AutoProxy_Cleanup frees already freed memory resulting in Excel crash

[japj]

WebHelpers AutoProxy_Cleanup will try to free any used memory without setting pointers to 0 after the free, see code at https://github.com/VBA-tools/VBA-Web/blob/master/src/WebHelpers.bas#L2785

This has been observed when having a proxy pac file that does not exist and the error handling code is triggered.

As a result the AutoProxy_Cleanup will raise an Error, which results in AutoProxy_Cleanup being called again. This in result will try to free memory again causing a crash in Excel

[timhall]

Thanks for bringing this up! (I'm wondering if that is what was happening here: #102)

I think I understand: By not zeroing them out after free, if an error is thrown it will re-run cleanup, see them as not freed (since they are non-zero), and will try to free them again (causing crash). I'll fix this shortly.

Do you know what caused the error in cleanup before the subsequent re-cleanup and crash?

[japj]

Yes, I believe we also had 10022 "Unable to download proxy auto-config script", reading #102 I think that indeed is the same problem.

Due to https://github.com/VBA-tools/VBA-Web/blob/master/src/WebHelpers.bas#L2805 an Error is thrown in case of an Error occured. However, the first time you execute AutoProxy_Cleanup might be due to 'normal' flow, while the second is due to the Err.Raise (which triggers calling AutoProxy_Cleanup again).

In this scenario, if you have a machine with a pac setup and the machine can not access the pac file (either because it is offline like in #102 or it is on a different network without access to the pac host) this problem will occur (Excel crashing).

I think you should be able to manually reproduce this behaviour by setting a dummy pac configuration in Internet Explorer, i.e. Internet Options, Connections, LAN settings, Use automatic configuration script with a non existing url and then try to autodetect proxy settings from Excel.

That would allow you to manually reproduce the problem and verify the correctness of a fix.