VulnerabilityPublished 29 Oct 2026

CVE-2026-116577: Tornado · Quadratic CPU usage when parsing HTTP headers with line folding

Affects jupyterlabSource GCVE Database

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Authoritative record:CVE-2026-116577· 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 version 4.6.4_3, which removes support for obsolete line folding and rejects header lines beginning with whitespace. Users should upgrade to this version to fully remediate the vulnerability. As a workaround, keeping max_header_size at the default value or lowering it can limit the impact but does not eliminate the vulnerability. Running Tornado behind a reverse proxy that rejects or normalizes obsolete line folding before forwarding requests can also mitigate the issue.

Description

Tornado HTTP servers have a vulnerability in their header parser that causes quadratic CPU usage when processing HTTP headers with obsolete line folding (line continuation). This can lead to significant event loop blocking and degraded service availability. The issue affects all Tornado HTTP servers before version 4.6.4_3. The vulnerability is fixed in Tornado 4.6.4_3 by removing support for obsolete line folding and rejecting header lines beginning with whitespace. The impact is limited to availability; confidentiality and integrity are not affected.

Impact

The vulnerability affects availability by causing the Tornado server's event loop to be blocked during header parsing, potentially leading to denial of service. No confidentiality or integrity impact is reported. The blocking duration increases quadratically with the size of the header block containing line folding. Default max_header_size (64 KB) causes tens of milliseconds of blocking per malicious request, while larger sizes (e.g., 1 MB) can block the process for several seconds.

Summary

CVE-2026-116577 describes a performance vulnerability in Tornado's HTTP header parser where parsing headers with many obsolete line folding continuations results in O(n²) CPU usage. An unauthenticated attacker can exploit this by sending requests with many short continuation lines, causing the Tornado IOLoop thread to be blocked, which in turn blocks all other connections handled by the same process. The severity of the impact depends on the configured max_header_size, with larger sizes causing longer blocking times. The vulnerability is fixed in Tornado 4.6.4_3 by completely removing support for obsolete line folding and rejecting headers starting with whitespace.

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Description

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

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