diff --git a/Cargo.lock b/Cargo.lock
index 6d2935702..1aef35fe0 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -974,6 +974,9 @@ dependencies = [
 [[package]]
 name = "craft-cfb"
 version = "0.1.0"
+dependencies = [
+ "craft-media-testkit",
+]
 
 [[package]]
 name = "craft-cms"
diff --git a/crates/cfb/Cargo.toml b/crates/cfb/Cargo.toml
index 04bdac867..e2684da0b 100644
--- a/crates/cfb/Cargo.toml
+++ b/crates/cfb/Cargo.toml
@@ -11,3 +11,6 @@ repository.workspace = true
 workspace = true
 
 [dependencies]
+
+[dev-dependencies]
+craft-media-testkit = { workspace = true }
diff --git a/crates/cfb/src/tests.rs b/crates/cfb/src/tests.rs
index 7625afb52..dda76fb24 100644
--- a/crates/cfb/src/tests.rs
+++ b/crates/cfb/src/tests.rs
@@ -3,6 +3,7 @@
 
 use super::*;
 
+mod hostile;
 mod ported;
 
 fn pattern(n: usize, seed: u8) -> Vec<u8> {
diff --git a/crates/cfb/src/tests/hostile.rs b/crates/cfb/src/tests/hostile.rs
new file mode 100644
index 000000000..c20c132e2
--- /dev/null
+++ b/crates/cfb/src/tests/hostile.rs
@@ -0,0 +1,30 @@
+//! Hostile-input hammer: truncation, bit flips and extreme field values must never panic or hang.
+
+use super::*;
+
+fn exercise(d: &[u8]) {
+    let Ok(f) = CompoundFile::open(d) else { return };
+    let _ = f.root();
+    let _ = f.sector_size();
+    for id in 0..2000 {
+        if f.entry(id).is_none() {
+            break;
+        }
+        let _ = f.read(id);
+        let _ = f.child(id, "s0");
+    }
+    let _ = f.read(usize::MAX);
+    let _ = f.find("Header-2/Content-3b03/s4");
+    let _ = f.read_path("Mobs-19010/properties");
+}
+
+#[test]
+fn hostile_input_never_panics() {
+    for (i, v) in [Version::V3, Version::V4].into_iter().enumerate() {
+        let (bytes, _) = sample(v, 5);
+        craft_media_testkit::hostile::hammer(&format!("cfb seed {i}"), &bytes, 15_000, exercise);
+        // The header and directory are where the structure lives: hammer a prefix harder.
+        let head = &bytes[..bytes.len().min(2048)];
+        craft_media_testkit::hostile::hammer(&format!("cfb head {i}"), head, 15_000, exercise);
+    }
+}
diff --git a/crates/media/src/matroska/synthetic_tests.rs b/crates/media/src/matroska/synthetic_tests.rs
index d667a0952..ba96d483c 100644
--- a/crates/media/src/matroska/synthetic_tests.rs
+++ b/crates/media/src/matroska/synthetic_tests.rs
@@ -10,6 +10,8 @@ use crate::matroska::mux::{el, el_float, el_str, el_uint};
 use crate::matroska::track::ContentEncoding;
 use crate::matroska::{ByteSource, Demuxer, MkvFile, OpenOptions, Packet, open_with};
 
