CVE-2026-48710VulnerabilityPublished 13 Aug 2026Updated 31 Oct 2026

Semgrep: Starlette has missing Host header validation that poisons request.url.path, bypassing path-based security

Affects semgrepSource GCVE Database

Compare

Authoritative record:CVE-2026-48710· Medium 6.5

Exploited in the wild · KEV due 16 Sep 2026 · public detection template (Nuclei) : upgrade to 1.165.0 now.

Data as of 31 Oct 2026, 16:23 UTC

CVSS 3.1 base score

6.5Medium

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

KEV deadline: 16 Sep 2026 US federal agencies must patch by this date; a sensible deadline for everyone else. Other KEV due dates
Patch: Available – fixed in 1.165.0

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

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

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

Disclosed: evidence found.
CVE published 26 May 2026
Detectable: evidence found.
nuclei template: Starlette - Improper Validation of Unsafe Equivalence in Input
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: evidence found.
Listed in CISA KEV 99 days after disclosure
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

CVE ID reserved
by GitHub, Inc.
22 May 202618:47 UTC, 4 days before disclosure
CVE published
on cve.org
26 May 202621:54 UTC, t = 0
In Threat Radar
record ingested
26 May 202622:18 UTC, 24 minutes after disclosure
Added to CISA KEV (opens in a new tab)
exploitation confirmed
2 Sep 2026, 99 days after disclosure
KEV remediation due
US federal deadline
16 Sep 2026, 113 days after disclosure

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), CISA KEV (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 30 Oct 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 semgrep to 1.165.0.

Patches and advisories

Affected software

Product	Affected	Fixed in
Homebrewsemgrep`pkg:brew/semgrep`Other threats	Affected >=1.137.0 <1.165.0	Fixed in 1.165.0

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 1.165.0 or later, which includes validation of the Host header per RFC 9112 and RFC 3986 and falls back to the server scope for malformed Host headers. Alternatively, deploy an RFC-compliant reverse proxy (e.g., nginx, Apache, HAProxy, Caddy) in front of the ASGI server to reject malformed Host headers before they reach the application. Review application code to ensure security decisions do not rely on request.url.path but instead use the canonical ASGI path or standard framework mechanisms.

Description

Starlette versions from 1.137.0 up to but not including 1.165.0 improperly validate the HTTP Host header when reconstructing request.url, allowing a malformed Host header to cause request.url.path to differ from the actual requested path. This discrepancy can lead to bypass of path-based security checks if middleware or endpoints rely on request.url.path rather than the canonical ASGI path. The vulnerability does not directly grant access or modify routing but can enable bypass only if applications depend on the flawed request.url.path for authorization. The issue is mitigated by validating the Host header per RFC 9112 and falling back to server scope for malformed values. A fix is available in version 1.165.0.

Impact

Applications using affected Starlette versions that rely on request.url.path for security-sensitive decisions, such as path-based authorization middleware, may have those security checks bypassed due to the mismatch between request.url.path and the actual requested path. The vulnerability does not itself grant access or alter routing but can enable bypass if the application trusts the reconstructed URL path. Deployments behind proxies that reject or normalize malformed Host headers are mitigated. The overall impact is rated moderate to low depending on application usage patterns.

References

Community reviews

Loading reviews…

Semgrep: Starlette has missing Host header validation that poisons request.url.path, bypassing path-based security

Exploitation evidence

What to do

Patches and advisories

Affected software

Description

Impact

Nuclei detection templates

Proofs of concept on GitHub

References

Community reviews

Exploitation evidence

Timeline

What to do

Patches and advisories

Affected software

Description

AI analysisMachine-generated, updated 31 Oct 2026, 16:23 UTC

Public exploits and detectiona Nuclei detection template, GitHub proofs of concept

Technical details

References

Community reviews

Related threats