MediumCVE-2026-115927

Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks (CVE-2026-115927)

Published 26.10.2026Last modified 27.10.2026Sources: OSV, Hersteller, CVE.org

What is affected, what to do

Update Tornado to version 6.6.0.

Affected: Tornado lt *..6.6.0
Fixed in: Tornado 6.6.0

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Metrics

5.3

Priority

XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

Source: Vendor (CNA), CVSS v3

5.3

CVSS v3

Severity Medium on the scale from 0 to 10.

Source: OSV, Hersteller, CVE.org

0.4 %

EPSS score

Low — model estimates < 1% exploitation likelihood.

Source: FIRST EPSS, percentile 32.7

Show all metrics

Severity: Medium
Priority (0–325) The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first? Read in glossary: 65.04 Methodology
Exploit maturity An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack. Read in glossary: no public PoC known
XIEM Effective CVSS XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command. Read in glossary: 5.3 Methodology
CVSS v3 CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions. Read in glossary: 5.3
EPSS score The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“. Read in glossary: 0.4 %; Low — model estimates < 1% exploitation likelihood.
EPSS percentile The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking. Read in glossary: 32.7; Elevated — this CVE ranks above at least 10% of all CVEs scored today.
KEV status The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching. Read in glossary: —
Published: 26.10.2026
CISA assessment (SSVC): Exploitation: none known · automatable · Technical impact: partial; Source: CISA Vulnrichment, as of 27 October 2026
CWE CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect. Read in glossary: CWE-400, CWE-770

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2 NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management. Read in glossary: Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
CRA The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates. Read in glossary: From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001 ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security. Read in glossary: Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

Summary

When Tornado reads an HTTP/1.1 body that uses chunked transfer encoding, it processes all chunks that have already arrived without yielding to the event loop. A client can send a body made of millions of 1-byte chunks, which blocks the event loop (and therefore every other connection served by that process) for the entire time it takes to parse the body. A body near the default max_body_size of 100 MB can block the process for close to a minute, and the attacker can repeat or parallelize these requests.

Bodies made of tiny chunks also consumed far more memory than their size: a handler that does not use @stream_request_body stored each chunk as a separate object, using roughly 20 times as much memory as the bytes received.

Impact

All Tornado HTTP servers are affected when they receive a request body for a handler that reads it, which includes any handler that does not reject the request before the body is read. No authentication is required. Lowering max_body_size reduces the maximum blocking time and memory use for each request but does not remove the issue.

SimpleAsyncHTTPClient is affected in the same way when it reads a response from a malicious server.

Patches

Fixed in Tornado 6.6. Tornado now yields to the event loop periodically while reading any body, combines small chunks before passing them to data_received, and stores non-streaming bodies more compactly.

Applications using @stream_request_body should note that the pieces passed to data_received no longer correspond to HTTP chunk boundaries.

Workarounds

Run Tornado behind a reverse proxy that buffers request bodies and forwards them with a Content-Length header instead of chunked encoding.
Lower max_body_size (for the whole server or for individual handlers) to limit the impact of each request.

Credit

Reported by uttej-badwane.

Source: OSV (Open Source Vulnerabilities), GitHub Security Advisories, cvelistv5, NVD (NIST)

Affected products

The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

Affected products
		Affected versions	Fixed in
tornadoweb	tornado	`< 6.6.0`	`6.6.0`
pip	tornado	`< 6.6`	`6.6`
pypi	tornado	`5.0` `6.4b1` `5.0a1` `3.0.2` `6.5.6` `6.0a1` `4.1` `1.1.1` `6.6a1` `4.0.2` `1.0` `6.5.4` `5.0.1` `6.4.1` `2.1` `6.5.9` `4.1b2` `2.1.1` `3.0` `4.4b1` `4.2.1` `3.0.1` `6.2b2` `5.1.1` `4.5b1` `6.3b1` `4.4` `3.2.2` `0.2` `6.1` `4.5b2` `2.3` `6.2` `4.4.2` `6.3` `4.0` `6.1b1` `4.5.3` `6.5b1` `2.4.1` `6.0.1` `1.2` `4.0.1` `6.2b1` `4.2b1` `5.0.2`	— — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — —

Vendors and products

Tornadoweb Tornado

Weakness classes (CWE)

CWE-400Uncontrolled Resource Consumption
Class
The product does not properly control the allocation and maintenance of a limited resource.
Entry at MITRE CWE-400
CWE-770Allocation of Resources Without Limits or Throttling
Base
The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.
Entry at MITRE CWE-770

References & sources

github.comtornadoweb/tornado/commit/ceab33581489751a66fba8566ad6d5960d4b0…x_refsource_MISC, web
github.comtornadoweb/tornado/pull/3773x_refsource_MISC, web
github.comtornadoweb/tornado/releases/tag/v6.6.0x_refsource_MISC, web
github.comtornadoweb/tornado/security/advisories/GHSA-r4j5-9xpp-3985x_refsource_CONFIRM, web
NVD (NIST)vuln/detail/CVE-2026-115927advisory
github.comtornadoweb/tornadopackage

Changes to this CVE

Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

27.10.2026Record modifiedCISA ADP

CISA assessment (SSVC): exploitation: none known, automatable: yes

Show the original change

Individual changes of this event as published by NVD
Type	Action	Before	After
SSVC assessment	added	—	{"id":"CVE-2026-115927","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-10-27T13:45:56.157686Z"}

Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

26.10.2026ReceivedGitHub

CVSS V3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Weakness type: CWE-400, CWE-770

Description added or changed

New references:

Show 9 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Description	added	—	Tornado is a Python web framework and asynchronous networking library. Prior to 6.6.0, Tornado decodes HTTP/1.1 chunked transfer encoding by processing body data that has already arrived without yielding to the event loop. A remote unauthenticated peer can send millions of one-byte chunks to monopolize the process event loop and stall every other connection served by that process, while lowering max_body_size only limits the duration of each request. For handlers that do not use @stream_request_body, retaining each chunk as a separate object can consume roughly twenty times the received body size, and SimpleAsyncHTTPClient has the same event-loop behavior when reading a response from a malicious server. This issue is fixed in version 6.6.0.
CVSS	added	—	AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Weakness type (CWE)	added	—	CWE-400
Weakness type (CWE)	added	—	CWE-770
Affected versions (publisher)	added	—	New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/115xxx/CVE-2026-115927.json">CVE-2026-115927</a>
Reference	added	—	https://github.com/tornadoweb/tornado/commit/ceab33581489751a66fba8566ad6d5960d4b01c8
Reference	added	—	https://github.com/tornadoweb/tornado/pull/3773
Reference	added	—	https://github.com/tornadoweb/tornado/releases/tag/v6.6.0
Reference	added	—	https://github.com/tornadoweb/tornado/security/advisories/GHSA-r4j5-9xpp-3985

Source identifier: security-advisories@github.com

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