Cisco’s Simple Network Management Protocol (SNMP) implementations in IOS and IOS XE have come under intense scrutiny following reports of active exploitation in the wild.
First disclosed in August 2025, CVE-2025-20352 describes a critical buffer overflow in the SNMP engine that allows unauthenticated remote attackers to execute arbitrary code.
The vulnerability arises when an oversized payload is sent in a GetBulk request, overrunning an internal buffer and redirecting control flow to attacker-supplied shellcode.
Initial indicators emerged when network operators began noticing unexplained device reboots and anomalous SNMP traffic patterns.
Subsequent forensic analysis revealed that compromised routers were pinging external command-and-control servers immediately after handling malformed SNMP requests.
CISA analysts identified this behavior within weeks of the vulnerability’s public disclosure, warning that adversaries are leveraging CVE-2025-20352 to establish persistent footholds in enterprise networks.
The impact spans a wide range of Cisco platforms, from ISR 4000 Series routers to Catalyst switches running IOS XE versions prior to 17.10.
Exploitation requires only network reachability to the SNMP service and no valid credentials, making exposed management interfaces particularly dangerous.
In one reported incident, attackers deployed a custom payload that established a reverse shell back to an attacker-controlled host, enabling full remote control of the device.
Infection Mechanism
Underneath the hood, the attack leverages a malformed PDU that triggers an out-of-bounds write in the SNMP engine’s stack.
Upon receiving a GetBulk request with a length field exceeding the maximum buffer size, the SNMP handler fails to validate the message size.
This overflow overwrites the saved return address on the stack, diverting execution to shellcode embedded in the packet.
Once execution begins, the payload initializes a socket connection back to the attacker’s IP address:-
from pysnmp.hlapi import *
payload = b"x90" * 100 + reverse_shell_shellcode
sendNotification(
SnmpEngine(),
CommunityData('public'),
UdpTransportTarget(('192.0.2.123', 161)),
ContextData(),
NotificationType(
ObjectIdentity('1.3.6.1.4.1.9.9.96'),
('1.3.6.1.4.1.9.9.96.1.1', OctetString(payload))
)
)The packet structure highlights how the oversized length field and embedded shellcode combine to hijack execution.
Network defenders are urged to apply the latest Cisco patches immediately and to restrict SNMP access to trusted hosts only.
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