Gateway serving invalid CAR files for non-existing CIDs

Checklist

• This is a bug report, not a question. Ask questions on discuss.ipfs.tech.
• I have searched on the issue tracker for my bug.
• I am running the latest kubo version or have an issue updating.

Installation method

docker image

Version

Kubo version: 0.34.1-4649554
Repo version: 16
System version: amd64/linux
Golang version: go1.24.1

Config

{
  "API": {
    "HTTPHeaders": {}
  },
  "Addresses": {
    "API": "/ip4/0.0.0.0/tcp/5001",
    "Announce": [],
    "AppendAnnounce": [],
    "Gateway": "/ip4/0.0.0.0/tcp/8080",
    "NoAnnounce": [
      "/ip4/10.0.0.0/ipcidr/8",
      "/ip4/100.64.0.0/ipcidr/10",
      "/ip4/169.254.0.0/ipcidr/16",
      "/ip4/172.16.0.0/ipcidr/12",
      "/ip4/192.0.0.0/ipcidr/24",
      "/ip4/192.0.2.0/ipcidr/24",
      "/ip4/192.168.0.0/ipcidr/16",
      "/ip4/198.18.0.0/ipcidr/15",
      "/ip4/198.51.100.0/ipcidr/24",
      "/ip4/203.0.113.0/ipcidr/24",
      "/ip4/240.0.0.0/ipcidr/4",
      "/ip6/100::/ipcidr/64",
      "/ip6/2001:2::/ipcidr/48",
      "/ip6/2001:db8::/ipcidr/32",
      "/ip6/fc00::/ipcidr/7",
      "/ip6/fe80::/ipcidr/10"
    ],
    "Swarm": [
      "/ip4/0.0.0.0/tcp/4001",
      "/ip6/::/tcp/4001",
      "/ip4/0.0.0.0/udp/4001/webrtc-direct",
      "/ip4/0.0.0.0/udp/4001/quic-v1",
      "/ip4/0.0.0.0/udp/4001/quic-v1/webtransport",
      "/ip6/::/udp/4001/webrtc-direct",
      "/ip6/::/udp/4001/quic-v1",
      "/ip6/::/udp/4001/quic-v1/webtransport"
    ]
  },
  "AutoNAT": {},
  "AutoTLS": {},
  "Bootstrap": [
    "/dnsaddr/bootstrap.libp2p.io/p2p/QmQCU2EcMqAqQPR2i9bChDtGNJchTbq5TbXJJ16u19uLTa",
    "/dnsaddr/bootstrap.libp2p.io/p2p/QmbLHAnMoJPWSCR5Zhtx6BHJX9KiKNN6tpvbUcqanj75Nb",
    "/dnsaddr/bootstrap.libp2p.io/p2p/QmcZf59bWwK5XFi76CZX8cbJ4BhTzzA3gU1ZjYZcYW3dwt",
    "/dnsaddr/va1.bootstrap.libp2p.io/p2p/12D3KooWKnDdG3iXw9eTFijk3EWSunZcFi54Zka4wmtqtt6rPxc8",
    "/ip4/104.131.131.82/tcp/4001/p2p/QmaCpDMGvV2BGHeYERUEnRQAwe3N8SzbUtfsmvsqQLuvuJ",
    "/ip4/104.131.131.82/udp/4001/quic-v1/p2p/QmaCpDMGvV2BGHeYERUEnRQAwe3N8SzbUtfsmvsqQLuvuJ",
    "/dnsaddr/bootstrap.libp2p.io/p2p/QmNnooDu7bfjPFoTZYxMNLWUQJyrVwtbZg5gBMjTezGAJN"
  ],
  "DNS": {
    "Resolvers": {}
  },
  "Datastore": {
    "BlockKeyCacheSize": null,
    "BloomFilterSize": 0,
    "GCPeriod": "1h",
    "HashOnRead": false,
    "Spec": {
      "mounts": [
        {
          "child": {
            "path": "blocks",
            "shardFunc": "/repo/flatfs/shard/v1/next-to-last/3",
            "sync": true,
            "type": "flatfs"
          },
          "mountpoint": "/blocks",
          "prefix": "flatfs.datastore",
          "type": "measure"
        },
        {
          "child": {
            "disableWAL": false,
            "path": "datastore",
            "type": "pebbleds"
          },
          "mountpoint": "/",
          "prefix": "pebble.datastore",
          "type": "measure"
        }
      ],
      "type": "mount"
    },
    "StorageGCWatermark": 90,
    "StorageMax": "9GB"
  },
  "Discovery": {
    "MDNS": {
      "Enabled": false
    }
  },
  "Experimental": {
    "FilestoreEnabled": false,
    "Libp2pStreamMounting": false,
    "OptimisticProvide": false,
