URGENT RCE ALERT! Nation-State Campaigns Are Hitting Cisco’s SNMP Zero-Day (CVE-2025-20352)
By CyberDudeBivash • September 26, 2025 • Critical Infrastructure Advisory
A high-severity flaw in the SNMP subsystem of Cisco IOS / IOS XE can lead to device reload (DoS) or—under common misconfigurations—root-level Remote Code Execution (RCE).
Cisco confirms exploitation in the wild. Treat all SNMP-enabled devices as at risk and apply the mitigations below immediately.
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Your Network Emergency Kit
Defensive-Only Advisory: This briefing focuses on mitigation, detection, and response. No exploit details are shared.
Executive Summary
CVE-2025-20352 is a stack-overflow vulnerability in the SNMP subsystem of Cisco IOS and IOS XE. With only read-only SNMP access, an attacker can trigger a device reload (DoS). In environments where the attacker also possesses device administrative/priv-15 credentials, the flaw can be pushed to root-level RCE on IOS XE, enabling full device takeover and a pivot deeper into your network. Cisco has confirmed real-world exploitation and released fixed builds; you must assume exposure and act now.
What is CVE-2025-20352?
A flaw in the SNMP subsystem of Cisco IOS / IOS XE that, when sent crafted SNMP packets, can cause a reload (DoS) or—if the attacker also holds device admin privileges—allow arbitrary code execution as root on IOS XE. It impacts all SNMP versions (v1/v2c/v3) when SNMP access exists. The risk is amplified in enterprises where SNMP is broadly enabled and device admin credentials are reused or poorly segmented.
Why it’s dangerous
- Device control: Root RCE on a router/switch central to your network gives an adversary packet visibility, policy manipulation, and stealthy persistence.
- Network pivot: With control of routing and ACLs, attackers can redirect traffic, siphon credentials, and stage further intrusions.
- Operational disruption: Even the DoS condition can cascade to outages across branches and data centers.
Affected software & devices
- Cisco IOS & IOS XE devices with SNMP enabled and without the affected OID excluded.
- Meraki MS390 & Catalyst 9300 (Meraki CS ≤ 17) called out by Cisco as impacted until patched.
- Not affected: Cisco IOS XR, NX-OS (per Cisco).
Exploitation prerequisites (as documented by Cisco):
- For DoS: SNMPv2c (or earlier) read-only community, or valid SNMPv3 user credentials.
- For RCE: the above plus device administrative / privilege-15 credentials (often present in real networks).
Quick exposure checks (safe, no scanning against others)
- Inventory: From your NMS or CMDB, list devices with SNMP enabled and reachable (mgmt VRF or otherwise). Verify which subnets can reach UDP/161.
- On device: Check
show snmp host, show running-config | include snmp, and confirm who can reach the SNMP listener.
- Network edge: Confirm SNMP is not reachable from untrusted networks/Internet. If in doubt, block it now and re-enable selectively.
Immediate fixes & mitigations
- Patch priority 0: Upgrade IOS XE to a fixed release (Cisco references 17.15.4a among fixed trains). If upgrade timing is constrained, progress to steps 2–5 first, then schedule the upgrade ASAP.
- Restrict who can talk SNMP: Limit UDP/161 to a dedicated, hardened management segment (PAWs → mgmt VRF → device).
- Disable/Exclude affected OIDs: Follow Cisco mitigation to exclude vulnerable OIDs where supported. Validate NMS behavior after exclusion.
- Enforce SNMPv3 only: Remove v1/v2c communities. Require auth+privacy. Rotate SNMP credentials and device admin creds.
- Zero Internet exposure: No SNMP from or to public networks. Block at perimeter, WAN edges, and inter-site firewalls.
Detection & hunting playbook
Prioritize devices where SNMP is reachable and where admin credentials may have been reused.
- NetFlow/PCAP: Spikes in UDP/161 to core devices from unusual sources; SNMP to devices outside the normal NMS IPs.
- Syslog/SNMP traps: Unexpected reloads; auth failures/bruteforce against SNMPv3; configuration changes shortly after SNMP access.
- SIEM idea: Correlate SNMP access → priv-15/AAA changes → routing/ACL changes within a short window.
Incident response workflow (compressed)
0–60 minutes (containment)
- Block UDP/161 to affected devices from all but PAWs/mgmt networks.
- Rotate device admin passwords and SNMPv3 creds; revoke stale TACACS/RADIUS tokens.
- Export configs and logs:
show tech, show logging, show run, clock/timebase.
1–8 hours (scope & eradicate)
- Audit startup/running configs for unknown users and privilege changes.
- Compare route/ACL tables with last known-good. Validate SNMP communities removed and v3 enforced.
- Stage upgrades to fixed versions; reboot windows coordinated with stakeholders.
Next 7 days (recover & harden)
- Move all network device management to an isolated mgmt VRF/VLAN with strict firewall ACLs.
- Centralize logs in a SIEM with retention; alert on SNMP access outside approved NMS/PAW IPs.
- Quarterly credential rotation for network infrastructure; ban shared admin accounts.
Build long-term resilience
- Privileged Access Workstations (PAWs) for all device admins; no browsing or email on PAWs.
- Network segmentation so NMS/PAWs are the only SNMP sources; egress controls everywhere else.
- Config drift detection and daily backups; immutable log storage.
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#CyberDudeBivash #Cisco #SNMP #CVE202520352 #IOS #IOSXE #Meraki #RCE #RootAccess #DoS #NetworkSecurity #IncidentResponse #ZeroTrust #EDR #SIEM #ThreatIntel
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