CVE-2026-16225: Snap7 Library RCE Vulnerability

CVE-2026-16225 Overview

CVE-2026-16225 is an out-of-bounds write vulnerability [CWE-119] in the davenardella snap7 library up to version 1.4.3. The flaw resides in the TSnap7Peer::NegotiatePDULength function within src/core/s7_peer.cpp. Manipulating the PDULength argument triggers an out-of-bounds write during PDU length negotiation. The attack can be executed remotely and requires low privileges with no user interaction. Public exploit code has been released, increasing the risk of opportunistic exploitation against exposed snap7 endpoints.

Critical Impact
Remote attackers can trigger an out-of-bounds write in snap7's S7 protocol peer negotiation, potentially corrupting adjacent memory in industrial automation applications that integrate the library.

Affected Products

davenardella snap7 up to and including version 1.4.3
Applications and industrial integrations that embed the snap7 S7 communication library
Systems exposing snap7-based S7 protocol services over the network

Discovery Timeline

2026-07-19 - CVE-2026-16225 published to NVD
2026-07-20 - Last updated in NVD database

Technical Details for CVE-2026-16225

Vulnerability Analysis

The vulnerability exists in the TSnap7Peer::NegotiatePDULength function inside src/core/s7_peer.cpp. snap7 implements the Siemens S7 communication protocol, which begins each session with a PDU (Protocol Data Unit) length negotiation exchange between peers. During this exchange, the peer parses a PDULength value supplied by the remote party. Inadequate bounds checking on this attacker-controlled value leads to a write past the end of an allocated buffer.

The issue is classified under [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer). An out-of-bounds write in a network-facing parser can corrupt heap or stack memory, destabilize the host process, and, depending on adjacent memory layout, influence control flow. EPSS currently rates the exploit probability at 0.228 percent.

Root Cause

The root cause is missing or insufficient validation of the PDULength argument before it is used as an index or size in a write operation. snap7 trusts the negotiated value supplied by the remote peer rather than clamping it to the maximum PDU size supported by the local buffer. Any value outside the expected range causes the function to write outside allocated bounds.

Attack Vector

An attacker with network reachability to a snap7 peer and the ability to authenticate at a low privilege level can initiate a session and send a crafted PDU negotiation message with a malicious PDULength. Because snap7 is widely embedded in operational technology and industrial automation tooling that communicates with Siemens S7 PLCs, exposed instances on internal or poorly segmented networks are the primary risk. Public exploit code is available, according to VulDB.

No verified proof-of-concept code is reproduced here. Refer to the GitHub Issue #17 for Snap7 and the VulDB CVE-2026-16225 entry for technical details.

Detection Methods for CVE-2026-16225

Indicators of Compromise

Unexpected crashes, segmentation faults, or restarts in applications linking against snap7 shortly after receiving S7 protocol traffic
Inbound S7 protocol connections (typically TCP port 102) from unexpected sources containing malformed PDU negotiation frames
Anomalous PDULength values in S7 COTP/TPKT session setup packets that exceed vendor-standard sizes

Detection Strategies

Deploy network intrusion detection signatures that inspect S7comm negotiation messages and flag PDULength values outside RFC- and vendor-documented ranges
Correlate process crash telemetry from hosts running snap7-based services with preceding S7 protocol traffic to identify exploitation attempts
Inventory software dependencies to identify applications statically or dynamically linking snap7 at version 1.4.3 or earlier

Monitoring Recommendations

Monitor TCP port 102 and other S7-related ports for connections originating outside authorized engineering workstations
Log and alert on repeated session initiations that fail after the PDU negotiation phase
Track outbound connections from OT and IT-OT boundary hosts running snap7 integrations for signs of post-exploitation activity

How to Mitigate CVE-2026-16225

Immediate Actions Required

Identify all applications and services that embed snap7 and record their linked version
Restrict network access to snap7-based endpoints so that only authorized engineering hosts can reach S7 protocol ports
Segment operational technology networks from general enterprise networks and remove any direct internet exposure of S7 services
Prioritize patching of internet-exposed or DMZ-hosted instances first

Patch Information

At the time of publication, no fixed release is referenced in the NVD entry. Track the upstream project via the GitHub Repository for Snap7 and the associated GitHub Issue #17 for Snap7 for a patched build. Rebuild and redistribute any downstream applications after upgrading the embedded library.

Workarounds

Place snap7 endpoints behind a firewall or industrial DMZ that enforces strict allowlisting of source addresses
Terminate S7 sessions at an inspecting proxy that validates and normalizes PDULength before forwarding traffic
Disable or remove snap7-based network services on hosts where S7 communication is not required

bash

# Example iptables allowlist restricting S7 (TCP/102) to authorized engineering workstations
iptables -A INPUT -p tcp --dport 102 -s 10.20.30.10 -j ACCEPT
iptables -A INPUT -p tcp --dport 102 -s 10.20.30.11 -j ACCEPT
iptables -A INPUT -p tcp --dport 102 -j DROP