CVE-2026-123096 in torrent
Summary
by MITRE • 11/05/2026
anacrolix/torrent through 1.61.0 contains a memory exhaustion vulnerability in the tracker/http package that allows malicious trackers to crash clients by returning unbounded responses. Attackers controlling an HTTP tracker in a torrent file or magnet link can return huge or gzip-compressed announce or scrape responses that Client.Announce and Client.Scrape buffer without limits, exhausting process memory.
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Analysis
by VulDB Data Team • 11/05/2026
The vulnerability identified in the anacrolix/torrent library through version 1.61.0 represents a critical resource exhaustion flaw within its HTTP tracker communication module. This issue stems from a fundamental design oversight where the client-side implementation fails to enforce strict limits on the size of responses received from external trackers. In BitTorrent ecosystems, clients regularly interact with trackers via HTTP requests for announce and scrape operations, which are essential for peer discovery and swarm health monitoring. The specific defect lies in how the Client.Announce and Client.Scrape functions handle incoming data streams. Instead of implementing a maximum read limit or validating response headers against expected bounds, these functions buffer the entire response body into memory without restriction. This architectural choice creates a direct pathway for denial-of-service attacks when interacting with untrusted or malicious tracker endpoints.
An attacker who controls an HTTP tracker referenced in a torrent file or magnet link can exploit this behavior by returning excessively large responses. The exploitation vector is straightforward yet effective: the malicious tracker responds to standard announce or scrape requests with payloads that are either inherently massive or compressed using algorithms like gzip to mask their true size until decompression occurs within the client's memory space. Because the anacrolix/torrent library does not impose a cap on the buffer size, it will continue reading and storing data from these responses until system resources are depleted. This unbounded buffering leads directly to severe memory exhaustion, causing the torrent client process to crash or become unresponsive due to out-of-memory conditions. The impact is particularly acute in environments where clients manage multiple torrents simultaneously, as each connection to a malicious tracker contributes cumulatively to resource depletion across the entire application instance.
From a classification perspective, this vulnerability aligns with CWE-400, which denotes Uncontrolled Resource Consumption. It also maps closely to ATT&CK technique T1498, specifically Network Denial of Service via volumetric attacks or resource exhaustion through protocol abuse. The flaw is not in the encryption or authentication mechanisms but rather in the fundamental handling of input data from untrusted sources. In the context of peer-to-peer networks, trust boundaries are often blurred because clients must communicate with arbitrary third-party trackers to function correctly. This vulnerability highlights the risk associated with processing external network inputs without rigorous validation and size constraints. The ability for a single malicious tracker to disrupt service demonstrates how lightweight protocol implementations can become significant attack surfaces if they assume all peer interactions are benign or well-behaved.
The operational impact of this flaw extends beyond simple application crashes. In production environments, such as seed boxes or automated torrent management systems, the sudden termination of client processes can lead to data integrity issues, interrupted downloads, and increased network overhead due to repeated reconnection attempts. Furthermore, if multiple instances are running on a single host with shared resource limits, one compromised instance could potentially affect others through system-wide memory pressure. For users relying on this library for critical file distribution or archival purposes, the reliability of their infrastructure is directly compromised by the presence of malicious trackers in the wild that may have been updated to exploit this weakness.
Mitigation strategies must focus on enforcing strict limits on network input sizes within the client implementation. Developers should modify the HTTP tracker handling logic to include a maximum response size limit for both announce and scrape operations. This can be achieved by wrapping the response body reader with an io.LimitedReader or similar construct that discards data exceeding a predefined threshold, typically aligned with standard tracker protocol specifications which rarely require responses larger than a few kilobytes. Additionally, implementing timeouts for HTTP connections ensures that slow-read attacks do not hang resources indefinitely. Upgrading to patched versions of the library where these limits are enforced is the primary remediation path for end-users. For developers integrating this library, auditing custom wrapper code to ensure no unbounded buffering occurs when processing tracker responses is essential to maintaining resilience against such resource exhaustion attacks.