1. Background and Core Concept

Gasper is the sub-protocol responsible for deterministic finality in Ethereum’s PoS consensus. Its core objectives are:

  • To continuously advance Justification and Finalization of the chain when the majority of honest participants are online;
  • To form a probabilistically secure chain selection through honest majority voting when forks exist.

However, if an attacker controls:

  • Close to 1/3 of the voting power;
  • The ability to delay attestation broadcasts (i.e., temporary network inconsistency);

They can create the following situation:

Make the last justified checkpoint repeatedly switch between two chains, preventing either chain from achieving Finalization.

This is known as the Bouncing Attack.

The core exploitation points are:

  1. Gasper requires blocks to receive 2/3 votes to be Justified;

  2. HLMD-GHOST directly prunes branches that conflict with the last Justified Checkpoint;

  3. If the attacker can:

    • Allow the honest chain to receive 1/3 votes (insufficient for Finalization);
    • Later release votes for their hidden chain to leapfrog Justification;

    Then they can:

    Force the entire network to “bounce back and forth” between two chains, with neither chain ever accumulating the 2/3 votes needed for finality.

2. Initial Attack Conditions

As shown in the diagram, with cp0 already Justified, two forked chains emerge:

  • c1: The main chain being built by honest nodes
    • Its checkpoint is cp1, which will extend to b0 → cp1' → ...
  • c2: The hidden fork maintained by the attacker
    • Its checkpoint is cp2, not yet public, but “justifiable” from the attacker’s perspective

Here, “justifiable” means:

Once the attacker releases their delayed attestations, cp2 can immediately achieve 2/3 votes and become the new last justified checkpoint.

3. Attack Process (Unfolding by Epoch)

3.1 Epoch e+1: Honest Nodes Continue Building Chain c1

Honest validators extend on c1:

cp0 → cp1 → b0 → cp1'

Meanwhile:

  • The attacker keeps new blocks and votes on c2 from being broadcast,
  • Ensuring cp2 can be Justified at any time, but doesn’t execute it yet.

At this point:

  • cp1 is the canonical checkpoint in the honest world,
  • cp2 is the “ready to activate” justifiable checkpoint controlled by the attacker.

3.2 Epoch e+2: Honest Chain cp1’ Receives Partial Votes

As the epoch progresses:

  • The honest chain cp1' receives over 1/3 attestations,
  • But hasn’t yet received the sufficient 2/3, therefore:

cp1' is justifiable but not justified.

In other words:

  • cp1’ has reached the potential threshold for Finalization,
  • But still requires consensus to progress further.

3.3 Attacker Reveals Hidden Votes, Making cp2 Overtake

At this moment, the attacker suddenly:

  • Releases all delayed votes for cp2,
  • cp2 immediately receives 2/3 attestations,

Thus:

cp2 becomes the new last justified checkpoint

4. Gasper’s Chain Pruning Mechanism is Triggered

Gasper (specifically HLMD-GHOST) stipulates:

All chains that conflict with the latest justified checkpoint must be pruned from the choice tree.

Therefore:

  • c1 conflicts with the newly Justified cp2,
  • c1 is immediately pruned,
  • c2 becomes the new canonical chain,

Honest nodes are forced to:

Abandon the chain c1 they just extended and switch to extending c2.

5. Attack Continues in a Loop, Achieving “Bouncing”

Now the attack state resets to:

  • Honest nodes begin continuing to build c2;
  • The honest chain’s new checkpoint (e.g., cp2’) starts accumulating votes;
  • When it reaches 1/3 but is not yet Justified:

The attacker can again:

  1. Create a justifiable checkpoint on the old chain c1;
  2. Withhold votes;
  3. Release them at the opportune moment;
  4. Make the last justified checkpoint bounce back to cp1'.

This repeats:

The last justified checkpoint “bounces” between c1 and c2.

The final effect:

  • Neither chain can accumulate two consecutive 2/3 votes,
  • Therefore:

The system cannot achieve Finalization for extended periods → liveness is critically threatened

6. Necessary Conditions for the Attack

In summary, the bouncing attack succeeds based on:

Condition ①: Attacker possesses at least 1/3 voting power

Ensuring:

  • Can prevent the honest chain from obtaining 2/3 attestations,
  • Can elevate the hidden chain to 2/3 at any time.

Condition ②: Attacker can temporarily control attestation propagation

For example:

  • The network is temporarily asynchronous (“can only be launched when the network is temporarily asynchronous”),
  • Causing honest validators to be unaware of the attacker’s hidden votes.

Condition ③: Gasper’s fork choice rule must enforce:

  • Conflicting chain pruning (HLMD-GHOST filter)

This is the fundamental protocol reason the attack can succeed.

7. Attack Impact

This attack does not:

  • Enable double-spending;
  • Rewrite state;
  • Compromise safety.

But it can:

Permanently block Finalization, causing deterministic finality to disappear.

In other words:

  • The chain continues running (safety is OK),
  • But from a protocol perspective:

The chain remains in a "suspended, unfinalized" state, and the value of on-chain certainty collapses.

For financial applications, this is a fatal liveness attack.

8. Can the Attack Continue After GST?

The paper points out:

The attack launch requires the network to be temporarily asynchronous (before GST), But once successful:

Even after GST (when the network returns to synchrony), the attack can continue to be maintained.

The reason is:

  • The attacker already controls:
    1. 1/3 voting power to prevent the honest chain from independently Finalizing;
    2. Another chain that can overtake at any time through hidden votes.

Therefore, the entire bouncing process no longer requires network anomalies to continue.

9. Summary

The Bouncing Attack is a structural weakness of the Gasper protocol under partially asynchronous network conditions. Its core mechanism lies in:

  1. The 2/3 voting requirement for Justification;
  2. HLMD-GHOST’s last justified checkpoint pruning rule;
  3. The attacker possessing 1/3 voting power and the ability to delay broadcasts.

Leveraging these conditions:

  • The attacker can repeatedly make the last Justified Checkpoint switch between two chains,
  • Preventing any single chain from completing two consecutive Justifications,
  • Ultimately causing the system to fail to achieve Finalization, impacting liveness but not compromising safety.