Thank you, the dev version works well!
A very quick fix. :-)
I am deleting millions of files, so this was quite a "big" leak.

The new version started at ~14 MB in task manager and after about half an hour it is at ~16 MB. So maybe there is another small leak, but could also be usual or wanted behavior.

Sorry, I wanted to write "Is this NEW problem in some recent version of WinSCP?".

Anyway, I hope I have identified the problem:
Issue 2476 – Memory leak in encrypted HTTP/S3/WebDAV connections
Though it involves rather a lot of small requests to the server, rather than large transfers (a small number of large requests). Still I hope it might be the same problem.

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I'm sending you an email with a development version of WinSCP to the address you have used to register on this forum.

Hello Martin!

Yes, as written above: Using WinSCP 6.7 (Build 17845 2026-10-14) on Windows 10 22H2 64-bit.

Thanks for your report. Is this problem in some recent version of WinSCP?
If it is, can you please try to identify what exact version did introduce the problem?
You can download all versions of WinSCP from:
https://sourceforge.net/projects/winscp/files/WinSCP/

When deleting a large number of foldes/subfolders/files in an S3 storage, the application runs out of memory:

• WinSCP shows an error message box (Cancel/Try again/Skip/...): "SSL handshake failed: SSL error: malloc failure"
  
• Windows Task Manager shows ~3.9 GB allocated for WinSCP.exe, slightly below the 4 GB limit.
  Cancelling the task results in expectable subsequent errors ("Invalid access to memory location.") but not in memory being released, according to Task Manager.
  

Using WinSCP 6.7 (Build 17845 2026-10-14) on Windows 10 22H2 64-bit.