---
id: CVE-2026-9546
title: |-
  A vulnerability in libcurl caused the HTTP `Referer:` header to persist even
  when explicitly cleared
summary: |-
  A vulnerability in libcurl caused the HTTP `Referer:` header to persist even
  when explicitly cleared. While the documentation states that passing NULL to
  `CURLOPT_REFERER` suppresses the header, the option failed to clear the
  internal st…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N'
cwe:
  - CWE-200
vendor: haxx
product: curl
affected:
  - 'curl >= 8.18.0, < 8.21.0'
patched:
  - curl 8.21.0
published: '2026-07-03'
updated: '2026-09-15'
sourceUpdated: '2026-09-15T07:16:35.130'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-9546'
references:
  - url: 'https://curl.se/docs/CVE-2026-9546.html'
    label: 2499f714-1537-4658-8207-48ae4bb9eae9
  - url: 'https://curl.se/docs/CVE-2026-9546.json'
    label: 2499f714-1537-4658-8207-48ae4bb9eae9
  - url: 'https://hackerone.com/reports/3754343'
    label: 2499f714-1537-4658-8207-48ae4bb9eae9
  - url: 'https://hackerone.com/reports/3754343'
    label: 134c704f-9b21-4f2e-91b3-4a467353bcc0
tags:
  - nvd
  - cve.org
  - exploit-available
exploitAvailable: true
ssvc:
  exploitation: poc
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-07-06T16:51:06.094113Z'
epss: 0.00494
epssPercentile: 0.41512
ingestedAt: '2026-07-04T06:55:48.805Z'
---

## Overview

A vulnerability in libcurl caused the HTTP `Referer:` header to persist even
when explicitly cleared. While the documentation states that passing NULL to
`CURLOPT_REFERER` suppresses the header, the option failed to clear the
internal state. As a result, the previous referrer string was erroneously
reused and sent in subsequent requests, potentially leaking sensitive
information to unintended servers.

## Affected

- `curl >= 8.18.0, < 8.21.0`

## Remediation

Upgrade past the affected range:

- `curl 8.21.0`