--- a/sys/vfs/tmpfs/tmpfs_vnops.c
+++ b/sys/vfs/tmpfs/tmpfs_vnops.c
@@ -686,7 +686,20 @@
 		return error;
 	}
 
-	node = VP_TO_TMPFS_NODE(vp);
+	node = VP_TO_TMPFS_NODE(vp);;
+
+	/*
+	 * DF-3026: re-check system flags on every write like UFS does
+	 * (ufs_readwrite.c): a writable descriptor opened before the
+	 * administrator set SF_APPEND/SF_IMMUTABLE must not keep writing.
+	 */
+	if (node->tn_flags & APPEND) {
+		if ((ioflag & IO_APPEND) == 0 &&
+		    uio->uio_offset != node->tn_size)
+			return (EPERM);
+	}
+	if (node->tn_flags & IMMUTABLE)
+		return (EPERM);
 
 	if (vp->v_type != VREG)
 		return (EINVAL);