diff --git a/crates/video/src/h264/slice.rs b/crates/video/src/h264/slice.rs
index 06a2bc67..5aec1207 100644
--- a/crates/video/src/h264/slice.rs
+++ b/crates/video/src/h264/slice.rs
@@ -1,7 +1,7 @@
 //! NAL unit header and slice header parsing (7.3.1, 7.3.3).
 
 use crate::bitstream::BitReader;
-use crate::h264::error::{Result, ensure};
+use crate::h264::error::{Error, Result, ensure};
 use crate::h264::params::{Pps, Sps};
 
 /// NAL unit types used by the decoder.
@@ -199,9 +199,9 @@ impl SliceHeader {
         let mut num_ref_idx_active = [pps.num_ref_idx_l0_default_active, pps.num_ref_idx_l1_default_active];
         if !slice_type.is_intra() {
             if r.read_flag()? {
-                num_ref_idx_active[0] = r.read_ue()? + 1;
+                num_ref_idx_active[0] = plus_one(r.read_ue()?)?;
                 if slice_type.is_b() {
-                    num_ref_idx_active[1] = r.read_ue()? + 1;
+                    num_ref_idx_active[1] = plus_one(r.read_ue()?)?;
                 }
             }
             let max = if field_pic { 32 } else { 16 };
@@ -223,7 +223,7 @@ impl SliceHeader {
             ensure!(cabac_init_idc <= 2, "cabac_init_idc out of range");
         }
         let slice_qp_delta = r.read_se()?;
-        let qp = pps.pic_init_qp + slice_qp_delta;
+        let qp = pps.pic_init_qp.checked_add(slice_qp_delta).ok_or_else(|| Error::Invalid("slice QP out of range".to_string()))?;
         ensure!((0..=51).contains(&qp), "slice QP {qp} out of range");
         let mut sp_for_switch = false;
         let mut slice_qs_delta = 0;
@@ -308,6 +308,11 @@ struct RefFields {
     mmcos: Vec<Mmco>,
 }
 
+/// `num_ref_idx_active_minus1 + 1`, or the error its range check gives.
+fn plus_one(v: u32) -> Result<u32> {
+    v.checked_add(1).ok_or_else(|| Error::Invalid("num_ref_idx_active out of range".to_string()))
+}
+
 fn parse_reference_fields(
     r: &mut BitReader<'_>,
     nal: NalHeader,
@@ -549,3 +554,88 @@ impl PocState {
         self.prev_frame_num = frame_num;
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::bitstream::BitWriter;
+
+    fn sps_pps() -> (Sps, Pps) {
+        let mut w = BitWriter::new();
+        w.write_bits(66, 8);
+        w.write_bits(0xC0, 8);
+        w.write_bits(30, 8);
+        w.write_ue(0);
+        w.write_ue(0);
+        w.write_ue(2);
+        w.write_ue(1);
+        w.write_bit(false);
+        w.write_ue(10);
+        w.write_ue(8);
+        w.write_bit(true);
+        w.write_bit(true);
+        w.write_bit(false);
+        w.write_bit(false);
+        w.rbsp_trailing();
+        let sps = Sps::parse(&w.finish()).unwrap();
+        let mut w = BitWriter::new();
+        w.write_ue(0);
+        w.write_ue(0);
+        w.write_bit(false);
+        w.write_bit(false);
+        w.write_ue(0);
+        w.write_ue(0);
+        w.write_ue(0);
+        w.write_bit(false);
+        w.write_bits(0, 2);
+        w.write_se(0);
+        w.write_se(0);
+        w.write_se(0);
+        w.write_bit(false);
+        w.write_bit(false);
+        w.write_bit(false);
+        w.rbsp_trailing();
+        let pps = Pps::parse(&w.finish(), &[Some(sps.clone())]).unwrap();
+        (sps, pps)
+    }
+
+    /// Parse a non-IDR slice header of `slice_type` whose override flag is set with the given counts.
+    fn parse_override(slice_type: u32, l0_minus1: u32, l1_minus1: u32, qp_delta: i32) -> Result<SliceHeader> {
+        let (sps, pps) = sps_pps();
+        let mut w = BitWriter::new();
+        w.write_ue(0);
+        w.write_ue(slice_type);
+        w.write_ue(0);
+        w.write_bits(0, 4);
+        if slice_type == 1 {
+            w.write_bit(true); // direct_spatial_mv_pred
+        }
+        w.write_bit(true); // num_ref_idx_active_override_flag
+        w.write_ue(l0_minus1);
+        if slice_type == 1 {
+            w.write_ue(l1_minus1);
+        }
+        w.write_bit(false); // ref_pic_list_modification_flag_l0
+        if slice_type == 1 {
+            w.write_bit(false);
+        }
+        w.write_bit(false); // adaptive_ref_pic_marking_mode_flag
+        w.write_se(qp_delta);
+        w.rbsp_trailing();
+        let nal = NalHeader { nal_ref_idc: 1, nal_unit_type: 1 };
+        SliceHeader::parse(&w.finish(), nal, |_| Ok((&pps, &sps))).map(|(h, _, _)| h)
+    }
+
+    #[test]
+    fn huge_num_ref_idx_override_is_invalid() {
+        assert!(matches!(parse_override(0, u32::MAX, 0, 0), Err(crate::h264::Error::Invalid(_))));
+        assert!(matches!(parse_override(1, 0, u32::MAX, 0), Err(crate::h264::Error::Invalid(_))));
+        assert!(matches!(parse_override(0, 16, 0, 0), Err(crate::h264::Error::Invalid(_))));
+        assert_eq!(parse_override(0, 15, 0, 0).unwrap().num_ref_idx_active, [16, 0]);
+    }
+
+    #[test]
+    fn huge_slice_qp_delta_is_invalid() {
+        assert!(matches!(parse_override(0, 0, 0, i32::MAX), Err(crate::h264::Error::Invalid(_))));
+    }
+}