diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
index b312ad27aa..ca994e3ef5 100644
--- a/docs/system/deprecated.rst
+++ b/docs/system/deprecated.rst
@@ -430,6 +430,26 @@ image).  Rather, any changes to the backing chain should be performed
 with ``qemu-img rebase -u`` either before or after the remaining
 changes being performed by amend, as appropriate.
 
+qemu-img backing file without format (since 5.1)
+''''''''''''''''''''''''''''''''''''''''''''''''
+
+The use of ``qemu-img create``, ``qemu-img rebase``, or ``qemu-img
+convert`` to create or modify an image that depends on a backing file
+now recommends that an explicit backing format be provided.  This is
+for safety: if QEMU probes a different format than what you thought,
+the data presented to the guest will be corrupt; similarly, presenting
+a raw image to a guest allows a potential security exploit if a future
+probe sees a non-raw image based on guest writes.
+
+To avoid the warning message, or even future refusal to create an
+unsafe image, you must pass ``-o backing_fmt=`` (or the shorthand
+``-F`` during create) to specify the intended backing format.  You may
+use ``qemu-img rebase -u`` to retroactively add a backing format to an
+existing image.  However, be aware that there are already potential
+security risks to blindly using ``qemu-img info`` to probe the format
+of an untrusted backing image, when deciding what format to add into
+an existing image.
+
 Backwards compatibility
 -----------------------
 
diff --git a/block.c b/block.c
index 5cfd10f6b4..aad3b3635e 100644
--- a/block.c
+++ b/block.c
@@ -6161,6 +6161,26 @@ void bdrv_img_create(const char *filename, const char *fmt,
             error_append_hint(&local_err, "Could not open backing image.\n");
             goto out;
         } else {
+            if (!backing_fmt) {
+                warn_report("Deprecated use of backing file without explicit "
+                            "backing format (detected format of %s)",
+                            bs->drv->format_name);
+                if (bs->drv != &bdrv_raw) {
+                    /*
+                     * A probe of raw deserves the most attention:
+                     * leaving the backing format out of the image
+                     * will ensure bs->probed is set (ensuring we
+                     * don't accidentally commit into the backing
+                     * file), and allow more spots to warn the users
+                     * to fix their toolchain when opening this image
+                     * later.  For other images, we can safely record
+                     * the format that we probed.
+                     */
+                    backing_fmt = bs->drv->format_name;
+                    qemu_opt_set(opts, BLOCK_OPT_BACKING_FMT, backing_fmt,
+                                 NULL);
+                }
+            }
             if (size == -1) {
                 /* Opened BS, have no size */
                 size = bdrv_getlength(bs);
@@ -6174,7 +6194,12 @@ void bdrv_img_create(const char *filename, const char *fmt,
             }
             bdrv_unref(bs);
         }
-    } /* (backing_file && !(flags & BDRV_O_NO_BACKING)) */
+        /* (backing_file && !(flags & BDRV_O_NO_BACKING)) */
+    } else if (backing_file && !backing_fmt) {
+        warn_report("Deprecated use of unopened backing file without "
+                    "explicit backing format, use of this image requires "
+                    "potentially unsafe format probing");
+    }
 
     if (size == -1) {
         error_setg(errp, "Image creation needs a size parameter");
diff --git a/qemu-img.c b/qemu-img.c
index 440c6aef67..cd19e9b0a7 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -2523,6 +2523,13 @@ static int img_convert(int argc, char **argv)
         goto out;
     }
 
+    if (out_baseimg_param) {
+        if (!qemu_opt_get(opts, BLOCK_OPT_BACKING_FMT)) {
+            warn_report("Deprecated use of backing file without explicit "
+                        "backing format");
+        }
+    }
+
     /* Check if compression is supported */
     if (s.compressed) {
         bool encryption =
@@ -3804,7 +3811,7 @@ static int img_rebase(int argc, char **argv)
      * doesn't change when we switch the backing file.
      */
     if (out_baseimg && *out_baseimg) {
-        ret = bdrv_change_backing_file(bs, out_baseimg, out_basefmt, false);
+        ret = bdrv_change_backing_file(bs, out_baseimg, out_basefmt, true);
     } else {
         ret = bdrv_change_backing_file(bs, NULL, NULL, false);
     }
diff --git a/tests/qemu-iotests/114 b/tests/qemu-iotests/114
index 26104fff6c..1436833813 100755
--- a/tests/qemu-iotests/114
+++ b/tests/qemu-iotests/114
@@ -42,9 +42,16 @@ _unsupported_proto vxhs
 # qcow2.py does not work too well with external data files
 _unsupported_imgopts data_file
 
