diff --git a/tools/uart-flash-server/Makefile b/tools/uart-flash-server/Makefile
index fe0b5b1f..03ccb855 100644
--- a/tools/uart-flash-server/Makefile
+++ b/tools/uart-flash-server/Makefile
@@ -16,6 +16,11 @@ $(EXE): $(EXE).o libwolfboot.o
 libwolfboot.o: ../../src/libwolfboot.c
 	$(Q)$(CC) $(CFLAGS) -c -o $(@) $(^)
 
+test_overflow: test_overflow.c
+	$(Q)$(CC) -Wall -g -o $@ $^ -lutil
+
+test: $(EXE) test_overflow
+	$(Q)./test_overflow ./$(EXE)
 
 clean:
-	$(Q)rm -f *.o $(EXE)
+	$(Q)rm -f *.o $(EXE) test_overflow
diff --git a/tools/uart-flash-server/test_overflow.c b/tools/uart-flash-server/test_overflow.c
new file mode 100644
index 00000000..c0c9bdf1
--- /dev/null
+++ b/tools/uart-flash-server/test_overflow.c
@@ -0,0 +1,112 @@
+/* Regression test for the uint32_t wraparound bounds-check bypass in
+ * uart_flash_erase()/read()/write() (finding F-4339).
+ *
+ * Drives the real ufserver binary over a pseudo-terminal and sends an ERASE
+ * command with address=0x1000 and len=0xFFFFFFFF. With the buggy uint32_t
+ * guard, address+len wraps to 0xFFF (< FIRMWARE_PARTITION_SIZE+SWAP_SIZE), the
+ * guard is bypassed and the erase loop walks far past the 0x21000-byte mmap,
+ * crashing the server with SIGSEGV. With the 64-bit guard the command is
+ * rejected and the server stays alive.
+ *
+ * Usage: test_overflow ./ufserver
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <string.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <signal.h>
+#include <pty.h>
+#include <sys/wait.h>
+
+#define CMD_HDR_WOLF  'W'
+#define CMD_HDR_ERASE 0x03
+
+int main(int argc, char *argv[])
+{
+    int master, slave;
+    char slavename[256];
+    pid_t pid;
+    char tmpl[] = "/tmp/ufserver_fw_XXXXXX";
+    int fd;
+    uint8_t buf[64];
+    int i;
+
+    if (argc != 2) {
+        fprintf(stderr, "usage: %s ./ufserver\n", argv[0]);
+        return 2;
+    }
+
+    /* Create a small (non-WOLF) firmware file; mmap_firmware() pads it to
+     * FIRMWARE_PARTITION_SIZE + SWAP_SIZE (0x21000). */
+    fd = mkstemp(tmpl);
+    if (fd < 0) { perror("mkstemp"); return 2; }
+    memset(buf, 0xA5, sizeof(buf));
+    if (write(fd, buf, sizeof(buf)) != (ssize_t)sizeof(buf)) { perror("write"); return 2; }
+    close(fd);
+
+    if (openpty(&master, &slave, slavename, NULL, NULL) != 0) {
+        perror("openpty");
+        return 2;
+    }
+
+    signal(SIGPIPE, SIG_IGN);
+
+    pid = fork();
+    if (pid < 0) { perror("fork"); return 2; }
+    if (pid == 0) {
+        /* Child: run the server attached to the slave pty. */
+        close(master);
+        close(slave);
+        fd = open("/dev/null", O_WRONLY);
+        if (fd >= 0) { dup2(fd, STDOUT_FILENO); }
+        execl(argv[1], argv[1], tmpl, slavename, (char *)NULL);
+        perror("execl");
+        _exit(127);
+    }
+
+    /* Parent: drive the server. */
+    close(slave);
+    usleep(200000); /* let the child open and configure the port */
+
+    /* STX selecting a flash command, then the ERASE opcode. */
+    buf[0] = CMD_HDR_WOLF;
+    buf[1] = CMD_HDR_ERASE;
+    /* address = 0x00001000 (little-endian raw bytes) */
+    buf[2] = 0x00; buf[3] = 0x10; buf[4] = 0x00; buf[5] = 0x00;
+    /* len = 0xFFFFFFFF -> address+len wraps to 0xFFF in uint32_t */
+    buf[6] = 0xFF; buf[7] = 0xFF; buf[8] = 0xFF; buf[9] = 0xFF;
+    if (write(master, buf, 10) != 10) { perror("write master"); }
+
+    /* Drain ack bytes the server sends back so it never blocks. */
+    fcntl(master, F_SETFL, O_NONBLOCK);
+
+    /* Give the server time to process (and crash, if vulnerable). */
+    for (i = 0; i < 20; i++) {
+        uint8_t drain[64];
+        (void)read(master, drain, sizeof(drain));
+        usleep(50000);
+        int status;
+        pid_t r = waitpid(pid, &status, WNOHANG);
+        if (r == pid) {
+            unlink(tmpl);
+            if (WIFSIGNALED(status)) {
+                fprintf(stderr, "FAIL: server died from signal %d "
+                        "(OOB access from wrapped bounds check)\n",
+                        WTERMSIG(status));
+                return 1;
+            }
+            fprintf(stderr, "FAIL: server exited unexpectedly (status %d)\n",
+                    status);
+            return 1;
+        }
+    }
+
+    /* Still alive after processing the malicious command: guard held. */
+    kill(pid, SIGKILL);
+    waitpid(pid, NULL, 0);
+    unlink(tmpl);
+    printf("PASS: server rejected wrapped address+len and stayed alive\n");
+    return 0;
+}
diff --git a/tools/uart-flash-server/ufserver.c b/tools/uart-flash-server/ufserver.c
index b6156ef5..bd159992 100644
--- a/tools/uart-flash-server/ufserver.c
+++ b/tools/uart-flash-server/ufserver.c
@@ -280,7 +280,7 @@ static void uart_flash_erase(uint8_t *base, int ud)
         return;
     if (read_word(ud,&len) != 4)
         return;
-    if (address + len > (FIRMWARE_PARTITION_SIZE + SWAP_SIZE))
+    if ((uint64_t)address + (uint64_t)len > (FIRMWARE_PARTITION_SIZE + SWAP_SIZE))
         return;
     if (address < FIRMWARE_PARTITION_SIZE) {
         printmsg(msgEraseUpdate);
@@ -319,7 +319,7 @@ static void uart_flash_read(uint8_t *base, int ud)
 #if LOG_FLASH_ADDRESS
     printf("Read @%x\n", address);
 #endif
-    if (address + len > (FIRMWARE_PARTITION_SIZE + SWAP_SIZE))
+    if ((uint64_t)address + (uint64_t)len > (FIRMWARE_PARTITION_SIZE + SWAP_SIZE))
         return;
     for (i = 0; i < len; i++) {
         write(ud, base + address + i, 1);
@@ -344,7 +344,7 @@ static void uart_flash_write(uint8_t *base, int ud)
 #if LOG_FLASH_ADDRESS
     printf("Write @%x\n", address);
 #endif
-    if (address + len > (FIRMWARE_PARTITION_SIZE + SWAP_SIZE))
+    if ((uint64_t)address + (uint64_t)len > (FIRMWARE_PARTITION_SIZE + SWAP_SIZE))
         return;
     for (i = 0; i < len; i++) {
         read(ud, base + address + i, 1);