diff --git a/sys/ufs/ffs/ffs_subr.c b/sys/ufs/ffs/ffs_subr.c
index 682dd60ebd4460..496de9913eda41 100644
--- a/sys/ufs/ffs/ffs_subr.c
+++ b/sys/ufs/ffs/ffs_subr.c
@@ -717,7 +717,19 @@ validate_sblock(struct fs *fs, int flags)
 		howmany(fs->fs_cssize, fs->fs_fsize), %jd);
 #endif
 	WCHK(fs->fs_metaspace, <, 0, %jd);
-	WCHK(fs->fs_metaspace, >, fs->fs_fpg / 2, %jd);
+	/*
+	 * FreeBSD and NetBSD FFSv2 layouts diverge in the superblock region
+	 * following fs_maxbsize.  NetBSD's WAPBL journal metadata occupies
+	 * the same bytes as FreeBSD's fs_metaspace and other fields.
+	 * fs_flags cannot serve as a discriminator: ffs_oldfscompat_read()
+	 * may overwrite it from the 8-bit fs_old_flags before this check runs.
+	 * Skip the upper-bound check for UFS2; ffs_mountfs() detects the
+	 * condition and enforces read-only access without modifying the
+	 * on-disk superblock.  The check is preserved for UFS1 where no such
+	 * layout divergence exists.
+	 */
+	if (fs->fs_magic != FS_UFS2_MAGIC)
+		WCHK(fs->fs_metaspace, >, fs->fs_fpg / 2, %jd);
 	WCHK(fs->fs_minfree, >, 99, %jd%%);
 	maxfilesize = fs->fs_bsize * UFS_NDADDR - 1;
 	for (sizepb = fs->fs_bsize, i = 0; i < UFS_NIADDR; i++) {
diff --git a/sys/ufs/ffs/ffs_vfsops.c b/sys/ufs/ffs/ffs_vfsops.c
index 11afa99bea7653..633a8fed90e64f 100644
--- a/sys/ufs/ffs/ffs_vfsops.c
+++ b/sys/ufs/ffs/ffs_vfsops.c
@@ -604,6 +604,16 @@ ffs_mount(struct mount *mp)
 			if (error) {
 				return (error);
 			}
+			/*
+			 * Refuse upgrade of NetBSD WAPBL filesystem to
+			 * read-write; see validate_sblock() for details.
+			 */
+			if (fs->fs_magic == FS_UFS2_MAGIC &&
+			    fs->fs_metaspace > fs->fs_fpg / 2) {
+				vfs_mount_error(mp, "NetBSD WAPBL filesystem "
+				    "must be mounted read-only");
+				return (EROFS);
+			}
 			fs->fs_flags &= ~FS_UNCLEAN;
 			if (fs->fs_clean == 0) {
 				fs->fs_flags |= FS_UNCLEAN;
@@ -926,6 +936,23 @@ ffs_mountfs(struct vnode *odevvp, struct mount *mp, struct thread *td)
 		    ffs_use_bread);
 	if (error != 0)
 		goto out;
+	/*
+	 * Per NetBSD's wapbl(4), systems without WAPBL support should mount it
+	 * read-only; see validate_sblock() for details.
+	 */
+	if (fs->fs_magic == FS_UFS2_MAGIC &&
+	    fs->fs_metaspace > fs->fs_fpg / 2) {
+		if ((mp->mnt_flag & MNT_RDONLY) == 0) {
+			vfs_mount_error(mp, "NetBSD WAPBL filesystem must be "
+			    "mounted read-only");
+			error = EROFS;
+			goto out;
+		}
+		printf("WARNING: %s: NetBSD WAPBL filesystem mounted "
+		    "read-only (fs_metaspace %jd, fs_fpg/2 %jd)\n",
+		    mp->mnt_stat.f_mntonname, (intmax_t)fs->fs_metaspace,
+		    (intmax_t)fs->fs_fpg / 2);
+	}
 	fs->fs_flags &= ~FS_UNCLEAN;
 	if (fs->fs_clean == 0) {
 		fs->fs_flags |= FS_UNCLEAN;