+# Intentionally specify backing file without backing format; demonstrate
+# the difference in warning messages when backing file could be probed.
+# Note that only a non-raw probe result will affect the resulting image.
+truncate --size=64M "$TEST_IMG.orig"
+_make_test_img -b "$TEST_IMG.orig" 64M
 
 TEST_IMG="$TEST_IMG.base" _make_test_img 64M
+$QEMU_IMG convert -O qcow2 -B "$TEST_IMG.orig" "$TEST_IMG.orig" "$TEST_IMG"
 _make_test_img -b "$TEST_IMG.base" 64M
+_make_test_img -u -b "$TEST_IMG.base" 64M
 
 # Set an invalid backing file format
 $PYTHON qcow2.py "$TEST_IMG" add-header-ext 0xE2792ACA "foo"
@@ -55,6 +62,11 @@ _img_info
 $QEMU_IO -c "open $TEST_IMG" -c "read 0 4k" 2>&1 | _filter_qemu_io | _filter_testdir
 $QEMU_IO -c "open -o backing.driver=$IMGFMT $TEST_IMG" -c "read 0 4k" | _filter_qemu_io
 
+# Rebase the image, to show that omitting backing format triggers a warning,
+# but probing now lets us use the backing file.
+$QEMU_IMG rebase -u -b "$TEST_IMG.base" "$TEST_IMG"
+$QEMU_IO -c "open $TEST_IMG" -c "read 0 4k" 2>&1 | _filter_qemu_io | _filter_testdir
+
 # success, all done
 echo '*** done'
 rm -f $seq.full
diff --git a/tests/qemu-iotests/114.out b/tests/qemu-iotests/114.out
index 67adef37a4..0a37d20c82 100644
--- a/tests/qemu-iotests/114.out
+++ b/tests/qemu-iotests/114.out
@@ -1,5 +1,11 @@
 QA output created by 114
+qemu-img: warning: Deprecated use of backing file without explicit backing format (detected format of raw)
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.orig
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
+qemu-img: warning: Deprecated use of backing file without explicit backing format
+qemu-img: warning: Deprecated use of backing file without explicit backing format (detected format of IMGFMT)
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+qemu-img: warning: Deprecated use of unopened backing file without explicit backing format, use of this image requires potentially unsafe format probing
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
 image: TEST_DIR/t.IMGFMT
 file format: IMGFMT
@@ -11,4 +17,7 @@ qemu-io: can't open device TEST_DIR/t.qcow2: Could not open backing file: Unknow
 no file open, try 'help open'
 read 4096/4096 bytes at offset 0
 4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+qemu-img: warning: Deprecated use of backing file without explicit backing format, use of this image requires potentially unsafe format probing
+read 4096/4096 bytes at offset 0
+4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 *** done
diff --git a/tests/qemu-iotests/293.out b/tests/qemu-iotests/293.out
index 3c612903f8..8bed89a67f 100644
--- a/tests/qemu-iotests/293.out
+++ b/tests/qemu-iotests/293.out
@@ -2,7 +2,8 @@ QA output created by 293
 
 == qcow backed by qcow ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=33554432
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT.base
+qemu-img: warning: Deprecated use of backing file without explicit backing format (detected format of IMGFMT)
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 image: TEST_DIR/t.IMGFMT
 file format: IMGFMT
 virtual size: 32 MiB (33554432 bytes)
@@ -35,6 +36,7 @@ cluster_size: 512
 backing file: TEST_DIR/t.IMGFMT.base
 
 == qcow backed by raw ==
+qemu-img: warning: Deprecated use of backing file without explicit backing format (detected format of raw)
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT.base
 image: TEST_DIR/t.IMGFMT
 file format: IMGFMT
