CVE-2026-53539VulnerabilityPublished 13 Aug 2026Updated 3 Nov 2026

fastmcp · python-multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service

Affects fastmcpSource GCVE Database

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Authoritative record:CVE-2026-53539· High 7.5

Not known to be exploited · CVSS 7.5 High : upgrade to 3.4.0 soon.

Data as of 3 Nov 2026, 22:58 UTC

CVSS 3.1 base score

7.5High

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

EPSS: 0.4% chance of exploitation in 30 days – higher than 39% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 3.4.0

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 22 Jun 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 3 Nov 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 fastmcp to 3.4.0.

Affected software

Product	Affected	Fixed in
Homebrewfastmcp`pkg:brew/fastmcp`Other threats	Affected >=2.12.3 <3.4.0	Fixed in 3.4.0

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

AI-suggested

Upgrade python-multipart to version 3.4.0 or later. This version treats only '&' as a field separator per the WHATWG URL standard, using a single bounded scan that ensures linear parsing time. No other mitigation is required.

Description

A vulnerability in python-multipart's QuerystringParser causes quadratic-time parsing when processing application/x-www-form-urlencoded bodies that use semicolon separators without ampersands. This leads to excessive CPU consumption and potential denial of service by exhausting worker processes. The issue affects versions >=2.12.3 and <3.4.0 and is fixed in version 3.4.0.

Impact

The vulnerability allows an attacker to cause uncontrolled CPU consumption leading to denial of service. Parsing is synchronous, so a single crafted request can occupy a worker for several seconds, blocking other requests. Multiple concurrent requests can exhaust all worker processes, degrading or denying service availability.

Summary

The python-multipart library's QuerystringParser performs a two-step lookup for field separators when parsing application/x-www-form-urlencoded bodies: it first scans for '&' and only if none is found does it scan for ';'. For bodies using ';' as the separator without any '&', this results in a full scan for '&' on every field iteration, causing O(B^2) byte comparisons for a chunk of size B. An attacker can exploit this by submitting a crafted body with many semicolon-separated fields, causing the parser to consume excessive CPU time and block worker processes. This parser is used by FastAPI and Starlette for form parsing. The issue is fixed in python-multipart version 3.4.0, which treats only '&' as a field separator, making parsing linear in body length.

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Exploitation evidence

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Affected software

Description

AI analysisMachine-generated, updated 3 Nov 2026, 21:33 UTC

Technical details

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