Azure Data Factory SFTP Service error: SftpAuthenticationFailure

Question

Azure Data Factory SFTP Service error: SftpAuthenticationFailure

Apurva Pathak 920

I'm using ADF Linked Service for SFTP connection to get data from an on-premises server.

We're running SHIR on containers, and keys/passphrases are passed by key vault.

Meet authentication failure when connect to Sftp server '' using 'SshPublicKey' authentication type. Please make sure you are using the correct authentication type and the credential is valid. For more details, see our troubleshooting docs.
Permission denied (publickey).

However, the weird part is that, if I import the same key to the SHIR container instance, and try an SFTP connection, it works fine.

Also, it was working all fine for months, and suddenly broke without any sensible reason.

Can anyone please help me go around this issue.

Srikanth Chavithina 0 Reputation points Microsoft External Staff Moderator

2026-07-06T12:47:58.76+00:00

Hi @Apurva Pathak

One possible cause of this issue is that the SFTP server's limit on concurrent SSH connections is being exceeded during the copy operation. You can try configuring the maxConcurrentConnections property in the SFTP connector to a lower value that is within the server's allowed connection limit.

According to the Azure Data Factory documentation, maxConcurrentConnections specifies the upper limit of concurrent connections established to the data store during the activity run:
https://learn.microsoft.com/en-us/azure/data-factory/connector-sftp?tabs=data-factory

Please let us know if you have any questions.
Apurva Pathak 920 Reputation points

2026-07-06T13:00:23.82+00:00

This is failing even on Test Connections.
Srikanth Chavithina 0 Reputation points Microsoft External Staff Moderator

2026-07-06T13:05:03.66+00:00
Since the issue occurs during Test Connection, this is unlikely to be related to the maxConcurrentConnections setting, as that property only applies during Copy Activity execution.

The error indicates that the SFTP server is rejecting the SSH public key authentication (Permission denied (publickey)). Could you please help confirm the following:

Has the SSH private key or its corresponding Azure Key Vault secret been changed or rotated recently?

Are you using the same private key in the linked service that works successfully when testing manually from the SHIR container?

Have there been any recent changes to the SFTP server configuration or authentication policies?

This information will help determine whether the issue is related to the SSH key, Key Vault secret, or the SFTP server configuration.
Apurva Pathak 920 Reputation points

2026-07-06T13:08:20.7533333+00:00
Has the SSH private key or its corresponding Azure Key Vault secret been changed or rotated recently?-- No

Are you using the same private key in the linked service that works successfully when testing manually from the SHIR container? Yes

Have there been any recent changes to the SFTP server configuration or authentication policies? No
Srikanth Chavithina 0 Reputation points Microsoft External Staff Moderator

2026-07-06T13:11:24.6166667+00:00
Since there have been no recent changes to the SSH key, Azure Key Vault secret, or the SFTP server configuration, and the same private key works successfully when tested manually from the SHIR container, the issue appears to be specific to how Azure Data Factory is authenticating during the linked service Test Connection.

Could you please help confirm the following:

Which SSH key algorithm is being used for the key pair (RSA, ECDSA, or ED25519)?

If you are using an RSA key, could you try generating an ED25519 key pair, add the corresponding public key to the SFTP server's authorized_keys file, update the private key in Azure Key Vault, and test the linked service again?

Please let us know the key algorithm currently in use and the outcome of the test.
Apurva Pathak 920 Reputation points

2026-07-06T13:13:24.21+00:00

I've tried with both.

Both works manually from SHIR, but fail from ADF Test Connection.

Your answer

Srikanth Chavithina 0 Reputation points Microsoft External Staff Moderator

2026-07-06T12:47:58.76+00:00

Hi @Apurva Pathak

One possible cause of this issue is that the SFTP server's limit on concurrent SSH connections is being exceeded during the copy operation. You can try configuring the maxConcurrentConnections property in the SFTP connector to a lower value that is within the server's allowed connection limit.

According to the Azure Data Factory documentation, maxConcurrentConnections specifies the upper limit of concurrent connections established to the data store during the activity run:
https://learn.microsoft.com/en-us/azure/data-factory/connector-sftp?tabs=data-factory

Please let us know if you have any questions.
Apurva Pathak 920 Reputation points

2026-07-06T13:00:23.82+00:00

This is failing even on Test Connections.
Srikanth Chavithina 0 Reputation points Microsoft External Staff Moderator

2026-07-06T13:05:03.66+00:00

Since the issue occurs during Test Connection, this is unlikely to be related to the maxConcurrentConnections setting, as that property only applies during Copy Activity execution.

The error indicates that the SFTP server is rejecting the SSH public key authentication (Permission denied (publickey)). Could you please help confirm the following:

Has the SSH private key or its corresponding Azure Key Vault secret been changed or rotated recently?

Are you using the same private key in the linked service that works successfully when testing manually from the SHIR container?

Have there been any recent changes to the SFTP server configuration or authentication policies?

This information will help determine whether the issue is related to the SSH key, Key Vault secret, or the SFTP server configuration.
Apurva Pathak 920 Reputation points

2026-07-06T13:08:20.7533333+00:00

Has the SSH private key or its corresponding Azure Key Vault secret been changed or rotated recently?-- No

Are you using the same private key in the linked service that works successfully when testing manually from the SHIR container? Yes

Have there been any recent changes to the SFTP server configuration or authentication policies? No
Srikanth Chavithina 0 Reputation points Microsoft External Staff Moderator

2026-07-06T13:11:24.6166667+00:00

Since there have been no recent changes to the SSH key, Azure Key Vault secret, or the SFTP server configuration, and the same private key works successfully when tested manually from the SHIR container, the issue appears to be specific to how Azure Data Factory is authenticating during the linked service Test Connection.

Could you please help confirm the following:

Which SSH key algorithm is being used for the key pair (RSA, ECDSA, or ED25519)?

If you are using an RSA key, could you try generating an ED25519 key pair, add the corresponding public key to the SFTP server's authorized_keys file, update the private key in Azure Key Vault, and test the linked service again?

Please let us know the key algorithm currently in use and the outcome of the test.
Apurva Pathak 920 Reputation points

2026-07-06T13:13:24.21+00:00

I've tried with both.

Both works manually from SHIR, but fail from ADF Test Connection.