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Besides Crypto.SE Q&As like Understanding the length extension attack, the SkullSecurity.org website offers an explanation in their article “Everything you need to know about hash length extension attacks”. Here's an excerpt:

This again lifts the length extension attack problem described above, since a HMAC construction (which securely hashes data with a secret key) prevents length extension attacks. For details on how a HMAC construction achieves that, you can check the Crypto.SE Q&A How does the secret key in an HMAC prevent modification of the HMAC?.

Besides Crypto.SE Q&As like “Understanding the length extension attack”, the SkullSecurity.org website offers an explanation in their article “Everything you need to know about hash length extension attacks”. Here's an excerpt:

This again lifts the length extension attack problem described above, since a HMAC construction (which securely hashes data with a secret key) prevents length extension attacks. For details on how a HMAC construction achieves that, you can check the Crypto.SE Q&A “How does the secret key in an HMAC prevent modification of the HMAC?” which explains that HMACs were designed to void length extension attack issues.

When trying to attack a HMAC,

…the attacker only controls variable-length input to the inner hash, not the outer hash…

The SkullSecurity.org website offers an explanation in their article “Everything you need to know about hash length extension attacks”. Here's an excerpt:

This again lifts the length extension attack problem described above, since a HMAC construction (which securely hashes data with a secret key) prevents length extension attacks. Therefore, the answer which practically considers your question title as well as your description is extended to:

Besides Crypto.SE Q&As like Understanding the length extension attack, the SkullSecurity.org website offers an explanation in their article “Everything you need to know about hash length extension attacks”. Here's an excerpt:

This again lifts the length extension attack problem described above, since a HMAC construction (which securely hashes data with a secret key) prevents length extension attacks. For details on how a HMAC construction achieves that, you can check the Crypto.SE Q&A How does the secret key in an HMAC prevent modification of the HMAC?.

So, the answer which practically considers your question title as well as your description is extended to:

This again lifts the length extension attack problem described above, since the HMAC construction prevents length extension attacks. Therefore, the answer which practically considers your question title as well as your description is extended to:

This again lifts the length extension attack problem described above, since a HMAC construction (which securely hashes data with a secret key) prevents length extension attacks. Therefore, the answer which practically considers your question title as well as your description is extended to: