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Net-BitTorrent-v2.0.1
⭐ Starred 13 / Net::BitTorrent::Protocol::MSE
Security Advisories (4)
CVE-2026-57079 (2026-06-30)

Net::BitTorrent versions through 2.0.1 for Perl write files outside the download directory via path traversal in peer-supplied metadata. Net::BitTorrent validates file path components only on the .torrent-file ingest path. The peer and magnet metadata path (_on_metadata_received, reached from the BEP09 ut_metadata extension) passes attacker-supplied file names straight to Storage::add_file and Storage::_parse_file_tree, where Path::Tiny's child() does not collapse "..". A v2 file tree key, a v1 files[].path element, or a single-file name containing ".." segments therefore resolves outside the download directory. Because the peer also controls the piece hashes and the served bytes, content verification passes, so a malicious magnet or peer writes attacker-chosen content to an attacker-chosen path on the downloading host.

CVE-2026-57080 (2026-06-30)

Net::BitTorrent versions through 2.0.1 for Perl allow remote memory exhaustion via an uncapped peer-wire message-length prefix. The peer-wire framing in _process_messages trusts the 4-byte length prefix sent by a connected peer with no upper bound, while receive_data appends every inbound byte to the input buffer. A peer announces a length prefix of up to about 4 GiB and then streams bytes; the decoder waits until the buffer holds the full message before processing it, so the buffer grows without limit. Peer connections are unauthenticated, so any peer in the swarm exhausts the downloading process's memory. The largest legitimate message is a 16 KiB piece block, so any announced length far above that is anomalous.

CVE-2026-57082 (2026-06-30)

Net::BitTorrent versions through 2.0.1 for Perl generate the MSE Diffie-Hellman private key with a non-cryptographic PRNG. The MSE (Message Stream Encryption) handshake derives its 160-bit Diffie-Hellman private key from Perl's rand(), a non-cryptographic drand48-class generator seeded once per process, in KeyExchange.pm. The shared secret and the RC4 keys derived from it (the SHA-1 of "keyA" or "keyB", the shared secret, and the infohash) therefore depend entirely on a predictable PRNG. The same handshake sends, in cleartext, random padding drawn from the same rand() sequence in _random_pad, immediately after the public key and the private-key draw. A passive observer of the handshake recovers the PRNG state from the cleartext padding, reconstructs the private key, computes the shared secret from the peer's public key on the wire, derives the RC4 keys, and decrypts the connection, defeating the passive-observation obfuscation MSE provides.

CVE-2026-57081 (2026-06-30)

Net::BitTorrent versions through 2.0.1 for Perl allow remote memory exhaustion via deeply nested bencoded input. bdecode recurses once per nested list or dictionary level with no depth cap, and each recursive call receives the remaining buffer by value while the list and dictionary branches capture the whole remainder, so every live recursion frame keeps its own copy of the shrinking buffer (O(N^2) bytes for an N-deep input). The decoder runs on every untrusted bencode source: .torrent files, BEP09 metadata fetched from peers, DHT messages, and tracker responses. A bencoded input of roughly 150,000 nested lists (about 150 KB on the wire) drives multi-gigabyte peak memory, so one short message from any peer, or one crafted .torrent file or magnet link, terminates the client.

NAME

Net::BitTorrent::Protocol::MSE - Message Stream Encryption (PE/MSE)

SYNOPSIS

use Net::BitTorrent::Protocol::MSE;

my $mse = Net::BitTorrent::Protocol::MSE->new(
    infohash     => $bin_ih,
    is_initiator => 1
);

# Step 1: Send initial Public Key
my $out = $mse->write_buffer( );

# Step 2: Feed data from socket until state is 'PAYLOAD'
while ($mse->state ne 'PAYLOAD') {
    $mse->receive_data($raw_incoming);
    my $reply = $mse->write_buffer( );
    # send $reply to peer
}

# Step 3: Transparently encrypt/decrypt PWP messages
my $encrypted = $mse->encrypt_data( $pwp_payload );
my $plain     = $mse->decrypt_data( $raw_from_socket );

DESCRIPTION

This module implements the Message Stream Encryption protocol used by BitTorrent to avoid traffic shaping and improve privacy. It provides a layer of obfuscation for BitTorrent traffic, helping to prevent passive traffic analysis and "throttling" by ISPs.

Security Disclaimer

MSE is not designed to provide strong cryptographic security or authentication. It is purely an obfuscation layer designed to make the BitTorrent protocol difficult to identify via simple pattern matching.

METHODS

new( %params )

Creates a new MSE handler object.

Expected parameters:

infohash

The 20-byte binary infohash of the torrent. Can be undef for initiators if using on_infohash_probe.

is_initiator

Boolean. True if we are starting the connection.

on_infohash_probe - optional

A code reference used by the responder to identify the infohash in incoming connections.

allow_plaintext - optional

Boolean. Whether to allow falling back to plaintext. Defaults to true.

receive_data( $data )

Handles incoming handshake or payload data.

my $decrypted = $mse->receive_data( $raw_chunk );

This method processes the MSE handshake phases. Once the handshake is complete (state is 'PAYLOAD'), it returns decrypted data.

Expected parameters:

$data

The raw data received from the transport.

encrypt_data( $data )

Encrypts outgoing data.

my $encrypted = $mse->encrypt_data( $pwp_message );

This method encrypts data using the negotiated RC4 key if the handshake is complete.

Expected parameters:

$data

The plaintext data to encrypt.

decrypt_data( $data )

Decrypts incoming data.

my $decrypted = $mse->decrypt_data( $raw_chunk );

Alias for receive_data.

Expected parameters:

$data

The encrypted data.

write_buffer( )

Returns pending bytes that must be sent to the remote peer to advance the handshake or provide encryption responses.

my $out = $mse->write_buffer();

supported( )

Returns true if MSE is supported by the system.

say "MSE enabled" if $mse->supported();

state( )

Returns the current MSE handshake state:

  • A_WAIT_PUBKEY, B_WAIT_PUBKEY: Waiting for Diffie-Hellman exchange.

  • A_WAIT_SELECT, B_WAIT_REQS: Synchronizing on the infohash.

  • PAYLOAD: Handshake complete; stream is now obfuscated.

buffer_in( )

Returns the internal input buffer.

buffer_out( )

Returns the internal output buffer.

ARCHITECTURE

MSE uses a 768-bit Diffie-Hellman key exchange to derive a shared secret, which is then used to seed two RC4 stream ciphers (one for each direction).

AUTHOR

Sanko Robinson <sanko@cpan.org>

COPYRIGHT

Copyright (C) 2008-2026 by Sanko Robinson.

This library is free software; you can redistribute it and/or modify it under the terms of the Artistic License 2.0.