Pumpkin-MC/Pumpkin · error · std::io::Error

user_data_len exceeds 1MB limit

Error message

user_data_len exceeds 1MB limit

What it means

Raised while reading a Bedrock network item's user data: the VarUInt-prefixed length field claims more than 1,048,576 bytes (1MB). The reader rejects it before allocating the buffer, protecting against corrupted or malicious packets that would otherwise trigger huge allocations.

Solutions

1. Confirm the packet stream is being read from the correct offset — desync makes garbage bytes look like lengths.
2. Ensure the client/server protocol versions match; mismatched versions change item encoding.
3. Treat it as a malformed-packet signal: drop the packet/connection rather than retrying the same bytes.
4. If sending side, keep user data under 1MB and clamp before writing.

Example fix

// before
let data = read_item_user_data(buf)?; // panics/allocs on huge len
// after
match read_item_user_data(buf) {
    Ok(d) => d,
    Err(e) if e.kind() == ErrorKind::InvalidData => { disconnect("malformed item"); }
    Err(e) => return Err(e),
}

match NetworkItem::read(buf) {
    Ok(item) => item,
    Err(e) if e.to_string().contains("1MB limit") => {
        log::warn!("malformed item packet rejected");
        drop_packet();
    }
    Err(e) => return Err(e),
}

When it happens

Trigger: Deserializing a network item (read path at network_item.rs:52) where the user_data_len varint exceeds 1MB — caused by stream desync, corrupt data, or crafted packets.

Common situations: Malicious client sends an inflated length field; packet parsing drifted out of sync so a data byte is misread as a length; corrupted save/network data.

Understand the failure class

Background: payload too large / request exceeds maximum size: why libraries cap bytes and how to fix oversize payloads — this error's family across 50 libraries.

Related errors

• extra_data_len exceeds 1MB limit
• Invalid ContainerName ID
• item string array length out of bounds
• String::from_utf8 error
• Nbt read error

AI-assisted analysis of Pumpkin-MC/Pumpkin@8d4639e25a (2026-09-09). Data as JSON: /api/errors/39c2970503fbaf73. Report an issue: GitHub.

Appendix: source

Thrown at crates/pumpkin-protocol/src/bedrock/network_item.rs:52

    }
}

impl PacketRead for NetworkItemDescriptor {
    fn read<R: Read>(buf: &mut R) -> Result<Self, Error> {
        let id = VarInt(i32::from(i16::read(buf)?));
        let stack_size = u16::read(buf)?;
        let aux_value = VarUInt::read(buf)?;

        let has_net_id = bool::read(buf)?;
        if has_net_id {
            let _net_id = VarInt::read(buf)?;
        }

        let block_runtime_id = VarInt(VarUInt::read(buf)?.0 as i32);

        let user_data_len = VarUInt::read(buf)?.0 as usize;
        if user_data_len > 1_048_576 {
            return Err(Error::new(
                ErrorKind::InvalidData,
                "user_data_len exceeds 1MB limit",
            ));
        }
        let mut user_data = vec![0u8; user_data_len];
        buf.read_exact(&mut user_data)?;

        let (nbt_data, place_on_blocks, destroy_blocks, shield_blocking_tick) =
            read_user_data(user_data, id.0 == i32::from(BedrockItem::SHIELD.id))?;

        Ok(Self {
            id,
            stack_size,
            aux_value,
            block_runtime_id,
            nbt_data,
            place_on_blocks,
            destroy_blocks,

View on GitHub (pinned to 8d4639e25a)