/ 
Net-BitTorrent-v2.0.1
⭐ Starred 13 / Net::BitTorrent::Storage::File
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::Storage::File - Individual File Handler with Merkle Tree Support

SYNOPSIS

use Net::BitTorrent::Storage::File;

my $file = Net::BitTorrent::Storage::File->new(
    path        => 'downloads/ubuntu.iso',
    size        => 2147483648,
    pieces_root => $sha256_merkle_root
);

# Read/Write
$file->write( 0, $data );
my $chunk = $file->read( 0, 16384 );

# Verify
my $valid = $file->verify_block( 0, $data );

DESCRIPTION

This class manages a single file on disk, handling sparse file allocation, Merkle tree verification (BEP 52), and raw I/O.

In BitTorrent v2 (BEP 52), every file has its own Merkle tree. This class manages the mapping of block indices to Merkle tree leaves, ensuring that every 16KiB chunk written to disk is cryptographically verified against the swarm's trusted pieces root.

METHODS

new( %params )

Creates a new File handler object.

my $file = Net::BitTorrent::Storage::File->new(
    path => $path,
    size => $size
);

Expected parameters:

path

The file system path to the file.

size

The expected size of the file in bytes.

pieces_root - optional

The 32-byte binary SHA-256 Merkle root of the file.

piece_size - optional

The BitTorrent piece size for this file.

verify_block( $index, $data )

Verifies a 16KiB block against the file's Merkle root.

my $valid = $file->verify_block( 0, $data );

Calculates the SHA-256 hash of $data and inserts it into the file's internal Merkle tree at block index $index. Returns true if the updated tree root matches the expected pieces_root.

Expected parameters:

$index

The zero-based block index within the file.

$data

The 16KiB block of data.

verify_block_audit( $index, $data, $audit_path )

Verifies a block using an audit path (branch nodes).

my $valid = $file->verify_block_audit( 0, $data, $audit );

Verifies a block using a provided audit path (a list of sibling hashes). This allows verifying data even if the rest of the Merkle tree has not been populated yet.

Expected parameters:

$index

The block index.

$data

The block data.

$audit_path

An array reference of hashes representing the branch nodes.

verify_piece_v2( $index, $data, $expected_hash )

Verifies a full BitTorrent v2 piece.

my $valid = $file->verify_piece_v2( 0, $data, $hash );

Expected parameters:

$index

The piece index within the file.

$data

The full piece data.

$expected_hash

The 32-byte binary SHA-256 hash from the piece layer.

read( $offset, $length )

Reads data from the file.

my $data = $file->read( 0, 16384 );

Reads $length bytes from the file starting at $offset. Returns the binary data or undef if the file does not exist.

Expected parameters:

$offset

The byte offset.

$length

The number of bytes to read.

write( $offset, $data )

Writes data to the file.

$file->write( 0, $data );

Writes the binary string $data to the file at $offset. Automatically creates the file and any parent directories if they are missing.

Expected parameters:

$offset

The byte offset.

$data

The data string to write.

dump_state( )

Exports the file's Merkle tree state.

my $state = $file->dump_state();

load_state( $state )

Restores the Merkle tree state.

$file->load_state( $state );

Expected parameters:

$state

The state hash reference.

path( )

Returns the Path::Tiny object for the file.

size( )

Returns the file size in bytes.

pieces_root( )

Returns the 32-byte Merkle root.

piece_size( )

Returns the configured piece size.

merkle( )

Returns the Digest::Merkle::SHA256 object.

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.