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CVE-2025-20352: Cisco IOS/IOS XE SNMP 0-Day — Fix Now

TL;DR (why this matters): CVE-2025-20352 is an actively exploited SNMP flaw in Cisco IOS/IOS XE that can cause device reloads (DoS) and, with higher privileges, remote code execution (RCE). You need a tight, auditable plan to find every SNMP exposure, patch/mitigate, restrict access, and verify the fixes across your fleet.

CVE-2025-20352: Cisco IOS/IOS XE SNMP 0-Day — Fix Now

What Cisco disclosed (and what it means)

  • Vulnerability: Stack-based buffer overflow in the SNMP subsystem of Cisco IOS and IOS XE.
    Impact: Low-privileged attackers can force a reload (DoS); with higher privileges, attackers can run code as root (RCE). CVSS 7.7 (High).
  • Exploit conditions: Requires SNMP access — e.g., v1/v2c read-only community strings or valid SNMPv3 user creds. RCE also needs admin/priv-15 creds. Crafted SNMP packets over IPv4/IPv6 trigger the flaw.
  • Status: Exploitation in the wild confirmed; updates released in Cisco’s September 24, 2025 bundled publication. Patch priority is immediate.

The playbook at a glance

  1. Rapid exposure check → Find where SNMP is enabled (core/edge, WAN, campus). Prioritize Catalyst access/aggregation, ISR/ASR WAN routers, and any management VLAN gateways. (Meraki & other campus stacks: still inventory SNMP exposure even if not in scope for this CVE.)
  2. Remediate fast → Upgrade to fixed trains, disable v1/v2c, enforce SNMPv3 (authPriv) only, and ACL-restrict sources to your NMS.
  3. Harden & monitor → Device ACLs, mgmt-only VRFs/VLANs, traps, and logging.
  4. Verify → Safe SNMP test packets, config diffing, and SIEM rules that catch suspicious SNMP access.

Throughout this guide, we’ll include ready-to-use commands, Ansible snippets, Nmap/net-SNMP checks, and SIEM rules you can drop in today.

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1) Rapid exposure check: find SNMP, everywhere

A. On-box (IOS/IOS XE) quick triage

! Find SNMP lines (communities, users, views, hosts)
show running-config | include ^snmp-server
show run | s snmp-server
show snmp
show snmp user
show snmp group
show access-lists | include SNMP|snmp
show ip interface brief | include up
show control-plane host open-ports

B. Network scans (authorized windows only)

# UDP 161 discovery + banner info (non-intrusive)
nmap -sU -p161 --script snmp-info <target/CIDR>

# If you must validate weak communities in a lab/staging range (not production):
nmap -sU -p161 --script snmp-brute --script-args snmp-brute.communitiesdb=/path/shortlist.txt <targets>
# net-snmp probes (safe reads)
snmpget -v2c -c <RO_STRING> <device> 1.3.6.1.2.1.1.1.0   # sysDescr.0
snmpget -v3 -l authPriv -u <user> -a SHA -A '<authpass>' -x AES -X '<privpass>' <device> 1.3.6.1.2.1.1.5.0  # sysName.0

C. Quick Python (inventory & classify)

# pip install netmiko
from netmiko import ConnectHandler
devices = [
  {"device_type":"cisco_xe","host":"10.0.10.1","username":"netops","password":"***"},
  {"device_type":"cisco_ios","host":"10.0.20.1","username":"netops","password":"***"},
]
report=[]
for d in devices:
    with ConnectHandler(**d) as c:
        out = c.send_command("show run | s snmp-server")
        v3 = "snmp-server group" in out or "snmp-server user" in out
        v2c = "snmp-server community" in out
        acl = any(("RO " in line or "RW " in line) and line.strip().split()[-1].isdigit() for line in out.splitlines())
        report.append({"device": d["host"], "v2c": v2c, "v3": v3, "acl_bound": acl})
print(report)

Triage rule of thumb: Internet-reachable SNMP, access from non-NMS subnets, or any v1/v2c exposure gets top priority.

2) Remediation: upgrade, restrict, and modernize

A. Upgrade to fixed trains

Consult the Cisco advisory for platform-specific fixed releases and upgrade in maintenance windows. Document device → target train mapping in your change ticket and record post-upgrade show version outputs for audit.

Change record template snippet:

- Device: C9500-48Y4C @ DC1
- Current: IOS-XE 17.6.4a
- Target fixed train: 17.x (per advisory)
- Pre-checks: config backup, bootvar, free flash
- Post-checks: SNMPv2 disabled, v3 authPriv only, ACL to NMS, traps ok

B. Disable legacy SNMP and enforce v3 (authPriv)

conf t
 ! kill v1/v2c
 no snmp-server community public
 no snmp-server community private
 ! if you keep any v2c (temporarily), bind to ACL only:
 ! snmp-server community <RO_ONLY> RO 99

 ! v3-only posture (authPriv)
 snmp-server view SECVIEW iso included
 snmp-server group SECGRP v3 priv read SECVIEW
 snmp-server user NMSUSR SECGRP v3 auth sha <AUTHPASS> priv aes 128 <PRIVPASS>

 ! restrict managers to mgmt VLAN/VRF only
 ip access-list standard 99
  permit 10.10.50.0 0.0.0.255   ! NMS subnet
  deny   any log
 exit
 snmp-server community <TEMP_RO> RO 99   ! remove after v3 cutover
end
wr mem

C. Fence SNMP to management networks

  • Use mgmt VLAN/VRF; disallow SNMP on user/data VLANs.
  • Edge/WAN: block UDP/161 inbound except from your NMS jump-boxes.
  • CoPP/CPPr: rate-limit control-plane SNMP if feasible.

