VulnerabilityPublished 22 Jun 2026

CVE-2026-53538: Kludex python-multipart · Interpretation Conflict

Affects Kludex python-multipartSource CVE Database V5

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Mentioned vulnerabilities

CVE-2021-23336

Not known to be exploited · CVSS 3.7 Low : upgrade to 0.0.30 when convenient.

Data as of 5 Nov 2026, 00:01 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 0.0.30

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 5 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 Kludex python-multipart to 0.0.30.

Affected software

Product	Affected	Fixed in
Kludexpython-multipart	Affected <0.0.30	Fixed in 0.0.30
PyPIpython-multipart`pkg:pypi/python-multipart`Other threats	Affected <0.0.30	Fixed in 0.0.30

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 0.0.30 or later, where this vulnerability is fixed. No other mitigation is required as the fix is available and addresses the issue directly.

Description

### Summary `QuerystringParser` treated `;` as a field separator in `application/x-www-form-urlencoded` bodies, in addition to `&`. The [WHATWG URL standard](https://url.spec.whatwg.org/#urlencoded-parsing), modern browsers, and Python's `urllib.parse` (since the CVE-2021-23336 fix) treat only `&` as a separator. This creates a parser differential: the same bytes are tokenized into different fields than a WHATWG compliant intermediary would produce, allowing an attacker to smuggle extra form fields past an upstream body inspecting component. ### Details In `python_multipart/multipart.py`, the `FIELD_NAME` and `FIELD_DATA` states located the next separator by scanning for `&` and, failing that, for `;`: ```python sep_pos = data.find(b"&", i) if sep_pos == -1: sep_pos = data.find(b";", i) ``` As a result, `;` acted as a field boundary. Because the fallback only triggered when no `&` remained in the current chunk, tokenization also depended on unrelated bytes later in the buffer and on how the body was split across `write()` calls. This is the same class of issue as CVE-2021-23336 in CPython's `urllib.parse`. For example, a body inspecting WAF or gateway that follows the WHATWG rule (only `&` separates fields) receives: ``` role=user&x=;role=admin ``` The upstream parses two fields, `role=user` and `x=";role=admin"`, sees a benign `role=user`, and forwards the request. `QuerystringParser` parsed the same bytes as three fields: `role="user"`, `x=""`, and `role="admin"`. The application (for example via Starlette/FastAPI `request.form()`, where the last value wins) then received `role=admin`, a value the upstream validator never saw. The parser is reachable through the public `QuerystringParser` class, the high level `FormParser`, `create_form_parser`, and `parse_form` APIs, and Starlette/FastAPI `request.form()` for url encoded bodies. ### Impact Interpretation conflict / HTTP parameter pollution. An attacker can smuggle extra or overriding form fields past an upstream component that applies the WHATWG separator rule, reaching the backend with parameters the intermediary did not observe. ### Mitigation Upgrade to `python-multipart` `0.0.30` or later, which treats only `&` as a field separator per the [WHATWG URL standard](https://url.spec.whatwg.org/#urlencoded-parsing). `;` is parsed as ordinary field data, matching `urllib.parse`, browsers, and other compliant parsers.

Impact

This vulnerability allows an attacker to bypass upstream body inspection by injecting extra form fields that are not recognized by intermediaries following the WHATWG URL standard. While it does not directly compromise confidentiality or availability, it can lead to integrity issues by enabling unauthorized data injection into form submissions.

Summary

The python-multipart library's QuerystringParser prior to version 0.0.30 incorrectly treated the semicolon character as a field separator in application/x-www-form-urlencoded bodies, diverging from the WHATWG URL standard and modern browser behavior, which recognize only the ampersand as a separator. This interpretation conflict creates a parser differential that can be exploited to smuggle additional form fields past upstream body inspection components. The issue is addressed and fixed in version 0.0.30 of python-multipart.

CWE-436Interpretation Conflict (opens in a new tab)
Class weaknessOWASP Top 10 A06:2025: Insecure Design (opens in a new tab)
Product A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state.
OWASP Cheat Sheets
REST Assessment (opens in a new tab)
REST Security (opens in a new tab)
Cross-Site Request Forgery Prevention (opens in a new tab)
Secure Design Principles (opens in a new tab)
OWASP ASVS 4.0.3
V13.2.5 (opens in a new tab)
Attack patterns
HTTP Request Splitting (opens in a new tab)
HTTP Response Smuggling (opens in a new tab)
HTTP Response Splitting (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-444Inconsistent Interpretation of HTTP Requests (HTTP Request Smuggling) (opens in a new tab)
Base weaknessOWASP Top 10 A06:2025: Insecure Design (opens in a new tab)
The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.
Implementation
Use a web server that employs a strict HTTP parsing procedure, such as Apache.
Implementation
Use only SSL communication.
Implementation
Terminate the client session after each request.
OWASP Cheat Sheets
REST Assessment (opens in a new tab)
REST Security (opens in a new tab)
Cross-Site Request Forgery Prevention (opens in a new tab)
Secure Design Principles (opens in a new tab)
OWASP ASVS 4.0.3
V13.2.5 (opens in a new tab)
Attack patterns
HTTP Response Smuggling (opens in a new tab)
HTTP Request Smuggling (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

github.com/Kludex/python-multipart/security/advisories/GHSA-6jv3-5f52-599m (opens in a new tab)

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 29 Jun 2026, 21:51 UTC

WeaknessesCWE-436, CWE-444

Technical details

References

Community reviews

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