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Response snapshot Oct 27, 2026, 06:26 UTC

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VulnerabilityCVE-2026-115774

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

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.

Published Updated

At a glance

Exploited
Not known
Severity
4.9 Medium (CVSS 3.1)
Chance of exploitation
0.4% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: Twisted: 26.10.0rc1. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-2qrf-pqgx-qh9v (opens a new tab)

What happened

## 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

Continue reading

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 runs in the reactor thread, and the connection timeout cannot fire because the reactor itself is frozen.

Affected product: Twisted (`twisted.mail`), the IMAP4 server `twisted.mail.imap4.IMAP4Server` Affected versions: 26.4.0 and earlier

## PoC

Log in, select a mailbox, and send a `SEARCH` whose literal size is negative:

```text C: t1 LOGIN testuser password-test C: t2 SELECT INBOX C: t3 SEARCH {-100}zzzzzzzzzz ```

The server never responds and one core stays at 100 percent. In a measured run a health service co-hosted on the same reactor stopped answering the instant the command arrived, showing the freeze hits the whole process and not just the attacking connection. A valid `{5}zzzzz` completes normally and a non-numeric `{abc}` is rejected with a parse error, which isolates the cause to the missing negative-size check.

## Impact

A remote denial of service. One command of about ten bytes freezes the single-threaded reactor, so it stops serving not only every other IMAP connection but every other service co-hosted in the same process (for example SMTP and POP3 under `twistd mail`), and no timeout can recover it until the process is restarted. It requires an authenticated session, so any account holder can trigger it; there is no confidentiality or integrity impact and no code execution.

Affected package ranges reported by GitHub: Twisted: <= 26.4.0

Scores and how they were measured

CVSS base severity

4.9

Version 3.1As of

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

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.4%

Scored Percentile 37.0%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-2qrf-pqgx-qh9v (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-2qrf-pqgx-qh9v, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-2qrf-pqgx-qh9v, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  4. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-2qrf-pqgx-qh9v, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  5. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-2qrf-pqgx-qh9v, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  6. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-115774, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 23). Twisted: Denial of Service in the Twisted IMAP4 Server via a Negative Literal Size in SEARCH (CVE-2026-115774). https://kevwire.com/record/CVE-2026-115774

MLA

"Twisted: Denial of Service in the Twisted IMAP4 Server via a Negative Literal Size in SEARCH (CVE-2026-115774)." KEVWire, 23 Oct. 2026, kevwire.com/record/CVE-2026-115774.

Chicago

KEVWire. "Twisted: Denial of Service in the Twisted IMAP4 Server via a Negative Literal Size in SEARCH (CVE-2026-115774)." Last modified October 23, 2026. https://kevwire.com/record/CVE-2026-115774.

Evidence summary (plain text)

Twisted: Denial of Service in the Twisted IMAP4 Server via a Negative Literal Size in SEARCH (Oct 23, 2026).
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.

Meeting an inline literal, `parseNestedParens` reads the size between the braces and advances the cursor past the body:
CVSS 4.9 (version 3.1; evidence as of Oct 23, 2026; https://github.com/advisories/GHSA-2qrf-pqgx-qh9v).
EPSS 0.4% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-115774&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-2qrf-pqgx-qh9v: https://github.com/advisories/GHSA-2qrf-pqgx-qh9v
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo1MTozMFrOAAf0Mg%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0OToxMlrOAAf0LQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NzoxN1rOAAf0Kg%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-115774&date=2026-10-26
Record ID: CVE-2026-115774.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 01:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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