VulnerabilityPublished 13 Aug 2026Updated 3 Nov 2026

CVE-2026-53538: python-multipart · Semicolon treated as querystring field separator enables parameter smuggling

Affects fastmcpSource GCVE Database

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Mentioned vulnerabilities

CVE-2021-23336

Authoritative record:CVE-2026-53538· Low 3.7

Not known to be exploited · CVSS 3.7 Low : upgrade to 3.4.0 when convenient.

Data as of 3 Nov 2026, 22:58 UTC

CVSS 3.1 base score

3.7Low

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

EPSS: 0.2% chance of exploitation in 30 days – higher than 17% 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:H/PR:N/UI:N/S:U/C:N/I:L/A:N

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

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: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
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: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on the impacted component.
Availability: None: There is no impact to availability within the impacted component.

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, which corrects the parsing behavior to treat only '&' as a field separator in compliance with the WHATWG URL standard. This update prevents parameter smuggling by ensuring consistent parsing between intermediaries and backend applications. No other mitigation is required as the issue is fully addressed by this official fix.

Description

The python-multipart library's QuerystringParser incorrectly treats the semicolon (';') as a field separator in application/x-www-form-urlencoded bodies, contrary to the WHATWG URL standard which only recognizes the ampersand ('&') as a separator. This discrepancy allows attackers to smuggle additional or overriding form fields past upstream components that follow the standard, potentially causing the backend to receive parameters unseen by the intermediary. The issue affects versions >=2.12.3 and <3.4.0 and is fixed in version 3.4.0.

Impact

An attacker can exploit this parser discrepancy to smuggle extra or overriding form fields past upstream components that enforce the WHATWG URL standard, causing the backend application to receive parameters that the intermediary did not observe or block. This can lead to HTTP parameter pollution, potentially altering application behavior or bypassing security controls that rely on upstream validation. The CVSS score is 3.7 (low severity) reflecting limited impact with no confidentiality or availability effects, only integrity impact with high attack complexity and no privileges required.

Summary

The vulnerability in python-multipart's QuerystringParser arises because it treats the semicolon character (';') as a field separator in URL-encoded form bodies, in addition to the ampersand ('&'). This behavior conflicts with the WHATWG URL standard, modern browsers, and Python's urllib.parse (post CVE-2021-23336 fix), which only treat '&' as a separator. The parser's fallback logic scans for '&' first and then ';' only if no '&' is found, causing tokenization to depend on the presence of unrelated characters and the chunking of input data. This leads to a parser differential where upstream components that follow the standard see fewer or different parameters than the backend application, enabling HTTP parameter pollution and form parameter smuggling. The flaw is reachable via public APIs including QuerystringParser, FormParser, create_form_parser, parse_form, and Starlette/FastAPI request.form() for URL-encoded bodies. The issue is fixed in python-multipart version 3.4.0, which aligns parsing with the WHATWG standard by treating only '&' as a separator.

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Description

AI analysisMachine-generated, updated 3 Nov 2026, 21:34 UTC

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