Polygon PoS Incident Postmortem: Milestone Bug & Hard Fork Resolution

Polygon PoS recently experienced a disruption where checkpoint finality on Ethereum remained active, but local fast finality (milestones) was delayed. While transactions were still secure, milestone confirmations faced instability due to a faulty proposal.

Polygon engineers acted swiftly to resolve the issue through coordinated fixes on Heimdall & Bor, successfully restoring consensus stability.

• What Happened
• Root Cause Analysis
• Fixes & Recovery
• Implications & Road Ahead

What Happened

Earlier yesterday, Polygon PoS validators observed delays in milestone-based fast finality. While Ethereum checkpointing continued normally, milestones lagged, creating temporary instability.

This led to forks among Bor nodes and short-term confusion within the network.

Milestones serve as a lightweight alternative to checkpoints, offering faster finality before transactions are secured on Ethereum.

• Before Heimdall v2: 60–90 seconds
• After Heimdall v2: 3–6 seconds

Each validator independently proposes milestones, enabling efficiency but also raising complexity in consensus coordination.

Root Cause Analysis

The incident was traced back to a faulty milestone proposal, which caused Bor nodes to diverge onto separate forks. Since this edge case was not anticipated, the network experienced temporary instability until corrective measures were applied.

Earlier today, Polygon PoS experienced a disruption where checkpoint finality on Ethereum was still active, but local fast finality on the chain (i.e. milestones) were delayed.

For context: milestones are a lightweight alternative to checkpoints, enabling faster, deterministic… https://t.co/ulINLtlurV

— Sandeep | CEO, Polygon Foundation (※,※) (@sandeepnailwal) September 10, 2025

Fixes & Recovery

Polygon engineers rolled out rapid fixes across both core layers:

• Heimdall v0.3.1 (Hard Fork): Removed the faulty milestone.
• Bor v2.2.11-beta2: Purged milestone data from local node databases.
  Following these updates, checkpoints & milestones finalized normally, and consensus stability was fully restored.

Implications & Road Ahead

This event underscores the challenges of ambitious upgrades in blockchain networks. Polygon emphasized that such incidents are part of its growing pains, but each strengthens resilience.

Polygon PoS has successfully overcome a milestone-related disruption with swift engineering fixes. With finality restored and stability ensured, the network continues to move forward toward scalability.

If you find any issues in this blog or notice any missing information, please feel free to reach out at yash@etherworld.co for clarifications or updates.

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