CVE-2026-8836: lwIP Buffer Overflow Vulnerability

CVE-2026-8836 Overview

CVE-2026-8836 is a stack-based buffer overflow in the lightweight IP stack (lwIP) versions up to 2.2.1. The flaw resides in the snmp_parse_inbound_frame function within src/apps/snmp/snmp_msg.c, part of the SNMPv3 User-based Security Model (USM) handler. An attacker can trigger the overflow by manipulating the msgAuthenticationParameters argument in an inbound SNMP frame. The vulnerability is remotely exploitable over the network without authentication or user interaction. lwIP is widely deployed in embedded systems, IoT devices, and real-time operating systems, expanding the potential exposure. The maintainers have published a fix in commit 0c957ec03054eb6c8205e9c9d1d05d90ada3898c.

Critical Impact

Unauthenticated remote attackers can corrupt the stack of devices running lwIP SNMPv3, enabling potential arbitrary code execution or denial of service against embedded and IoT systems.

Affected Products

• lwIP versions up to and including 2.2.1
• Embedded firmware and IoT devices integrating lwIP with the SNMPv3 USM handler enabled
• Real-time operating systems and bare-metal applications shipping the affected snmp_msg.c

Discovery Timeline

• 2026-05-18 - CVE-2026-8836 published to NVD
• 2026-05-18 - Last updated in NVD database

Technical Details for CVE-2026-8836

Vulnerability Analysis

The vulnerability is a classic stack-based buffer overflow [CWE-119] in lwIP's SNMPv3 message parser. When snmp_parse_inbound_frame processes an inbound SNMPv3 packet, it reads the msgAuthenticationParameters field from the USM security header. The function copies this attacker-controlled data onto a fixed-size stack buffer without sufficient length validation. Crafting an SNMPv3 request with an oversized msgAuthenticationParameters value overruns the buffer and corrupts adjacent stack memory, including saved frame pointers and return addresses. Because lwIP commonly runs on memory-constrained embedded devices with limited or absent exploit mitigations such as stack canaries, ASLR, and non-executable stack enforcement, exploitation can lead to arbitrary code execution at the privilege of the network stack.

Root Cause

The root cause is missing bounds checking on the length of the msgAuthenticationParameters field during SNMPv3 USM message decoding. The parser trusts attacker-controlled length values when copying authentication parameter data into a stack-allocated buffer. The patch identified by commit hash 0c957ec03054eb6c8205e9c9d1d05d90ada3898c adds the validation required to reject oversized inputs before they reach the vulnerable copy operation.

Attack Vector

The attack is initiated remotely over UDP port 161 by default, the standard SNMP listener port. No authentication is required because the overflow occurs during the parsing of the USM security parameters, prior to any cryptographic verification of the message. An attacker on the same network segment, or any routable path reaching the SNMP agent, can send a single crafted SNMPv3 packet with a malformed msgAuthenticationParameters value to trigger the corruption.

No verified public proof-of-concept code is available. Technical details are documented in the upstream commits referenced in the GNU lwIP Commit Update and the Savannah Bug Report #68194.

Detection Methods for CVE-2026-8836

Indicators of Compromise

• Unexpected crashes, watchdog resets, or reboots on embedded devices running lwIP shortly after receiving inbound SNMP traffic
• Inbound SNMPv3 UDP packets to port 161 containing abnormally large msgAuthenticationParameters fields
• SNMP agent log entries indicating malformed USM headers or truncated message processing

Detection Strategies

• Deploy network intrusion detection signatures that inspect SNMPv3 USM headers and alert on msgAuthenticationParameters lengths exceeding RFC 3414 expectations (12 bytes for HMAC-MD5/SHA)
• Inventory firmware images and source trees for lwIP versions at or below 2.2.1 with SNMPv3 enabled
• Correlate device crash telemetry with inbound SNMP traffic from untrusted sources

Monitoring Recommendations

• Enable flow logging on management VLANs and inspect UDP/161 traffic originating from unauthorized sources
• Monitor SNMP agent availability and restart counts on critical embedded infrastructure
• Capture and review packet samples of SNMPv3 traffic for malformed ASN.1 structures

How to Mitigate CVE-2026-8836

Immediate Actions Required

• Apply the upstream lwIP patch from commit 0c957ec03054eb6c8205e9c9d1d05d90ada3898c and rebuild affected firmware images
• Restrict SNMP access at the network perimeter and on management interfaces using ACLs that permit only trusted management hosts
• Disable SNMPv3 on devices that do not require it, particularly on internet-exposed or untrusted network segments

Patch Information

The lwIP maintainers released a fix in commit 0c957ec03054eb6c8205e9c9d1d05d90ada3898c. Vendors integrating lwIP must rebase or backport the change into their firmware. References include the GitHub lwIP Commit Change, the VulDB Vulnerability #364474, and the Savannah Bug Report #68194.

Workarounds

• Block inbound UDP port 161 traffic from untrusted networks at firewalls and switch ACLs
• Disable the SNMP agent or the SNMPv3 USM module on devices where the feature is unused
• Segment management traffic onto isolated VLANs accessible only from authorized network management stations

# Example iptables rule restricting SNMP to a trusted management host
iptables -A INPUT -p udp --dport 161 -s 10.0.0.10 -j ACCEPT
iptables -A INPUT -p udp --dport 161 -j DROP