libp2p is a JavaScript implementation of the libp2p networking stack. From 15.0.0 until 16.0.5, @libp2p/gossipsub uses the default StrictSign policy in packages/gossipsub/src/utils/buildRawMessage.ts, where validateToRawMessage verifies a signature with attacker-controlled msg.key but skips binding that key to msg.from when the claimed author is an RSA peer ID that does not inline a public key. An unauthenticated attacker can place a victim RSA peer ID in msg.from, sign the message with the attacker's private key, and supply the attacker's public key in msg.key, causing the message to be accepted and propagated as authored by the victim. Applications that trust message.from for validators, authorization, accounting, moderation, reputation, or audit logging can process attacker-controlled data under false origin attribution. The issue is fixed in version 16.0.5.

Affected: npm: @libp2p/gossipsub

### Summary `@libp2p/gossipsub` `StrictSign` validation does not bind a supplied message public key to the claimed `from` peer ID when `from` is an RSA-style peer ID that does not inline its public key. An attacker can set `from` to a victim RSA peer ID, sign the message with the attacker's own private key, include the attacker's public key in `msg.key`, and have the message accepted as a valid signed message from the victim.

### Details The vulnerable code is in `packages/gossipsub/src/utils/buildRawMessage.ts` inside `validateToRawMessage`.

When `msg.key` is present:

```ts publicKey = publicKeyFromProtobuf(msg.key) if (fromPeerId.publicKey !== undefined && !publicKey.equals(fromPeerId.publicKey)) { return { valid: false, error: ValidateError.InvalidPeerId } } ```

For RSA peer IDs parsed from the wire `from` multihash, `fromPeerId.publicKey` is undefined because the RSA public key is not inlined in the peer ID. This means the key-to-`from` comparison is skipped. The code then verifies the signature with the attacker-supplied `msg.key` and returns a signed message whose `from` is the victim RSA peer ID.

The missing invariant is:

```ts peerIdFromPublicKey(publicKey).equals(fromPeerId) ```

This check must be performed whenever a public key is supplied, including keyless peer ID representations such as RSA peer IDs.

`StrictSign` is the default gossipsub signature policy in `packages/gossipsub/src/gossipsub.ts`:

```ts this.globalSignaturePolicy = opts.globalSignatureP…