Twisted: Denial of Service in the Twisted IMAP4 Server via a Negative Literal Size in SEARCH

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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.

Source published

23 Oct 2026 20:14:07 UTC

Source modified

23 Oct 2026 20:14:07 UTC

Observed here

23 Oct 2026 21:48:20 UTC

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CVSS 3.1 · 4.9 CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H