    "OptimisticProvideJobsPoolSize": 0,
    "P2pHttpProxy": false,
    "StrategicProviding": false,
    "UrlstoreEnabled": false
  },
  "Gateway": {
    "DeserializedResponses": null,
    "DisableHTMLErrors": null,
    "ExposeRoutingAPI": null,
    "HTTPHeaders": {},
    "NoDNSLink": true,
    "NoFetch": true,
    "PublicGateways": {
      "localhost": {
        "Paths": [
          "/ipfs"
        ],
        "UseSubdomains": false
      }
    },
    "RootRedirect": ""
  },
  "Identity": {
    "PeerID": <omitted>
  },
  "Import": {
    "BatchMaxNodes": null,
    "BatchMaxSize": null,
    "CidVersion": null,
    "HashFunction": null,
    "UnixFSChunker": null,
    "UnixFSRawLeaves": null
  },
  "Internal": {},
  "Ipns": {
    "RecordLifetime": "",
    "RepublishPeriod": "",
    "ResolveCacheSize": 128
  },
  "Migration": {
    "DownloadSources": [],
    "Keep": ""
  },
  "Mounts": {
    "FuseAllowOther": false,
    "IPFS": "/ipfs",
    "IPNS": "/ipns"
  },
  "Peering": {
    "Peers": null
  },
  "Pinning": {
    "RemoteServices": {}
  },
  "Plugins": {
    "Plugins": null
  },
  "Provider": {
    "Strategy": ""
  },
  "Pubsub": {
    "DisableSigning": false,
    "Router": ""
  },
  "Reprovider": {},
  "Routing": {
    "AcceleratedDHTClient": true,
    "Methods": null,
    "Routers": null
  },
  "Swarm": {
    "AddrFilters": [
      "/ip4/10.0.0.0/ipcidr/8",
      "/ip4/100.64.0.0/ipcidr/10",
      "/ip4/169.254.0.0/ipcidr/16",
      "/ip4/172.16.0.0/ipcidr/12",
      "/ip4/192.0.0.0/ipcidr/24",
      "/ip4/192.0.2.0/ipcidr/24",
      "/ip4/192.168.0.0/ipcidr/16",
      "/ip4/198.18.0.0/ipcidr/15",
      "/ip4/198.51.100.0/ipcidr/24",
      "/ip4/203.0.113.0/ipcidr/24",
      "/ip4/240.0.0.0/ipcidr/4",
      "/ip6/100::/ipcidr/64",
      "/ip6/2001:2::/ipcidr/48",
      "/ip6/2001:db8::/ipcidr/32",
      "/ip6/fc00::/ipcidr/7",
      "/ip6/fe80::/ipcidr/10"
    ],
    "ConnMgr": {
      "GracePeriod": "30s",
      "HighWater": 4096,
      "LowWater": 2024
    },
    "DisableBandwidthMetrics": false,
    "DisableNatPortMap": true,
    "RelayClient": {
      "Enabled": true
    },
    "RelayService": {
      "Enabled": false
    },
    "ResourceMgr": {},
    "Transports": {
      "Multiplexers": {},
      "Network": {},
      "Security": {}
    }
  },
  "Version": {}
}

Description

Hi,

I see some weird behaviour in my Kubo node Gateway. I am running with Gateway.NoFetch set to true, when i try to fetch a CID that i haven't already pinned to the node it works as expected returning a 404, but when it try requesting the same CID but also passing query param ?format=car i then get 200 response and start downloading in invalid CAR file with some hex data.

This is a weird behaviour which i have replicated in browsers but also using wget/curl etc.

To reproduce:

1. Set Gateway.NoFetch to true to the node doesn't start searching for the CID
2. Do a GET request to a non-existing CID (e.g. http://localhost:8080/ipfs/Qmey4V7cYZ6nDtMNdnxmwVvyyjncn7eGG494f8WjDjYrCf) -> get 404
3. Make the same request but add the format query param (e.g. http://localhost:8080/ipfs/Qmey4V7cYZ6nDtMNdnxmwVvyyjncn7eGG494f8WjDjYrCf?format=car) -> get 200

This should return 404 in step 3.

Thanks!