We've found a problem in the restricted python compiler on Python 2.1 which can significantly undercalculate the stack size of a dynamically generated python code object (such as those used for DTML and Python scripts in Zope). What that means is that when such an object overflows its stack, it begins clobbering things in memory. As the amount of entropy in the system increases, it heads for a crash. We've got a patch into the Zope 2.4 branch and 2.5 branch which corrects a large number of misestimations (although we believe there are a few cases yet unhandled) and has resulted in a "fix" for our test case reproducing the problem. You can download an "on-the-fly" tarball from our cvs.zope.org server, the URL for this is http://cvs.zope.org/Zope/Zope.tar.gz?tarball=1&only_with_tag=Zope-2_4-branch We intend on integrating a few further refinements early next week and putting out an official release at that time; however, applying the new code earlier may significantly reduce your exposure to the bug. Also, we believe that anyone using Python 2.1 with garbage collection turned on should recompile Python to turn off garbage collection, using the flags --without-pymalloc and --without-cycle-gc to avoid additional exposures caused by these options. We are evaluating a modification at this time to z2.py which would also disable garbage collection if it is enabled; users who cannot rebuild Python could modify z2.py to include the additonal lines import gc gc.disable() to disable garbage collection at runtime.