Powys Council has apologised to people whose personal data was published on the authority's web page.
Names and addresses attached to a letter on direct payments were shown on the site amid a rush to put information about coronavirus support online on April 22.
The letters were taken down the following day when the council was informed of the issue.
But, those affected have now complained about the breach, and it was brought to the attention of Coun Joy Jones (Non-Aligned – Newtown East).
Coun Jones said: “Powys have dropped the ball again and I have had to deal with some very upset residents.
“This comes at a time when there is enough stress about. They don’t need this.”
A council spokesman confirmed that a template letter containing a number of service users’ names and addresses was placed on the council’s website by mistake.
They explained it had been part of a move to ensure information about Covid-19 support was made available as widely and quickly as possible.
Dylan Owen, the head of commissioning for social services, said: “We felt it appropriate to inform you of the breach in order that you are able to be extra vigilant over unexpected callers or letters.
“We sincerely apologise for this personal data breach which has been formally investigated in line with council policy.
“It has been determined that the breach occurred due to human error when publishing template letters.
“Measures have now been put in place to reduce the risk of this occurring again.
“This includes all information leaving the department now being checked by two people before it is published.”
Direct payments allow people to arrange care or support services themselves instead of receiving a package of care or support arranged by the local authority.
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