Dragonfly is an open source P2P-based file distribution and image acceleration system. Prior to 2.1.0, the DragonFly2 uses a variety of hash functions, including the MD5 hash, for downloaded files. This allows attackers to replace files with malicious ones that have a colliding hash. This vulnerability is fixed in 2.1.0.

Affected: Go: github.com/dragonflyoss/dragonfly, Go: d7y.io/dragonfly/v2

### Impact The DragonFly2 uses a variety of hash functions, including the MD5 hash. This algorithm does not provide collision resistance; it is secure only against preimage attacks. While these security guarantees may be enough for the DragonFly2 system, it is not completely clear if there are any scenarios where lack of the collision resistance would compromise the system. There are no clear benefits to keeping the MD5 hash function in the system.

```golang var pieceDigests []string for i := int32(0); i < t.TotalPieces; i++ { pieceDigests = append(pieceDigests, t.Pieces[i].Md5) } digest := digest.SHA256FromStrings(pieceDigests...) if digest != t.PieceMd5Sign { t.Errorf("invalid digest, desired: %s, actual: %s", t.PieceMd5Sign, digest) t.invalid.Store(true) return ErrInvalidDigest } ```

Alice, a peer in the DragonFly2 system, creates two images: an innocent one, and one with malicious code. Both images consist of two pieces, and Alice generates the pieces so that their respective MD5 hashes collide (are the same). Therefore, the PieceMd5Sign metadata of both images are equal. Alice shares the innocent image with other peers, who attest to their validity (i.e., that it works as expected and is not malicious). Bob wants to download the image and requests it from the peer-to-peer network. After downloading the image, Bob checks its integrity with a SHA256 hash that is known to him. Alice, who is participating in the network, had already provided Bo…

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