I hope I've 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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Can you send me an email, so I can send you back a development version of WinSCP for testing? Please include a link back to this topic in your email. Also note in this topic that you have sent the email. Thanks.

Thanks for your report.
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/

Hello,
I would like to report a possible memory management issue in WinSCP 6.7 ((Build 17845 2026-10-14) on Windows 11 Pro 25H2 64-bit when uploading large files (about 15 GB/file x 70 files = 1,050 GB) to Amazon S3.
After upgrading from WinSCP 6.5.7 to 6.7, I started experiencing several memory-related errors during S3 operations. These problems did not occur with version 6.5.7.

1. SSL handshake failure during file uploads
   When uploading a large file to Amazon S3, the transfer initially proceeds but eventually fails with the following error:
   

   SSL handshake failed: SSL error: malloc failure
   

   
   
2. Out of memory during file list retrieval
   An Out of memory error also occurs while WinSCP is retrieving or preparing the list of files to transfer, even when uploading only a single large file.
   When using WinSCP 6.5.7, the upload completed successfully without any of the errors described above.
   

This suggests that the issue may have been introduced in WinSCP 6.7.
Is this a known issue in WinSCP 6.7?
Have there been any changes in memory management or SSL/TLS handling that could explain these symptoms?
Thank you for your assistance.