ensure the parser progresses.

fixes #103

pdf/src/backend.rs

@@ -51,6 +51,10 @@ pub trait Backend: Sized {
     /// Used internally by File, but could also be useful for applications that want to look at the raw PDF objects.
     fn read_xref_table_and_trailer(&self, start_offset: usize) -> Result<(XRefTable, Dictionary)> {
         let xref_offset = t!(self.locate_xref_offset());
+        if xref_offset < start_offset {
+            bail!("Invalid PDF. xref before header.");
+        }
+
         let mut lexer = Lexer::new(t!(self.read(start_offset + xref_offset..)));
 
         let (xref_sections, trailer) = t!(read_xref_and_trailer_at(&mut lexer, &NoResolve));

pdf/src/parser/lexer/mod.rs

@@ -155,8 +155,9 @@ impl<'a> Lexer<'a> {
         }
         let mut pos = self.skip_whitespace(self.pos)?;
         while self.buf.get(pos) == Some(&b'%') {
-            if let Some(off) = self.buf[pos+1..].iter().position(|&b| b == b'\n') {
-                pos += off+2;
+            pos += 1;
+            if let Some(off) = self.buf[pos..].iter().position(|&b| b == b'\n') {
+                pos += off+1;
             }
 
             // Move away from eventual whitespace

pdf/tests/xref.rs

@@ -0,0 +1,4 @@
+#[test]
+fn infinite_loop_invalid_file() {
+    assert!(pdf::file::File::from_data(b"startxref%PDF-".as_ref()).is_err());
+}