vendor ships one (no fixed version is named in the current data). As interim mitigation, firewall the SMUX port (TCP 199) from untrusted sources or disable SMUX (remove smuxpeer/smuxsocket directives) if you do not use SMUX subagents, and verify exposure by checking whether snmpd is listening on port 199.