+mod hostile;
+
 fn block(track: u8, rel: i16, flags: u8, data: &[u8]) -> Vec<u8> {
     let mut b = vec![0x80 | track];
     b.extend_from_slice(&rel.to_be_bytes());
diff --git a/crates/media/src/matroska/synthetic_tests/hostile.rs b/crates/media/src/matroska/synthetic_tests/hostile.rs
new file mode 100644
index 000000000..e8180fd3b
--- /dev/null
+++ b/crates/media/src/matroska/synthetic_tests/hostile.rs
@@ -0,0 +1,75 @@
+//! Hostile-input hammer: truncation, bit flips and extreme field values must never panic or hang.
+
+use super::*;
+use crate::matroska::mux::{LacingMode, MkvWriter, MuxOptions, TrackSpec};
+use crate::matroska::{MkvFile, TrackKind};
+use std::io::Cursor;
+
+fn muxed(lacing: LacingMode) -> Vec<u8> {
+    let mut v = TrackSpec::new(TrackKind::Video, "V_MJPEG");
+    v.video_size = Some((16, 16));
+    v.default_duration_ns = Some(40_000_000);
+    let mut a = TrackSpec::new(TrackKind::Audio, "A_PCM/INT/LIT");
+    a.audio = Some((48000.0, 1, Some(16)));
+    let mut s = TrackSpec::new(TrackKind::Subtitle, "S_TEXT/UTF8");
+    s.language = Some("fre".into());
+    let opts = MuxOptions { lacing, cluster_duration_ns: 100_000_000, ..Default::default() };
+    let mut m = MkvWriter::new(Cursor::new(Vec::new()), vec![v, a, s], opts).unwrap();
+    for i in 0..12i64 {
+        m.write_frame(0, i * 40_000_000, i % 4 == 0, &vec![i as u8; 30 + i as usize], None).unwrap();
+        for k in 0..3 {
+            m.write_frame(1, (i * 40 + k * 10) * 1_000_000, true, &[k as u8; 48], None).unwrap();
+        }
+        if i % 5 == 0 {
+            m.write_frame(2, i * 40_000_000, true, b"hello", Some(500_000_000)).unwrap();
+        }
+    }
+    m.finish().unwrap().into_inner()
+}
+
+fn walk(f: &MkvFile, d: &[u8]) {
+    for t in 0..f.tracks.len() {
+        let n = f.tracks[t].samples.len();
+        for k in [0, n / 2, n.saturating_sub(1), n, usize::MAX] {
+            let _ = f.read_sample(d, t, k);
+            let _ = f.read_sample_addition(d, t, k);
+        }
+        let _ = f.keyframe_before(t, i64::MIN);
+        let _ = f.keyframe_before(t, i64::MAX);
+    }
+    let _ = f.duration_ns();
+}
+
+fn exercise(d: &[u8]) {
+    for opts in
+        [OpenOptions { index: true, verify_crc: true, ..Default::default() }, OpenOptions { index: false, verify_crc: false, ..Default::default() }]
+    {
+        if let Ok(f) = open_with(d, &opts) {
+            walk(&f, d);
+        }
+        if let Ok(mut dm) = Demuxer::with_options(d, opts) {
+            let mut n = 0;
+            while let Ok(Some(_)) = dm.next_packet() {
+                n += 1;
+                if n > 100_000 {
+                    break;
+                }
+            }
+            for t in 0..dm.tracks().len() {
+                let _ = dm.seek(t, 20_000_000);
+                let _ = dm.seek(t, i64::MAX);
+                let _ = dm.seek(t, i64::MIN);
+                let _ = dm.keyframe_index(t);
+            }
+            let _ = dm.build_index();
+        }
+    }
+}
+
+#[test]
+fn hostile_input_never_panics() {
+    let seeds = [synthetic(), muxed(LacingMode::None), muxed(LacingMode::Xiph), muxed(LacingMode::Ebml), muxed(LacingMode::FixedOrEbml)];
+    for (i, s) in seeds.iter().enumerate() {
+        craft_media_testkit::hostile::hammer(&format!("matroska seed {i}"), s, 20_000, exercise);
+    }
+}
diff --git a/crates/media/tests/fuzz.rs b/crates/media/tests/fuzz.rs
index 3c302ac6b..29cdbf7c7 100644
--- a/crates/media/tests/fuzz.rs
+++ b/crates/media/tests/fuzz.rs
@@ -175,6 +175,15 @@ fn mutated_ffmpeg_fixtures_never_panic() {
     }
 }
 
+#[test]
+fn hostile_hammer_never_panics() {
+    for (i, f) in synthetic_files().iter().enumerate() {
+        craft_media_testkit::hostile::hammer(&format!("isobmff seed {i}"), f, 20_000, |d| {
+            exercise(d);
+        });
+    }
+}
+
 #[test]
 fn truncations_never_panic() {
     for f in synthetic_files() {