VulnerabilityPublished 29 Oct 2026

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

Affects jupyterlabSource 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 4.6.4_3 when convenient.

Data as of 30 Oct 2026, 09:00 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 4.6.4_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 jupyterlab to 4.6.4_3.

Affected software

Product	Affected	Fixed in
Homebrewjupyterlab`pkg:brew/jupyterlab`Other threats	Affected >=1.2.0 <4.6.4_3	Fixed in 4.6.4_3

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

AI-suggested

A fix is available in Tornado 6.6. Users should upgrade to Tornado 6.6 or later to resolve this issue. As workarounds, running Tornado behind a reverse proxy that buffers request bodies and forwards them with a Content-Length header instead of chunked encoding can mitigate the issue. Additionally, lowering max_body_size for the server or individual handlers can limit the impact of each request but does not fully remove the vulnerability.

Description

Tornado HTTP servers are vulnerable to event loop blocking and excessive memory use when processing HTTP/1.1 chunked transfer encoding bodies composed of many small chunks. An attacker can send a request body with millions of 1-byte chunks, causing the server to block the event loop for extended periods and consume significantly more memory than the actual body size. This affects all Tornado HTTP servers that read request bodies without prior rejection and also impacts SimpleAsyncHTTPClient when reading responses from malicious servers. The issue is fixed in Tornado 6.6.

Impact

The vulnerability causes denial of service by blocking the Tornado event loop for extended periods, impacting all connections handled by the server process. It also leads to excessive memory consumption, approximately 20 times the size of the received data, increasing resource exhaustion risk. No authentication is required to exploit this. Lowering max_body_size reduces but does not eliminate the impact. Both Tornado HTTP servers and SimpleAsyncHTTPClient are affected.

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Affected software

Description

AI analysisMachine-generated, updated 30 Oct 2026, 09:00 UTC

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