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Net-BitTorrent-v2.1.0
⭐ Starred 13 / Net::BitTorrent::SSRF
Security Advisories (2)
CVE-2026-57080 (2026-06-30)

Net::BitTorrent versions through 2.1.0 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-2026-57081 (2026-06-30)

Net::BitTorrent versions through 2.1.0 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::SSRF - Server-Side Request Forgery Protection

SYNOPSIS

use Net::BitTorrent::SSRF qw[is_safe_ip is_safe_host is_safe_url resolve_and_pin];

# Check a raw IP address
die 'unsafe' unless is_safe_ip('8.8.8.8');

# Check a hostname (resolves DNS and validates all IPs)
die 'unsafe' unless is_safe_host('tracker.example.com');

# Check a full URL (extracts host, resolves, validates)
die 'unsafe' unless is_safe_url('http://tracker.example.com/announce');

# Resolve a hostname to a validated safe IP + port (DNS pinning)
my ($ip, $port) = resolve_and_pin('tracker.example.com', 80)
    or die 'could not resolve to a safe address';

DESCRIPTION

Net::BitTorrent::SSRF provides utility functions to detect and block outbound connections to RFC 1918 private ranges, loopback, link-local, multicast, cloud metadata endpoints, and other address spaces that should never be contacted by a BitTorrent client on behalf of untrusted input.

This module is used by the tracker, web seed, and DHT subsystems to prevent Server-Side Request Forgery (SSRF) attacks where a malicious torrent file, magnet link, tracker response, or DHT node tricks the client into probing internal networks.

FUNCTIONS

is_safe_ip( $ip )

Returns true if $ip is a public (non-private) IP address string. Returns false for loopback, RFC 1918 private, link-local, ULA, multicast, unspecified, and unparseable inputs.

if ( is_safe_ip( $ip ) ) {
    # safe to connect
}

Blocked ranges:

  • 127.0.0.0/8 - IPv4 loopback

  • 10.0.0.0/8 - RFC 1918 private

  • 172.16.0.0/12 - RFC 1918 private

  • 192.168.0.0/16 - RFC 1918 private

  • 169.254.0.0/16 - Link-local (includes 169.254.169.254 cloud metadata)

  • 224.0.0.0/4 - IPv4 multicast

  • 0.0.0.0 - Unspecified

  • ::1 - IPv6 loopback

  • fe80::/10 - IPv6 link-local

  • fc00::/7 - IPv6 ULA

  • ff00::/8 - IPv6 multicast

  • :: - IPv6 unspecified

is_safe_host( $host )

Returns true if $host (an IP address or hostname) resolves exclusively to public IP addresses. If $host is already a valid IP, this behaves identically to is_safe_ip. Otherwise, $host is resolved via getaddrinfo and every resulting IP is checked with is_safe_ip.

unless ( is_safe_host( $tracker_host ) ) {
    warn 'refusing to connect to ' .$tracker_host;
}

is_safe_url( $url )

Extracts the hostname from $url using URI and passes it to is_safe_host. Returns true if the URL's host resolves to only public IPs.

my $http = HTTP::Tiny->new();
if ( is_safe_url( $target_url ) ) {
    my $res = $http->get( $target_url );
}

resolve_and_pin( $host, $port = undef )

Resolves $host via getaddrinfo, validates every resolved IP with is_safe_ip, and returns ($safe_ip, $port) for the first safe result. Returns an empty list on failure (DNS error, no results, or all resolved IPs are unsafe).

my ($ip, $port) = resolve_and_pin('tracker.example.com', 80)
    or die 'could not resolve to a safe address';
# $ip is now a validated public IP, $port is the resolved or original port

If $host is already a valid safe IP (passes is_safe_ip), it is returned immediately without DNS resolution. This function is intended for DNS pinning: resolve once, validate, and use the result for the actual connection to minimize the TOCTOU window between resolution and connection.

A full TOCTOU elimination would require overriding HTTP::Tiny's internal connection logic. The residual window when using this function is sub-millisecond and requires attacker-controlled DNS.

INTEGRATION

This module is consumed by the following classes:

All tracker and DHT classes accept a ssrf_bypass constructor parameter for testing. This parameter defaults to 0 (protection enabled) and should never be set to 1 in production.

SPECIFICATIONS

  • RFC 1918 - Address Allocation for Private Internets

  • RFC 3927 - Dynamic Configuration of IPv4 Link-Local Addresses

  • RFC 4193 - Unique Local IPv6 Unicast Addresses

  • BEP 55 - Holepunching via UDP (SSRF vector via HP_CONNECT)

AUTHOR

Sanko Robinson - https://github.com/sanko

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.