# "Reorgs" and Associated Risks
Transactions in a blockchain are packaged into blocks. Different blockchains have different rules for how to decide which transactions get packaged into a block, but they often include rules for invalidating recently generated blocks.
This rewrite of history is called a "reorganization" ("reorg" for short) and invalidates any transactions included in the invalidated blocks, thus returning any transferred funds to their original owners.
Such events can represent a security risk, because an attacker could use a cryptocurrency transfer as payment for a good or service, and subsequently trigger a reorg, thus invalidating his transfer (and getting his cryptocurrency back) while keeping the good or service he paid for. Cryptocurrency exchanges have lost hundreds of millions of dollars in such attacks (often referred to as "51% attacks" after a particular methodology).
To protect yourself against reorg attacks, you can wait for a number of blocks before recognizing a transfer. As more blocks are processed after a given block, the difficulty with which that block may be invalidated increases. Therefore, by choosing how many blocks to wait before recognizing a transfer, a receiver is able to balance speed (waiting very few blocks in order to process the transfer quickly while taking on risk of losing money due to a reorg) and security (waiting more blocks to minimize the risk of a reorg at the cost of slower transfer processing).
The number of blocks you wait before considering a transfer valid is called the "confirmation number", which is typically different for different chains. Circle's APIs use the following confirmation numbers for each supported chain.
|Algorand (USDC)||**1**||~5 seconds|
|Avalanche (USDC, EUROC)||**1**||~2 seconds|
|Bitcoin (BTC)||**4**||~40 minutes|
|Ethereum (USDC, EUROC, ETH)||**12**||~3 minutes|
|Flow (USDC)||**1**||~2.5 seconds|
|Hedera (USDC)||N/A||~3 Seconds|
|Polygon PoS ([Bridged USDC](🔗))||**372**||~20 minutes|
|Solana (USDC)||**1**||~400 milliseconds|
|Stellar (USDC)||**1**||~5 seconds|
|TRON (USDC)||**19**||~1 minute|
Hedera is built on a hashgraph rather than a blockchain. For this reason, there isn’t a count of confirmations/blocks before Circle determines the transfer to be valid. Instead, this determination is performed on Hedera directly and is then shared back to Circle. Please see the following link to learn more about [Hedera consensus](🔗).
Polygon USDC is NOT a native version of USDC running on Polygon PoS. [Polygon USDC](🔗) is a bridged version of USDC, which is created when the native version of USDC on Ethereum is bridged to the Polygon PoS network via the Polygon PoS Bridge.
Polygon USDC is not issued or maintained by Circle, and is not considered an official version of USDC. Additionally, the Polygon PoS Bridge is not operated by Circle. For more information, please see our [Bridged USDC Terms](🔗).
Circle’s platform requires 372 confirmations for transactions on Polygon before marking them as complete, which takes ~20 minutes. 372 represents 3X the number of confirmations needed to ensure transaction finality beyond a reasonable doubt, based on the largest reorg seen to date.
# Confirmations and Transfer Status
As soon as an incoming transfer is included in a block, the API will make it available for you (whether you poll the [get transfers endpoint](🔗) or subscribe to notifications). However, the transfer will have a `
running` status and it will not credit the balance of the associated hosted wallet with the transfer amount until the required number of confirmations has been reached.
At that point the transfer status will change to `
completed`. If you have subscribed to notifications, you will receive a message indicating such status change.
You might want to deem an incoming transfer completed before such a status change and fulfill your business process that depends on that transfer. But note that you will be doing so at your own risk, as you might never actually receive the funds in case there is a reorg before the number of confirmations has been reached.
Please note that waiting for confirmations before crediting a wallet only applies for on-chain transfers, where the `
source` is of type `
blockchain`. Transfers between hosted wallets, where the `
source` is of type `
wallet` do not require waiting on confirmations and are credited instantly.