Some non-standard implementations of error stack properties have idiosyncratic safety problems that need idiosyncratic solutions. Thus, the ses-shim can only repair the safety problems that fit into the categories it knows about.
Firefox/SpiderMonkey, Moddable/XS, and the Error Stack proposal all agree on the safest behavior, to have an Error.prototype.stack accessor property that is inherited by error instances, enabling an initial-load library like the ses-shim to virtualize this behavior across all errors.
Safari/JSC and v8 up through Node 20 both have stack appear as an own data property on error instances. This was safe enough for integrity purposes. In addition, v8 has a magic error-stack initialization API that enabled us to hide the stack for confidentiality and determinism purposes.
Starting with the v8 of Node 21, v8 makes a per-instance stack own accessor property. Fortunately, for all errors in the same realm, all their stack own properties use the same getter, and they all use the same setter. This enables the ses-shim to repair some of their safety problems.
Before doing the v8 repair described above, the ses-shim first does a sanity check that we're on a platform whose stack own property misbehaves in precisely this way. If we see that error instances have own stack accessor properties that fail this sanity check, then we have encountered another idiosyncratic case that do not yet know about, and thus do not yet know how to secure. In that case, ses-shim initialization should fail with this SES_UNEXPECTED_ERROR_OWN_STACK_ACCESSOR diagnostic.
If you see this diagnostic, PLEASE let us know, and let us know what platform (JavaScript engine and version) you saw this on. Thanks!
Root cause at https://chromium-review.googlesource.com/c/v8/v8/+/4459251
First reported at tc39/proposal-error-stacks#26 (comment)
v8 issue at https://issues.chromium.org/issues/40279506
Endo issue at #2198
Failed attempt at mitigation Agoric/endojs-vespera#1
Endo workaround at #2232
v8 discussion of a proper fix https://chromium-review.googlesource.com/c/v8/v8/+/4459251/10..15/src/execution/messages.cc#b1131