VulnerabilityPublished 13 Aug 2026Updated 9 Nov 2026

CVE-2026-40347: Fastapi · python-multipart affected by Denial of Service via large multipart preamble or epilogue data

Affects fastapiSource GCVE Database

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Authoritative record:CVE-2026-40347· Medium 5.3

Not known to be exploited · CVSS 5.3 Medium : upgrade to 0.135.3_1 when convenient.

Data as of 9 Nov 2026, 20:03 UTC

CVSS 3.1 base score

5.3Medium

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

Patch: Available – fixed in 0.135.3_1

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

Attack Vector: Network
Attack Complexity: Low
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: 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: 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 17 Apr 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 9 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 fastapi to 0.135.3_1.

Affected software

Product	Affected	Fixed in
Homebrewfastapi`pkg:brew/fastapi`Other threats	Affected >=0.111.0 <0.135.3_1	Fixed in 0.135.3_1

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

AI-suggested

Upgrade FastAPI to version 0.135.3_1 or later, which includes fixes that skip ahead to the next boundary candidate when processing leading CR/LF data and immediately discard epilogue data after the closing boundary, mitigating the denial of service condition.

Description

A denial of service vulnerability in FastAPI's python-multipart parser allows attackers to send crafted multipart/form-data requests with large preamble or epilogue sections. This causes excessive CPU consumption during request parsing, degrading availability but not typically causing a full denial of service. The issue is fixed in version 0.135.3_1.

Impact

An attacker can cause excessive CPU usage by sending oversized malformed multipart bodies, reducing the application's capacity to handle legitimate requests and degrading service availability. This does not typically cause a complete denial of service for the entire application but can delay request processing.

Summary

FastAPI's python-multipart parser has two inefficient parsing paths that can be exploited by attacker-controlled input. Leading CR and LF bytes before the first multipart boundary are handled inefficiently, and trailing epilogue data after the closing boundary is processed instead of discarded immediately. This results in parsing time increasing with the size of crafted data placed before or after multipart boundaries, enabling denial of service via resource exhaustion. The vulnerability affects versions >=0.111.0 and <0.135.3_1 and is resolved in 0.135.3_1 by optimizing the handling of these sections.

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What to do

Affected software

Description

AI analysisMachine-generated, updated 9 Nov 2026, 20:03 UTC

Technical details

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