VulnerabilityPublished 13 Aug 2026Updated 9 Nov 2026

CVE-2026-54282: Starlette · Unvalidated request path concatenated into authority poisons request.url.hostname

Affects fastapiSource GCVE Database

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Authoritative record:CVE-2026-54282· Low 3.7

Not known to be exploited · CVSS 3.7 Low : upgrade to 0.137.0 when convenient.

Data as of 9 Nov 2026, 20:02 UTC

CVSS 3.1 base score

3.7Low

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

Patch: Available – fixed in 0.137.0

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 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 0.137.0.

Affected software

Product	Affected	Fixed in
Homebrewfastapi`pkg:brew/fastapi`Other threats	Affected >=0.111.0 <0.137.0	Fixed in 0.137.0

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

AI-suggested

Upgrade to Starlette version 0.137.0 or later, where the request path is validated to prevent crossing into the URL authority. This ensures that malformed paths like '@google.com' are correctly interpreted as part of the path (e.g., 'http://localhost/@google.com') with the hostname remaining trustworthy. No other mitigations are indicated.

Description

A vulnerability in Starlette versions >=0.111.0 and <0.137.0 allows an attacker to craft an HTTP request path that lacks a leading slash, causing the reconstructed request URL's hostname to be attacker-controlled. This occurs because the path is concatenated directly after the host without validation, enabling the path to poison the authority portion of the URL. This can mislead code that relies on request.url.hostname for security decisions, particularly in middleware or 404/exception handlers. The issue is fixed in version 0.137.0.

Impact

Applications using affected Starlette versions that rely on request.url.hostname or related properties for security decisions may be misled by attacker-controlled hostnames. This can affect host-based authorization, redirect or callback base validation, SSRF target determination, cache key generation, or audit logging. However, the vulnerability is limited because the malformed path causes routing to fail with a 404, preventing endpoint handlers from running. The impact is primarily on middleware or error handlers that inspect request.url before routing.

Summary

In affected Starlette versions, the HTTP request path is not validated before reconstructing request.url. If the path does not start with '/', such as '@google.com', the URL is rebuilt as 'http://localhost@google.com'. According to RFC 3986, this causes the authority to be parsed with 'localhost' as userinfo and 'google.com' as hostname, making request.url.hostname attacker-controlled. This can mislead security-sensitive code that uses request.url.hostname, request.url.netloc, or request.url for decisions like host-based authorization or SSRF prevention. The vulnerability requires an ASGI server forwarding a malformed request-target without a leading slash. The exposure is limited to code executing before routing or in error handlers, as the malformed path does not match any route and results in a 404 response. The issue is fixed in Starlette 0.137.0, which ensures the path cannot cross into the URL authority.

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Affected software

Description

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

Technical details

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