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CVE-2026-56111 is an out-of-bounds write vulnerability in Marlin Firmware through 2.1.2.7 in the M421 G-code handler when the firmware is built with MESH_BED_LEVELING enabled. By supplying out-of-range X and Y mesh grid indices in a crafted M421 G-code command, an attacker can cause the handler to write an attacker-controlled 32-bit float past the bounds of the z_values array. This results in memory corruption of adjacent firmware variables. The issue was fixed in commit 1f255d1.
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1 valid exploit after Mallory filtered fakes, detection scripts, and README-only repos.
This repository is a small standalone proof-of-concept exploit for CVE-2026-56111 affecting Marlin Firmware <= 2.1.2.7. The repository contains two files: a README describing the vulnerability, impact, build/use instructions, and one C source file implementing the exploit client. The exploit does not execute code on the target directly; instead, it connects to a vulnerable printer over either a local serial/USB interface or a network TCP service and sends crafted M421 G-code commands that trigger the vulnerable Mesh Bed Leveling handler inside the firmware. The core capability is a constrained out-of-bounds write: the attacker controls the IX/IY indices and Z float value, allowing overwrite of memory adjacent to z_values at an offset computed from the indices. The tool offers two modes: 'write', which sends attacker-specified I/J/Z values for targeted corruption of nearby firmware state, and 'dos', which sends 'M421 I120 J120 Znan' to propagate NaN into motion calculations and reliably crash or hang the firmware. Code structure is simple: usage/help output, serial setup via termios, TCP connection setup via getaddrinfo/socket/connect, a send_line helper, and main argument parsing plus payload construction. No framework is used, no shellcode or post-exploitation logic is present, and the exploit is best classified as an operational PoC for denial-of-service and state corruption against exposed Marlin printer command channels.
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