https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=7872

diff --git a/MagickCore/fx.c b/MagickCore/fx.c
index 28b4f255bedc9a1aca2f619a5b4ae527d7bc9d44..99ef1f746d65fe79963d7dd878c087c542df1c66 100644 (file)
--- a/MagickCore/fx.c
+++ b/MagickCore/fx.c
@@ -2547,7 +2547,10 @@ static double FxEvaluateSubexpression(FxInfo *fx_info,
               case AlphaPixelChannel: type="opacity"; break;
               default: type="unknown"; break;
             }
-          (void) CopyMagickString(subexpression,expression+6,MagickPathExtent);
+          *subexpression='\0';
+          if (strlen(expression) > 6)
+            (void) CopyMagickString(subexpression,expression+6,
+              MagickPathExtent);
           if (strlen(subexpression) > 1)
             subexpression[strlen(subexpression)-1]='\0';
           if (fx_info->file != (FILE *) NULL)