Vulnerability-Lookup

CVE-2026-115778 (GCVE-0-2026-115778)

Vulnerability from cvelistv5 – Published: 2026-10-23 20:20 – Updated: 2026-10-23 20:55

Title

Twisted: Unauthenticated Remote Denial of Service in the Twisted conch Telnet Transport via an Unbounded Subnegotiation Buffer

Summary

Twisted is an event-based framework for internet applications, supporting Python 3.6+. Prior to 26.10.0rc1, twisted.conch.telnet.Telnet.dataReceived appends every byte received after IAC SB to a persistent subnegotiation buffer until IAC SE is received, without applying a maximum length. Telnet option negotiation occurs before authentication, so an unauthenticated peer can omit the terminator and continuously grow the buffer. The resulting memory exhaustion can terminate the process and every service it hosts. This issue is fixed in version 26.10.0rc1.

Severity

7.5 (High)


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

SSVC

Exploitation: none Automatable: yes Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-10-23 20:52 UTC

CWE

CWE-400 - Uncontrolled Resource Consumption
CWE-770 - Allocation of Resources Without Limits or Throttling

Assigner

GitHub_M CNA under the mitre root

CNA scorecard C 72/100 over 7761 records in the last 180 days details

References

5 references

URL	Tags
https://github.com/twisted/twisted/security/advis…	x_refsource_CONFIRM
https://github.com/twisted/twisted/pull/12750	x_refsource_MISC
https://github.com/twisted/twisted/commit/073ba87…	x_refsource_MISC
https://github.com/twisted/twisted/commit/5d91cc6…	x_refsource_MISC
https://github.com/twisted/twisted/releases/tag/t…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
twisted	twisted	Affected: < 26.10.0rc1	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-115778

Vulnerability from fkie_nvd - Published: 2026-10-23 21:17 - Updated: 2026-10-23 21:17

Severity

7.5 (High) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/twisted/twisted/commit/073ba878c6c07d569cb61abc5ae76d46eff1b529
	security-advisories@github.com	https://github.com/twisted/twisted/commit/5d91cc66a3591faba758a80a1479e0103602facc
	security-advisories@github.com	https://github.com/twisted/twisted/pull/12750
	security-advisories@github.com	https://github.com/twisted/twisted/releases/tag/twisted-26.10.0rc1
	security-advisories@github.com	https://github.com/twisted/twisted/security/advisories/GHSA-cwm5-jh59-mwxr

Impacted products

	Vendor	Product	Version

JSON

To clipboard

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GHSA-CWM5-JH59-MWXR

Vulnerability from github – Published: 2026-10-23 20:45 – Updated: 2026-10-23 20:45

Summary

Twisted: Unauthenticated Remote Denial of Service in the Twisted conch Telnet Transport via an Unbounded Subnegotiation Buffer

Details

Summary

An allocation-without-limits vulnerability (CWE-770) in the Twisted conch telnet transport lets any unauthenticated remote peer exhaust server memory over a single connection. When the peer opens a telnet subnegotiation with IAC SB and then streams bytes without ever sending the IAC SE terminator, every byte is appended to a per-connection buffer that has no maximum length. The buffer persists across reads and is released only on the terminator the attacker withholds, so it grows without bound until the process is killed.

Details

The option-negotiation state machine in Telnet.dataReceived enters the subnegotiation state on IAC SB and sets an empty command list. In that state every non-IAC byte is appended to the list with no length check:

# src/twisted/conch/telnet.py  (Telnet.dataReceived)
elif self.state == "subnegotiation":
    if b == IAC:
        self.state = "subnegotiation-escaped"
    else:
        self.commands.append(b)      # unbounded: no len(self.commands) check

self.commands is an instance attribute that persists across dataReceived calls and is deleted only when IAC SE completes the subnegotiation. Negotiation runs at the transport layer before authentication, and the bytes are consumed inside dataReceived before any application callback, so an application cannot impose a bound. Twisted caps other parser buffers, for example NetstringReceiver.MAX_LENGTH, but not this one.

Affected product: Twisted, twisted.conch.telnet transport Affected versions: confirmed against 26.4.0

PoC

On any endpoint that exposes the Twisted telnet transport, such as the Twisted manhole telnet service, open one connection and send the two bytes IAC SB (0xFF 0xFA), then stream any non-0xFF bytes continuously and never send the IAC SE terminator (0xFF 0xF0). No credentials are required, because option negotiation runs before authentication.

