diff --git a/crates/polkavm-common/src/program.rs b/crates/polkavm-common/src/program.rs
index bc22a4c6c..45c8bd8b9 100644
--- a/crates/polkavm-common/src/program.rs
+++ b/crates/polkavm-common/src/program.rs
@@ -7813,6 +7813,42 @@ fn test_jump_table_entry_width_with_interior_zero_byte() {
     assert_eq!(blob.jump_table().get_by_index(0), Some(ProgramCounter(TARGET_POSITION)));
 }
 
+#[cfg(feature = "alloc")]
+#[test]
+fn test_from_parts_rejects_non_canonical_varints() {
+    for code_and_jump_table in [
+        // |j| = [80, 01]
+        alloc::vec![0x80, 0x01, 0x01, 0x00, 0x00],
+        // |j| = [c0, 01, 00]
+        alloc::vec![0xc0, 0x01, 0x00, 0x01, 0x00, 0x00],
+        // |j| = [e0, 01, 00, 00]
+        alloc::vec![0xe0, 0x01, 0x00, 0x00, 0x01, 0x00, 0x00],
+    ] {
+        let mut parts = ProgramParts::empty(InstructionSetKind::Latest64);
+        parts.code_and_jump_table = ArcBytes::from(code_and_jump_table);
+        assert!(ProgramBlob::from_parts(parts).is_err());
+    }
+}
+
+#[cfg(feature = "alloc")]
+#[test]
+fn test_from_parts_rejects_varints_outside_u32() {
+    let mut parts = ProgramParts::empty(InstructionSetKind::Latest64);
+    parts.code_and_jump_table = ArcBytes::from(alloc::vec![0xf1, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00]);
+    assert!(ProgramBlob::from_parts(parts).is_err());
+}
+
+#[cfg(feature = "alloc")]
+#[test]
+fn test_from_parts_accepts_canonical_varints() {
+    // |j| = 1, z = 1, j = [0], c = [trap]
+    let mut parts = ProgramParts::empty(InstructionSetKind::Latest64);
+    parts.code_and_jump_table = ArcBytes::from(alloc::vec![0x01, 0x01, 0x00, 0x00]);
+    let blob = ProgramBlob::from_parts(parts).unwrap();
+    assert_eq!(blob.jump_table().len(), 1);
+    assert_eq!(blob.instructions().count(), 1);
+}
+
 /// The source location.
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub enum SourceLocation<'a> {
diff --git a/crates/polkavm-common/src/varint.rs b/crates/polkavm-common/src/varint.rs
index 31b6c173b..a9f6f2800 100644
--- a/crates/polkavm-common/src/varint.rs
+++ b/crates/polkavm-common/src/varint.rs
@@ -11,18 +11,22 @@ pub const MAX_VARINT_LENGTH: usize = 5;
 pub(crate) fn read_varint(input: &[u8], first_byte: u8) -> Option<(usize, u32)> {
     let length = (!first_byte).leading_zeros();
     let upper_mask = 0b11111111_u32 >> length;
-    let upper_bits = (upper_mask & u32::from(first_byte)).wrapping_shl(length * 8);
+    let upper_bits = u64::from(upper_mask & u32::from(first_byte)).wrapping_shl(length * 8);
     let input = input.get(..length as usize)?;
     let value = match input.len() {
         0 => upper_bits,
-        1 => upper_bits | u32::from(input[0]),
-        2 => upper_bits | u32::from(u16::from_le_bytes([input[0], input[1]])),
-        3 => upper_bits | u32::from_le_bytes([input[0], input[1], input[2], 0]),
-        4 => upper_bits | u32::from_le_bytes([input[0], input[1], input[2], input[3]]),
+        1 => upper_bits | u64::from(input[0]),
+        2 => upper_bits | u64::from(u16::from_le_bytes([input[0], input[1]])),
+        3 => upper_bits | u64::from(u32::from_le_bytes([input[0], input[1], input[2], 0])),
+        4 => upper_bits | u64::from(u32::from_le_bytes([input[0], input[1], input[2], input[3]])),
         _ => return None,
     };
 
-    Some((length as usize, value))
+    if (length != 0 && value < (1u64 << (7 * length))) || value > u64::from(u32::MAX) {
+        return None;
+    }
+
+    Some((length as usize, value as u32))
 }
 
 #[inline]
@@ -75,6 +79,63 @@ proptest::proptest! {
     }
 }
 
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn varint_rejects_non_canonical_encodings() {
+        assert_eq!(read_varint(&[0x01], 0x80), None);
+        assert_eq!(read_varint(&[0x01, 0x00], 0xc0), None);
+        assert_eq!(read_varint(&[0x01, 0x00, 0x00], 0xe0), None);
+        assert_eq!(read_varint(&[0x01, 0x00, 0x00, 0x00], 0xf0), None);
+        assert_eq!(read_varint(&[0x00], 0x80), None);
+    }
+
+    #[test]
+    fn varint_rejects_values_outside_u32() {
+        assert_eq!(read_varint(&[0x00, 0x00, 0x00, 0x00], 0xf1), None);
+        assert_eq!(read_varint(&[0xff, 0xff, 0xff, 0xff], 0xf7), None);
+        assert_eq!(read_varint(&[0x00, 0x00, 0x00, 0x00, 0x00], 0xf8), None);
+        assert_eq!(read_varint(&[0x00; 8], 0xff), None);
+    }
+
+    #[test]
+    fn varint_accepts_canonical_encodings() {
+        assert_eq!(read_varint(&[], 0x00), Some((0, 0)));
+        assert_eq!(read_varint(&[], 0x7f), Some((0, 127)));
+        assert_eq!(read_varint(&[0x80], 0x80), Some((1, 128)));
+        assert_eq!(read_varint(&[0x00, 0x00, 0x00, 0x80], 0xf0), Some((4, 1 << 31)));
+        assert_eq!(read_varint(&[0xff, 0xff, 0xff, 0xff], 0xf0), Some((4, u32::MAX)));
+    }
+}
+
+#[cfg(kani)]
+mod kani {
+    use super::{read_varint, write_varint, MAX_VARINT_LENGTH};
+
+    #[kani::proof]
+    fn verify_read_varint_is_canonical() {
+        let first_byte: u8 = kani::any();
+        let input: [u8; 4] = kani::any();
+        if let Some((length, value)) = read_varint(&input, first_byte) {
+            if length != 0 {
+                assert!(u64::from(value) >= (1u64 << (7 * length)));
+            }
+        }
+    }
+
+    #[kani::proof]
+    fn verify_read_varint_round_trip() {
+        let value: u32 = kani::any();
+        let mut buffer = [0u8; MAX_VARINT_LENGTH];
+        let length = write_varint(value, &mut buffer);
+        let (parsed_length, parsed_value) = read_varint(&buffer[1..], buffer[0]).unwrap();
+        assert_eq!(parsed_value, value);
+        assert_eq!(parsed_length + 1, length);
+    }
+}
+
 static LENGTH_TO_SHIFT: [u32; 256] = {
     let mut output = [0; 256];
     let mut length = 0_u32;