VulnerabilityPublished 13 Aug 2026Updated 3 Nov 2026

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

Affects fastmcpSource 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 2.14.4_1 as a priority.

Data as of 3 Nov 2026, 22:58 UTC

CVSS 3.1 base score

8.6High

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

EPSS: 2.2% chance of exploitation in 30 days – higher than 82% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.14.4_1

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 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 2.14.4_1.

Affected software

Product	Affected	Fixed in
Homebrewfastmcp`pkg:brew/fastmcp`Other threats	Affected >=2.12.3 <2.14.4_1	Fixed in 2.14.4_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 fastmcp version 2.14.4_1 or later, where the vulnerability is fixed. Alternatively, avoid using the configuration option UPLOAD_KEEP_FILENAME=True with UPLOAD_DIR set to prevent exploitation. The default configuration without these settings is not vulnerable.

Description

Fastmcp's Python-Multipart library has a path traversal vulnerability (CVE-2026-24486) when using a non-default configuration with UPLOAD_DIR set and UPLOAD_KEEP_FILENAME=True. This allows an attacker to write files to arbitrary filesystem locations by crafting filenames starting with '/'. The vulnerability only triggers if the uploaded file exceeds MAX_MEMORY_FILE_SIZE, causing a flush to disk. The default configuration is not vulnerable. A fix is available in version 2.14.4_1.

Impact

An attacker can write files to arbitrary locations on the filesystem, potentially overwriting critical files or placing malicious files, leading to integrity compromise and possible further system exploitation. The CVSS score is 8.6 (high severity), reflecting network attack vector, low complexity, no privileges required, no user interaction, and impacts on integrity and availability.

Summary

The vulnerability arises from the use of os.path.join() in Python-Multipart when UPLOAD_DIR is set and UPLOAD_KEEP_FILENAME=True. If an uploaded filename starts with a '/', os.path.join discards the intended upload directory and uses the absolute path from the filename, enabling arbitrary file write. This affects projects using these specific configuration options and uploading files larger than MAX_MEMORY_FILE_SIZE. The issue is fixed in version 2.14.4_1.

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Exploitation evidence

Timeline

What to do

Affected software

Description

AI analysisMachine-generated, updated 3 Nov 2026, 21:33 UTC

Public exploits and detection1 Exploit-DB entry

Technical details

References

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