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Net-BitTorrent-v2.0.1
⭐ Starred 13 / Net::BitTorrent::Torrent::PiecePicker
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::Torrent::PiecePicker - Intelligent Piece Selection

SYNOPSIS

use Net::BitTorrent::Torrent::PiecePicker;
use Net::BitTorrent::Types qw[:pick];

my $picker = Net::BitTorrent::Torrent::PiecePicker->new(
    bitfield => $my_bitfield,
    strategy => PICK_RAREST_FIRST
);

# Update global swarm availability
$picker->update_availability($peer_bitfield, 1); # peer joined
$picker->update_availability($peer_bitfield, -1); # peer left

# Pick the next piece to request from a peer
my ($idx, $off, $len) = $picker->pick_block($peer, \%blocks_pending);

DESCRIPTION

Net::BitTorrent::Torrent::PiecePicker implements the selection logic used to decide which torrent piece or block to request next. A good picker is essential for swarm health and performance.

SELECTION STRATEGIES

  • PICK_RAREST_FIRST (Default)

    Prioritizes pieces that have the lowest availability in the local swarm. This ensures that rare pieces are replicated quickly, preventing "stalled" torrents where the last few pieces are missing.

  • PICK_SEQUENTIAL

    Picks pieces in simple numerical order. Useful for linear media playback where early data is needed first.

  • PICK_STREAMING

    A hybrid approach that prioritizes early pieces while still respecting per-piece priorities.

METHODS

new( %params )

Creates a new PiecePicker object.

my $picker = Net::BitTorrent::Torrent::PiecePicker->new(
    bitfield => $bitfield
);

Expected parameters:

bitfield

The Acme::Bitfield object representing the local client's pieces.

piece_priorities - optional

An array reference of priority levels for each piece.

strategy - optional

The selection strategy constant. Defaults to PICK_RAREST_FIRST.

update_availability( $peer_bitfield, $delta )

Updates internal availability counts when a peer's bitfield changes.

$picker->update_availability( $bf, 1 );

Expected parameters:

$peer_bitfield

The bitfield object representing the peer's pieces.

$delta

The change in availability (usually 1 when a peer connects/has, -1 when a peer disconnects).

set_priority( $index, $priority )

Sets the priority for a specific piece.

$picker->set_priority( 0, 2 );

Expected parameters:

$index

The piece index.

$priority

The priority level (0 = skip, 1 = normal, higher = higher).

get_priority( $index )

Returns the priority of a piece.

my $p = $picker->get_priority( 0 );

Expected parameters:

$index

The piece index.

get_availability( $index )

Returns the number of peers that have a specific piece.

my $count = $picker->get_availability( 0 );

Expected parameters:

$index

The piece index.

is_interesting( $peer )

Checks if a peer has any pieces that we lack and want.

if ($picker->is_interesting( $peer )) { ... }

Expected parameters:

$peer

The Peer object to check.

pick_piece( $peer_bitfield, $blocks_pending )

Selects a piece to request from a peer based on the current strategy.

my $idx = $picker->pick_piece( $p_bf, $pending );

Expected parameters:

$peer_bitfield

The peer's bitfield.

$blocks_pending

Hash reference of currently pending blocks.

pick_block( $peer, $blocks_pending )

Selects a specific block (16KiB) to request from a specific peer.

my ($idx, $off, $len) = $picker->pick_block( $peer, $pending );

Iterates through prioritized piece candidates and returns the first block that is not already pending or received.

Expected parameters:

$peer

The Peer object.

$blocks_pending

Hash reference of currently pending blocks.

enter_end_game( )

Enables End Game mode.

$picker->enter_end_game();

In this mode, the picker becomes more aggressive, allowing requests for the same block to be sent to multiple peers simultaneously to finish the torrent faster.

strategy( )

Returns or sets the picking strategy.

my $s = $picker->strategy();
$picker->strategy( PICK_SEQUENTIAL );

Expected parameters:

$strategy - optional

The new strategy constant.

end_game( )

Returns true if in end game mode.

if ($picker->end_game) { ... }

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.