Twisted is an event-based framework for internet applications, supporting Python 3.6+. Prior to 26.10.0rc1, twisted.mail.imap4.IMAP4Server reaches twisted.mail.imap4.parseNestedParens with a negative literal size in an authenticated SEARCH command after the client enters the selected mailbox state. The negative size moves the parser cursor below zero, and Python's negative indexing prevents the synchronous scanning loop from terminating. The loop freezes the single-threaded reactor, stopping other IMAP connections and any SMTP or POP3 services hosted in the same process until restart. This issue is fixed in version 26.10.0rc1.

Affected: PyPI: Twisted

## Summary

An infinite-loop vulnerability (CWE-835) in the Twisted IMAP4 server lets an authenticated client freeze the whole process with one small command. `twisted.mail.imap4.parseNestedParens` reads an IMAP literal size with `int()` and never rejects a negative value. A negative size such as `{-100}` drives the parser cursor negative, and because a negative Python index wraps around the end of the string instead of raising, the scanning loop never terminates. It runs synchronously on the single-threaded reactor, so one `SEARCH` command holds the reactor at full CPU forever and every other connection stops being served.

## Details

Meeting an inline literal, `parseNestedParens` reads the size between the braces and advances the cursor past the body:

```python # twisted/mail/imap4.py parseNestedParens literalSize = int(s[i + 1 : end]) # no sign check contentStack[-1].append((s[end + 3 : end + 3 + literalSize],)) i = end + 3 + literalSize # negative size -> i goes negative ```

`int(b"-100")` is `-100`, so `i` goes negative; the loop condition `i < len(s)` stays true and `s[i : i + 1]` on a negative index never raises, so the cursor cycles forever. The `_literalStringLimit` guard only covers the synchronizing-literal continuation path, not this inline branch. The call is reached from `select_SEARCH` through `arg_searchkeys`, dispatched only in the selected state, so a valid login and a selected mailbox (any low-privilege account) suffice. It…