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 nvcheckerSource 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 2.22 soon.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 3.1 base score

7.5High

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

Patch: Available – fixed in 2.22

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 nvchecker to 2.22.

Affected software

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

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

Description

## Summary Tornado parses `application/x-www-form-urlencoded` bodies with `urllib.parse.parse_qs` and does not pass `max_num_fields`. A body made almost entirely of separators produces tens of millions of fields, and the parse happens on the event loop before the handler runs, so a single request stalls the whole server. ## Where it is `tornado/escape.py`, at HEAD `e530031405e2154654dedc4c84d5656b557ea310`: ```python result = urllib.parse.parse_qs( qs, keep_blank_values, strict_parsing, encoding="latin1", errors="strict" ) ``` `max_num_fields` is the parameter CPython added for exactly this, and it is absent. The path to it is entirely server-side and pre-dispatch. `RequestHandler._execute` parses the body at `tornado/web.py:1821`, which reaches `HTTPServerRequest._parse_body` at `tornado/httputil.py:636`, and the urlencoded branch of `parse_body_arguments` calls `parse_qs_bytes` at `tornado/httputil.py:1030`. The size that reaches it is bounded only by the body cap, which defaults to the stream's `max_buffer_size` of 104857600 at `tornado/iostream.py:239`, applied as the request body default at `tornado/http1connection.py:136-140`. A 100 MB body of separators is around fifty million fields. ## Impact Denial of service against the whole process, not one request. Tornado is single-threaded and the parse is synchronous on the event loop, so every other connection waits. No authentication is needed if any route accepts a form post, which is the normal case. ## Suggested fix Pass a bound: ```python result = urllib.parse.parse_qs( qs, keep_blank_values, strict_parsing, encoding="latin1", errors="strict", max_num_fields=max_num_fields, ) ``` with a conservative default and a way for applications to raise it. CPython raises `ValueError` when the limit is exceeded, which maps cleanly onto a 400. Lowering the default body cap for urlencoded specifically would help too, since 100 MB of form fields is not a shape any real client sends. ## Why I do not think this is a duplicate The published tornado advisories cover out-of-bounds access in the C extension, unbounded accumulation of decompressed chunks in `AsyncHTTPClient`, the Authorization header surviving cross-origin redirects, credential leakage on curl handle reuse, and cookie attribute validation. The decompression one is the nearest in spirit and is on the client side; this is the server parsing a request body. The call is unchanged at HEAD.

References

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