diff --git a/unshackle/core/drm/playready.py b/unshackle/core/drm/playready.py
index 107adbc..f23e1b8 100644
--- a/unshackle/core/drm/playready.py
+++ b/unshackle/core/drm/playready.py
@@ -356,25 +356,6 @@ class PlayReady:
             key_hex = key if isinstance(key, str) else key.hex()
             key_args.extend(["--key", f"{kid_hex}:{key_hex}"])
 
-        # Also pass keys by track ID as fallback for CBCS content where the
-        # tenc box may have an all-zeros default_KID (common with HLS/FairPlay),
-        # causing mp4decrypt to fail matching KID-based keys silently.
-        if self.content_keys:
-            first_key = next(iter(self.content_keys.values()))
-            first_key_hex = first_key if isinstance(first_key, str) else first_key.hex()
-            try:
-                with open(path, "rb") as f:
-                    header = f.read(200_000)
-                tenc_idx = header.find(b"tenc")
-                if tenc_idx >= 0:
-                    tenc_data = header[tenc_idx + 4:]  # after box type
-                    version = tenc_data[0]
-                    default_kid = tenc_data[8:24] if version >= 1 else tenc_data[6:22]
-                    if default_kid == b"\x00" * 16:
-                        key_args.extend(["--key", f"1:{first_key_hex}"])
-            except OSError:
-                pass
-
         cmd = [
             str(binaries.Mp4decrypt),
             "--show-progress",
diff --git a/unshackle/core/drm/widevine.py b/unshackle/core/drm/widevine.py
index c9cb9fd..6ca4fb5 100644
--- a/unshackle/core/drm/widevine.py
+++ b/unshackle/core/drm/widevine.py
@@ -276,25 +276,6 @@ class Widevine:
             key_hex = key if isinstance(key, str) else key.hex()
             key_args.extend(["--key", f"{kid_hex}:{key_hex}"])
 
-        # Also pass keys by track ID as fallback for CBCS content where the
-        # tenc box may have an all-zeros default_KID (common with HLS/FairPlay),
-        # causing mp4decrypt to fail matching KID-based keys silently.
-        if self.content_keys:
-            first_key = next(iter(self.content_keys.values()))
-            first_key_hex = first_key if isinstance(first_key, str) else first_key.hex()
-            try:
-                with open(path, "rb") as f:
-                    header = f.read(200_000)
-                tenc_idx = header.find(b"tenc")
-                if tenc_idx >= 0:
-                    tenc_data = header[tenc_idx + 4:]  # after box type
-                    version = tenc_data[0]
-                    default_kid = tenc_data[8:24] if version >= 1 else tenc_data[6:22]
-                    if default_kid == b"\x00" * 16:
-                        key_args.extend(["--key", f"1:{first_key_hex}"])
-            except OSError:
-                pass
-
         cmd = [
             str(binaries.Mp4decrypt),
             "--show-progress",