Example interface scoping:

conf t
interface Vlan50
 description MGMT
 ip access-group ACL-SNMP-MGMT in
exit

ip access-list extended ACL-SNMP-MGMT
 permit udp 10.10.50.0 0.0.0.255 any eq 161
 deny   udp any any eq 161 log
 permit ip any any

D. Emergency monitoring (until everything’s patched)

Traps & logs

conf t
snmp-server enable traps snmp authentication linkdown linkup coldstart warmStart
snmp-server host 10.10.50.10 version 3 priv NMSUSR
logging host 10.10.60.10 transport udp port 514
logging trap warnings

EEM tripwire (optional) — log any config write touching SNMP

event manager applet SNMP_CHANGE_ALERT
 event cli pattern "conf t" sync yes
 action 1.0 cli command "enable"
 action 2.0 cli command "show archive config differences nvram:startup-config system:running-config | include snmp-server"
 action 3.0 syslog msg "SNMP stanza changed - review immediately"

SIEM rules (examples)

Splunk

index=network syslog ("SNMP" OR "udp/161" OR "Denied-UDP-161")
| stats count by src_ip, dest_ip, msg
| where src_ip != "10.10.50.0/24"

Elastic (KQL)

(event.dataset : "ios.log" and (message : "*SNMP*" or destination.port : 161))
and not source.ip : 10.10.50.0/24

3) Verification: safe tests, diffs, and alerts

A. Safe SNMP test packets (post-change)

# Expect success only from your NMS; all others should fail/deny
snmpget -v3 -l authPriv -u NMSUSR -a SHA -A '<authpass>' -x AES -X '<privpass>' <device> 1.3.6.1.2.1.1.1.0

# Negative test from a non-NMS box should timeout or log ACL deny
snmpget -v2c -c public <device> 1.3.6.1.2.1.1.1.0

B. Config diffing that auditors love

IOS built-in

show archive
show archive config differences nvram:startup-config system:running-config | include snmp|access-list

Git-style diff via Netmiko

from netmiko import ConnectHandler
import difflib

dev={"device_type":"cisco_xe","host":"10.0.10.1","username":"netops","password":"***"}
with ConnectHandler(**dev) as c:
    running = c.send_command("show run")
    with open("prechange_10.0.10.1.txt") as f:
        before = f.read().splitlines()
    after = running.splitlines()
    for line in difflib.unified_diff(before, after, lineterm=""):
        if "snmp" in line or "access-list" in line:
            print(line)

C. Alerting rules to keep

  • Any new SNMP community line → page NetOps.
  • SNMP from non-NMS subnets (v4/v6) → high severity.
  • Burst of SNMP errors or authentication failures → investigate.

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Risk & remediation help (done-for-you)

  • Get a prioritized posture review for CVE-2025-20352 with our Risk Assessment Services — mappings to PCI DSS, ISO 27001, SOC 2, HIPAA, GDPR included.
  • Need hands-on fixes and validation? Our Remediation Services team will disable legacy SNMP, roll out ACLs, upgrade to fixed trains, and provide verification artifacts.
  • Self-serve checks: Run our free scanner to quickly spot exposed services on your web estate: free.pentesttesting.com.

Prefer email? [email protected]

Step-by-step: from risk to verifiable remediation

  1. Inventory (24–48h): Enumerate SNMP on all IOS/IOS XE devices; tag Internet-reachable and v1/v2c usage.
  2. Contain: Block UDP/161 from untrusted networks; limit to mgmt VLAN or out-of-band mgmt.
  3. Upgrade: Move to fixed releases per Cisco advisory; track each device’s before/after state.
  4. Modernize: SNMPv3 authPriv only, device ACLs bound to NMS subnets.
  5. Verify: Safe snmpget tests from NMS and non-NMS, show archive diff, SIEM dashboards.
  6. Document: Keep evidence bundle (configs, logs, screenshots) for compliance and leadership.

Deep-dive code: automate safer SNMP at scale

Ansible — disable v2c, enforce v3, bind ACL

- name: Harden SNMP on IOS/IOS-XE
  hosts: cisco
  gather_facts: no
  connection: network_cli
  tasks:
    - ios_config:
        lines:
          - no snmp-server community public
          - no snmp-server community private
          - ip access-list standard 99
          -  permit 10.10.50.0 0.0.0.255
          -  deny any log
          - snmp-server view SECVIEW iso included
          - snmp-server group SECGRP v3 priv read SECVIEW
          - snmp-server user NMSUSR SECGRP v3 auth sha AUTHPASS priv aes 128 PRIVPASS
          - snmp-server enable traps snmp authentication linkdown linkup coldstart warmStart
          - snmp-server host 10.10.50.10 version 3 priv NMSUSR

Bash — verify only NMS can read

#!/usr/bin/env bash
DEVICE=$1
NMS_USER=NMSUSR
AUTH=SHA
PRIV=AES
APASS='***'
PPASS='***'
OID=1.3.6.1.2.1.1.1.0
timeout 5 snmpget -v3 -l authPriv -u $NMS_USER -a $AUTH -A "$APASS" -x $PRIV -X "$PPASS" $DEVICE $OID || echo "Blocked or misconfigured"

Splunk saved search — Non-NMS SNMP

index=network (udp.port=161 OR message="*SNMP*") NOT src_ip=10.10.50.0/24
| stats count by _time, src_ip, dest_ip, dest_port, message

Read next (from our blog)

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