lwIP SNMPv3 USM Handler Stack-Based Buffer Overflow (CVE-2026-8836)

A stack-based buffer overflow vulnerability, identified as CVE-2026-8836, has been discovered in lwIP versions up to 2.2.1. The vulnerability resides within the snmpv3 USM Handler component, specifically in the snmp_parse_inbound_frame function of the src/apps/snmp/snmp_msg.c file. By manipulating the msgAuthenticationParameters argument, a remote attacker can trigger a buffer overflow, potentially leading to arbitrary code execution. The patch addressing this vulnerability is identified by the commit hash 0c957ec03054eb6c8205e9c9d1d05d90ada3898c. This vulnerability poses a significant risk as it can be exploited remotely without authentication.

Attack Chain

1. The attacker identifies a vulnerable lwIP instance with SNMPv3 USM enabled.
2. The attacker crafts a malicious SNMPv3 packet targeting the snmp_parse_inbound_frame function.
3. The crafted packet includes a msgAuthenticationParameters argument designed to exceed the buffer’s capacity.
4. The snmp_parse_inbound_frame function processes the malformed SNMPv3 packet without proper bounds checking.
5. The oversized msgAuthenticationParameters argument overwrites adjacent memory on the stack, including return addresses.
6. Upon function return, the overwritten return address is used, redirecting execution flow to attacker-controlled code.
7. The attacker gains arbitrary code execution within the context of the lwIP process.
8. The attacker can then use this code execution to further compromise the system, potentially leading to data exfiltration or denial of service.

Impact

Successful exploitation of CVE-2026-8836 allows a remote attacker to execute arbitrary code on the vulnerable system. Given the widespread use of lwIP in embedded devices and network appliances, a large number of devices are potentially affected. A successful attack could lead to complete system compromise, allowing the attacker to steal sensitive data, disrupt network services, or use the compromised device as a bot in a larger botnet. The CVSS v3.1 score of 9.8 highlights the critical severity of this vulnerability.

Recommendation

• Apply the patch identified by commit hash 0c957ec03054eb6c8205e9c9d1d05d90ada3898c to address the buffer overflow.
• Monitor network traffic for malformed SNMPv3 packets, especially those with unusually large msgAuthenticationParameters using the provided Sigma rules.
• Consider disabling SNMPv3 USM if it is not required to reduce attack surface.
• Deploy the Sigma rule “Detect CVE-2026-8836 Exploitation Attempt via Malformed SNMP Packet” to detect potential exploitation attempts.