anacrolix torrent has unbounded recursion in the bencode decoder that causes stack-overflow crash and CPU-exhaustion DoS

Why it is Yellow

• CVSS 6.5 medium
• On your watchlist: Go

Affected packages

References

• https://github.com/advisories/GHSA-2wrx-84qj-4pcg
• https://github.com/anacrolix/torrent/security/advisories/GHSA-2wrx-84qj-4pcg
• https://github.com/anacrolix/torrent/commit/76452a2c8a2fd9c59d4d4f10fc3495723d538c7b
• https://github.com/advisories/GHSA-2wrx-84qj-4pcg

Original description

## Summary anacrolix/torrent's bencode decoder (`bencode/`) applies **no limit on the nesting depth** of nested dictionaries and lists. Two distinct denial-of-service mechanisms follow from that single root cause, both reachable from a single crafted `.torrent` file or from metadata obtained from a peer on a magnet-link/v2 download: 1. **Stack-overflow process crash.** `Decoder.readOneValue` recurses once per nesting level with no depth bound. A `.torrent` whose `info` value is a list nested a few million levels deep (≈8 MB file) drives the recursion past the 1 GiB goroutine-stack limit and the process dies with an unrecoverable `fatal error: stack overflow`. 2. **Quadratic CPU exhaustion.** `metainfo.FileTree.UnmarshalBencode` (BitTorrent v2) re-parses each subtree at every level, making parse time grow ~O(n²). A v2 info dict with a ~30,000-level file tree (≈0.2 MB) takes ~66 s to decode; 60,000 levels exceeds 90 s of decode time. Affected: every application embedding `github.com/anacrolix/torrent` that ingests attacker-supplied metainfo - the CLI tools (`cmd/torrent`, `cmd/torrent2`), any server that opens `.torrent` files, or any client that accepts metadata from a peer for a magnet/v2 download. No depth limit has ever existed in the decoder. ## Vulnerability Details All file:line references are against the pinned commit `4ad31c5170787215c51c6ddaa54abc06b623c11c`. ### Root cause - no recursion/depth limit in the bencode decoder `bencode/decode.go` implements three independently unbounded recursive parsers. There is no depth counter and no `MaxDepth`/nesting guard anywhere in `bencode/` or `metainfo/`. **Defect 1 - `readOneValue` recursion (`bencode/decode.go:546-549`)** ```go case 'd', 'l': // read until there is nothing to read for d.readOneValue() { } // <- recurses once per nested dict/list level, unbounded ``` `readOneValue` is invoked by `Decoder.parseUnmarshaler` (`bencode/decode.go:591`) whenever a decoded field implements `bencode.Unmarshaler`, which includes `bencode.Bytes` - the type of `MetaInfo.InfoBytes` (`metainfo/metainfo.go:17`). So decoding a normal `.torrent` routes the entire `info` value through this recursion. Stack usage is ~256 bytes per level (consistent with the observed ~4M-level crash at the 1 GiB goroutine-stack limit), so nesting `l` a few million levels deep exhausts the 1 GiB default goroutine-stack limit. Go's stack overflow is a `fatal error` - it is not a recoverable panic, so the `recover()` in `Decoder.Decode` (`bencode/decode.go:35-55`) cannot intercept it, and the whole process terminates. Entry points that reach `readOneValue` with attacker-controlled bytes: - `metainfo.Load` (`metainfo/metainfo.go:35`) / `metainfo.LoadFromFile` (`metainfo.go:50`) - decoding a `.torrent` file; the `info` value becomes a `bencode.Bytes` unmarshaler field. - `Client.AddTorrent` / `Client.AddTorrentFromFile` -> `metainfo.LoadFromFile` (`client.go:1776` inside `AddTorrentFromFile`), and `TorrentSpec.InfoBytes` -> `setInfoBytesLocked` -> `bencode.Unmarshal(b, &info)` (`torrent.go:638`) - peer-fetched metadata. For the **remote-peer path** (magnet-link/v2 download), peer-supplied metadata pieces are reassembled and passed to `setInfoBytesLocked` once complete (`torrent.go:1874`). The accepted metadata size is capped at `maxMetadataSize = 16 MiB` (`global.go:18`), which comfortably admits the ≈8 MB Defect-1 payload and the ≈0.2 MB Defect-2 payload, so both defects are reachable from an untrusted peer, not only from a locally-opened `.torrent`. **Defect 2 - `FileTree.UnmarshalBencode` recursive re-parse (`metainfo/file-tree.go:42-49`)** ```go for key, bytes := range dir { var sub FileTree err = sub.UnmarshalBencode(bytes) // <- recurses once per file-tree level, and each call re-runs // bencode.Unmarshal over its entire subtree ``` `FileTree` implements `bencode.Unmarshaler` (`file-tree.go:53`), so the `file tree` field of a v2 info dict (`metainfo/info.go:33`) routes through it. Each level calls `sub.UnmarshalBencode`, which starts with `bencode.Unmarshal(bytes, &dir)` over the *whole remaining subtree*. A file tree nested `D` levels deep therefore performs O(D) `bencode.Unmarshal` calls, each over an O(D)-sized payload - O(D²) total parse work. The steady state is a CPU-bound single goroutine, so the embedding process simply hangs. Reachability: any `bencode.Unmarshal` into `metainfo.Info`, including the peer-metadata path `setInfoBytesLocked` (`torrent.go:638`) for a magnet/v2 download, and `MetaInfo.UnmarshalInfo` (`metainfo/metainfo.go:61-64`). ### Sanitizers / existing limits that do NOT apply - `Decoder.MaxStrLen` (`bencode/decode.go:787-791`, default 128 MiB) bounds a *single string value*; it is never consulted by `readOneValue` (`decode.go:564` does `io.CopyN(&d.buf, d.r, length)` with no `MaxStrLen` check) and does not bound dict/list nesting depth. - The existing fuzzers (`bencode/fuzz_test.go`) only round-trip `interface{}` values; small fuzz inputs cannot synthesize the millions of nesting levels needed to trip the stack limit, which is why this has evaded them. ## Impact - **Denial of service / availability (complete process crash).** A single ~8 MB `.torrent` file, or ~8 MB of metadata from a malicious peer, crashes the embedding application with `fatal error: stack overflow` (Defect 1). The crash is not recoverable in-process. For a server that ingests user-submitted torrents or runs a long-lived swarm, this is a trivial remote DoS: seed a torrent whose metainfo nests lists deeply, and any client that fetches the metadata dies. - **Denial of service / availability (CPU starvation).** A ~0.2-0.6 MB v2 info dict with a deeply nested file tree pins a core for ~66 s to minutes and blocks any other decode work sharing the process (Defect 2). Threat actor: an unauthenticated remote peer (v2/magnet metadata, within the allowed 16 MiB metadata size - `globa