VulnerabilityPublished 28 Oct 2026

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

Affects tornadoSource GCVE Database

Authoritative record:CVE-2026-116577· Medium 5.3

Not known to be exploited · CVSS 5.3 Medium : upgrade to 6.6 when convenient.

Data as of 28 Oct 2026, 23:14 UTC

CVSS 3.1 base score

5.3Medium

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

Patch: Available – fixed in 6.6

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
Weaponised: 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 26 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.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, 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 tornado to 6.6.

Affected software

Product	Affected	Fixed in
PyPItornado	Affected <6.6	Fixed in 6.6

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, which removes support for obsolete line folding and rejects headers starting with whitespace. Users should upgrade to Tornado 6.6 or later. As a workaround, keeping max_header_size at the default (64 KB) or lowering it can limit the impact but not eliminate it. Running Tornado behind a reverse proxy that rejects or normalizes obsolete line folding headers before forwarding requests can also mitigate the issue.

Description

Tornado HTTP servers have a vulnerability in their HTTP header parser that causes quadratic CPU usage when parsing headers with obsolete line folding (line continuation with leading whitespace). This can be exploited by unauthenticated attackers sending specially crafted requests with many short continuation lines, leading to significant CPU consumption and blocking the event loop. The issue affects all Tornado versions prior to 6.6. The vulnerability impacts availability only and has been fixed in Tornado 6.6 by rejecting headers with leading whitespace. Mitigations include keeping the max_header_size at default or lower and using a reverse proxy to normalize or reject such headers.

Impact

The vulnerability causes denial of service by blocking the Tornado event loop during header parsing, leading to degraded service or unavailability. No authentication or specific handler is required to trigger the issue, as header parsing occurs before application code. The impact severity scales with the max_header_size configuration, with larger header sizes causing longer blocking times. There is no impact on confidentiality or integrity.

Summary

Tornado's HTTP header parser exhibits O(n²) CPU time complexity when processing HTTP headers that use obsolete line folding (obs-fold), where header values are continued across multiple lines starting with whitespace. An unauthenticated attacker can exploit this by sending requests with many short continuation lines, causing the IOLoop thread to consume excessive CPU and block other connections. The severity of the impact depends on the max_header_size setting, with the default 64 KB causing tens of milliseconds of blocking per request, and larger sizes (e.g., 1 MB) causing multi-second blocking. This vulnerability affects all Tornado versions before 6.6. The issue was fixed in Tornado 6.6 by removing support for obsolete line folding and rejecting headers that begin with whitespace. This vulnerability only affects availability; confidentiality and integrity are not impacted.

CWE-407Inefficient Algorithmic Complexity (opens in a new tab)
Class weaknessLikelihood of exploit: Low
An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.1 (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-770Allocation of Resources Without Limits or Throttling (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #25
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.
Requirements
Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.
Architecture and Design
Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.
Architecture and Design
Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
OWASP ASVS 4.0.3
V8.1.4 (opens in a new tab)
V13.4.1 (opens in a new tab)
V11.1.4 (opens in a new tab)
Attack patterns
XML Ping of the Death (opens in a new tab)
Exponential Data Expansion (opens in a new tab)
Serialized Data Parameter Blowup (opens in a new tab)
Serialized Data with Nested Payloads (opens in a new tab)
Oversized Serialized Data Payloads (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 28 Oct 2026, 23:14 UTC

WeaknessesCWE-407, CWE-770

Technical details

References

Community reviews

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