VulnerabilityPublished 13 Aug 2026Updated 9 Nov 2026

CVE-2026-24486: Fastapi · Python-Multipart has Arbitrary File Write via Non-Default Configuration

Affects fastapiSource GCVE Database

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Authoritative record:CVE-2026-24486· High 8.6

Public exploit available (Exploit-DB) · CVSS 8.6 High : upgrade to 0.128.0_2 as a priority.

Data as of 9 Nov 2026, 20:02 UTC

CVSS 3.1 base score

8.6High

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

Patch: Available – fixed in 0.128.0_2

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: Low
Integrity: High
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: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to 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

t = 0 is the CVE’s publication on cve.org

Disclosed: evidence found.
CVE published 27 Jan 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: evidence found.
1 Exploit-DB entry, 93 days after disclosure
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

CVE ID reserved
by GitHub, Inc.
23 Jan 202600:38 UTC, 4 days before disclosure
CVE published
on cve.org
27 Jan 202600:34 UTC, t = 0
In Threat Radar
record ingested
27 Jan 202600:50 UTC, 17 minutes after disclosure
First Exploit-DB entry (opens in a new tab)
Python-Multipart 0.0.22 - Path Traversal
30 Apr 2026, 93 days after disclosure

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 0.128.0_2.

Affected software

Product	Affected	Fixed in
Homebrewfastapi`pkg:brew/fastapi`Other threats	Affected >=0.111.0 <0.128.0_2	Fixed in 0.128.0_2

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 0.128.0_2 or later where this vulnerability is fixed. Alternatively, avoid setting UPLOAD_KEEP_FILENAME to True in project configurations to prevent exploitation.

Description

A path traversal vulnerability in Fastapi's Python-Multipart library allows arbitrary file write when using non-default configuration options. Specifically, if UPLOAD_DIR is set and UPLOAD_KEEP_FILENAME is True, an attacker can craft filenames starting with '/' to write files outside the intended upload directory. This affects versions from 0.111.0 up to but not including 0.128.0_2. The vulnerability is fixed in version 0.128.0_2.

Impact

An attacker can write files to arbitrary locations on the filesystem, potentially overwriting critical files or placing malicious files that could lead to further system compromise. The CVSS score of 8.6 indicates high severity with network attack vector, low attack complexity, no privileges or user interaction required, and impacts confidentiality, integrity, and availability.

Summary

The vulnerability arises from the use of os.path.join(file_dir, fname) when UPLOAD_DIR is set and UPLOAD_KEEP_FILENAME is True. Because os.path.join discards preceding path components if the filename starts with a '/', an attacker can upload files with malicious filenames to arbitrary filesystem locations, bypassing the intended upload directory. This only occurs if the uploaded file exceeds MAX_MEMORY_FILE_SIZE, triggering a flush to disk. The default configuration is not vulnerable.

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Exploitation evidence

Timeline

What to do

Affected software

Description

AI analysisMachine-generated, updated 9 Nov 2026, 20:02 UTC

Public exploits and detection1 Exploit-DB entry

Technical details

References

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