Every byte is appended to the per-connection subnegotiation buffer with no maximum, and the buffer is released only by the terminator the attacker withholds, so it grows for the life of the connection. Because each byte is retained as a separate element the memory consumed is several times the traffic sent, so a modest-bandwidth peer, or several such connections, drives the process to memory exhaustion.

Impact

This is an unauthenticated remote denial of service (CWE-770, CWE-400) by memory exhaustion. A single connection grows an in-memory buffer without bound at a cost proportional to the bytes sent, amplified by per-byte object overhead, until the operating system kills the server and every service it hosts. There is no confidentiality or integrity impact and no code execution. The affected surface is any deployment that exposes a Twisted telnet or manhole transport to untrusted peers.

Severity

7.5 (High)


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

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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 26.4.0"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "Twisted"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "26.10.0rc1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-115778"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-400",
      "CWE-770"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-10-23T20:45:26Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "## Summary\n\nAn allocation-without-limits vulnerability (CWE-770) in the Twisted conch telnet transport lets any unauthenticated remote peer exhaust server memory over a single connection. When the peer opens a telnet subnegotiation with `IAC SB` and then streams bytes without ever sending the `IAC SE` terminator, every byte is appended to a per-connection buffer that has no maximum length. The buffer persists across reads and is released only on the terminator the attacker withholds, so it grows without bound until the process is killed.\n\n## Details\n\nThe option-negotiation state machine in `Telnet.dataReceived` enters the `subnegotiation` state on `IAC SB` and sets an empty command list. In that state every non-`IAC` byte is appended to the list with no length check:\n\n```python\n# src/twisted/conch/telnet.py  (Telnet.dataReceived)\nelif self.state == \"subnegotiation\":\n    if b == IAC:\n        self.state = \"subnegotiation-escaped\"\n    else:\n        self.commands.append(b)      # unbounded: no len(self.commands) check\n```\n\n`self.commands` is an instance attribute that persists across `dataReceived` calls and is deleted only when `IAC SE` completes the subnegotiation. Negotiation runs at the transport layer before authentication, and the bytes are consumed inside `dataReceived` before any application callback, so an application cannot impose a bound. Twisted caps other parser buffers, for example `NetstringReceiver.MAX_LENGTH`, but not this one.\n\nAffected product: Twisted, `twisted.conch.telnet` transport\nAffected versions: confirmed against 26.4.0\n\n## PoC\n\nOn any endpoint that exposes the Twisted telnet transport, such as the Twisted manhole telnet service, open one connection and send the two bytes `IAC SB` (`0xFF 0xFA`), then stream any non-`0xFF` bytes continuously and never send the `IAC SE` terminator (`0xFF 0xF0`). No credentials are required, because option negotiation runs before authentication.\n\nEvery byte is appended to the per-connection subnegotiation buffer with no maximum, and the buffer is released only by the terminator the attacker withholds, so it grows for the life of the connection. Because each byte is retained as a separate element the memory consumed is several times the traffic sent, so a modest-bandwidth peer, or several such connections, drives the process to memory exhaustion.\n\n## Impact\n\nThis is an unauthenticated remote denial of service (CWE-770, CWE-400) by memory exhaustion. A single connection grows an in-memory buffer without bound at a cost proportional to the bytes sent, amplified by per-byte object overhead, until the operating system kills the server and every service it hosts. There is no confidentiality or integrity impact and no code execution. The affected surface is any deployment that exposes a Twisted telnet or manhole transport to untrusted peers.",
  "id": "GHSA-cwm5-jh59-mwxr",
  "modified": "2026-10-23T20:45:26Z",
  "published": "2026-10-23T20:45:26Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/twisted/twisted/security/advisories/GHSA-cwm5-jh59-mwxr"
    },
    {
      "type": "WEB",
      "url": "https://github.com/twisted/twisted/pull/12750"
    },
    {
      "type": "WEB",
      "url": "https://github.com/twisted/twisted/commit/073ba878c6c07d569cb61abc5ae76d46eff1b529"
    },
    {
      "type": "WEB",
      "url": "https://github.com/twisted/twisted/commit/5d91cc66a3591faba758a80a1479e0103602facc"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/twisted/twisted"
    },
    {
      "type": "WEB",
      "url": "https://github.com/twisted/twisted/releases/tag/twisted-26.10.0rc1"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Twisted: Unauthenticated Remote Denial of Service in the Twisted conch Telnet Transport via an Unbounded Subnegotiation Buffer"
}

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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
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CVE-2026-115778 (GCVE-0-2026-115778)

FKIE_CVE-2026-115778

GHSA-CWM5-JH59-MWXR

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