VulnerabilityPublished 3 Sep 2026Updated 29 Oct 2026

CVE-2026-82397: Tornado · Urlencoded body parsing omits max_num_fields, so one request can stall the event loop

Affects livereloadSource GCVE Database

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Authoritative record:CVE-2026-82397· High 7.5

Not known to be exploited · CVSS 7.5 High : upgrade to 2.7.1_3 soon.

Data as of 30 Oct 2026, 06:59 UTC

CVSS 3.1 base score

7.5High

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

EPSS: 0.3% chance of exploitation in 30 days – higher than 28% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.7.1_3

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

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

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: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 31 Aug 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 livereload to 2.7.1_3.

Affected software

Product	Affected	Fixed in
Homebrewlivereload`pkg:brew/livereload`Other threats	Affected >=2.7.1 <2.7.1_3	Fixed in 2.7.1_3

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

AI-suggested

A patch is available in Tornado version 2.7.1_3 that adds a max_num_fields parameter to the URL-encoded body parser, limiting the number of fields parsed and preventing event loop stalling. Users should upgrade to this fixed version. Additionally, lowering the maximum allowed request body size for URL-encoded forms can help mitigate the risk. No other actions are required once patched.

Description

Tornado's URL-encoded body parser does not limit the number of fields parsed, allowing a specially crafted request with many separators to produce tens of millions of fields. This parsing occurs synchronously on the single-threaded event loop before request handling, causing the entire server to stall and resulting in a denial of service. The issue affects Tornado versions from 2.7.1 up to but not including 2.7.1_3, where a fix has been applied.

Impact

This vulnerability allows an unauthenticated attacker to cause a denial of service against the entire Tornado server process by sending a single large URL-encoded request body with excessive separators. The server stalls because the parsing blocks the event loop, preventing it from handling other requests. There is no impact on confidentiality or integrity, but availability is severely affected.

Summary

Tornado uses Python's urllib.parse.parse_qs to parse application/x-www-form-urlencoded request bodies but omits the max_num_fields parameter, which limits the number of fields parsed. An attacker can send a large request body composed mostly of separators, causing parse_qs to generate tens of millions of fields. Since Tornado is single-threaded and the parsing happens synchronously on the event loop before the request handler executes, this stalls the entire server process, denying service to all clients. The default maximum request body size is 100 MB, which can contain around fifty million fields of separators. The vulnerability is fixed in Tornado version 2.7.1_3 by adding a max_num_fields limit to the parsing call.

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Affected software

Description

AI analysisMachine-generated, updated 30 Oct 2026, 06:44 UTC

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