VulnerabilityPublished 13 Aug 2026Updated 3 Nov 2026

CVE-2026-53540: python-multipart · Negative Content-Length in parse_form buffers the entire body in memory

Affects fastmcpSource GCVE Database

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Authoritative record:CVE-2026-53540· Low 3.7

Not known to be exploited · CVSS 3.7 Low : upgrade to 3.4.1 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: 0.3% chance of exploitation in 30 days – higher than 27% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 3.4.1

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

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

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: None: There is no loss of integrity within the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to 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 3.4.1.

Affected software

Product	Affected	Fixed in
Homebrewfastmcp`pkg:brew/fastmcp`Other threats	Affected >=2.12.3 <3.4.1	Fixed in 3.4.1

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

AI-suggested

Upgrade to version 3.4.1 or later, which rejects negative Content-Length headers with a ValueError before reading the stream. If using bespoke WSGI or http.server handlers that call parse_form() directly, ensure that Content-Length headers are validated and normalized to prevent negative values.

Description

A vulnerability in the parse_form() function allows a negative Content-Length header to cause the entire request body to be buffered in memory at once instead of being read in chunks. This can degrade availability under concurrent requests but does not cause a complete denial of service. No known popular packages are affected, and the issue mainly impacts bespoke WSGI or http.server handlers that forward raw client headers without normalization. The issue is fixed in version 3.4.1.

Impact

The vulnerability can cause excessive memory consumption by buffering the entire request body in memory at once when a negative Content-Length header is used. This can degrade service availability under concurrent requests but does not lead to complete denial of service or compromise confidentiality or integrity. No known popular packages are affected, limiting the practical impact.

Summary

The parse_form() function reads the request body in chunks bounded by the Content-Length header. However, it does not validate that Content-Length is non-negative before using it. A negative Content-Length (e.g., -1) causes the chunk size calculation to become negative, resulting in input_stream.read(-1), which reads until EOF. This causes the entire request body to be loaded into memory in a single read rather than in fixed-size chunks. The vulnerability only affects code that calls parse_form() directly with attacker-controlled negative Content-Length headers without normalization. Popular frameworks like Starlette and FastAPI do not call parse_form() directly with raw headers, and other known consumers normalize or recompute Content-Length. The impact is limited to bespoke handlers that forward raw headers. The issue is fixed in version 3.4.1, which rejects negative Content-Length values with a ValueError.

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What to do

Affected software

Description

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

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