VulnerabilityPublished 29 Oct 2026

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

Affects nvcheckerSource GCVE Database

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Authoritative record:CVE-2026-115927· Medium 5.3

Not known to be exploited · CVSS 5.3 Medium : upgrade to 2.22_3 when convenient.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 3.1 base score

5.3Medium

How severe the flaw is on the 0–10 CVSS scale.

Thin evidence

One scorer or none

CVSS scores by source
CNA (GitHub_M)	CVSS 3.1	5.3

Patch: Available – fixed in 2.22_3

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: Low

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 26 Oct 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade nvchecker to 2.22_3.

Affected software

Product	Affected	Fixed in
Homebrewnvchecker`pkg:brew/nvchecker`Other threats	Affected >=2.7 <2.22_3	Fixed in 2.22_3

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

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](https://github.com/uttej-badwane).

References

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