Now let's see the actual vulnerable code:

def get_safe_return_url(return_url: str, fallback_url: str):
    # Use fallback if URL is none or URL is not on same domain
    if not return_url or not return_url.startswith('/'):
        return fallback_url
    return return_url

what happens here is that the return_url variable is checked using startswith('/') method, it's definitely vulnerable as the attacker doesn't need to supply HTTP OR HTTPS in front of a url to redirect a user, he will simply use two forwarded slahes //evil.com and since it starts with a slash / it passes the check and the browser still understand that //evil.com is a URI that he should navigate to ...

Remediation:

I think that using a proper regex should fix the issue, but the better solution in this case is implementing a whitelist of pre-defined values, so the attacker's can't bypass the restriction, for example look for this snippet:

def get_safe_return_url(return_url: str, fallback_url: str):
    # Use fallback if URL is none or URL is not on same domain
   allowed_values = ['/', 'about', 'home', 'anythingelse']
    if not return_url in allowed_values:
        return fallback_url
    return return_url

so here the if statement will check if the supplied return_url parameter is one of the allowed values, If it's not the fallback_url will be returned.

The first occurrence of this issue is withing remove() function defined in bookmarks.py view:

def remove(request, bookmark_id: int):
    try:
        bookmark = Bookmark.objects.get(pk=bookmark_id, owner=request.user)
    except Bookmark.DoesNotExist:
        raise Http404('Bookmark does not exist')

    bookmark.delete()
    return_url = get_safe_return_url(request.GET.get('return_url'), reverse('bookmarks:index'))
    return HttpResponseRedirect(return_url)

as we can see it checks first if the bookmarks is exist and the user is authorized to delete it, it removes it using delete() method. Now he will get the return_url GET parameter and passes it to get_safe_return_url() function which is the actual vulnerable function, after that it uses HttpReponsesRedirect() function to redirect the user.