CVE-2026-9546 Details
Description
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.
A vulnerability in libcurl versions 8.18.0 through 8.20.0 allows the HTTP Referer header to persist even after being explicitly cleared. Although the documentation states that setting CURLOPT_REFERER to NULL disables the header, this option failed to update the internal state. Consequently, the previous referrer string was incorrectly reused in subsequent requests, potentially leaking sensitive information to unintended servers. This issue is particularly problematic for applications that pool or reuse easy handles across different jobs, users, or tenants.
Users are advised to upgrade libcurl to version 8.21.0 or later, or to apply the available patch and rebuild. Alternatively, avoid using CURLOPT_REFERER.
Metrics
CVSS 4.0 Severity and Vector Strings:
No CVSS 4.0 data is available for this CVE.
CVSS 3.x Severity and Vector Strings:
No data available for CVSS Version 2.0 on this CVE.
CISA-ADP
Assessed Jul 6, 2026References to Advisories, Solutions, and Tools
By selecting these links, you will be leaving this site. These are references gathered from the official CVE record and are not endorsed by Volerion.
| URL | Source(s) | Tag(s) |
|---|---|---|
| https://hackerone.com/reports/3754343 | CISA-ADP | ExploitIssue TrackingThird Party Advisory |
| https://curl.se/docs/CVE-2026-9546.html | curl | PatchVendor Advisory |
| https://curl.se/docs/CVE-2026-9546.json | curl | Third Party Advisory |
| https://hackerone.com/reports/3754343 | curl | ExploitIssue TrackingThird Party Advisory |
Weakness Enumeration
| CWE-ID | CWE Name | Source |
|---|---|---|
| NVD-CWE-noinfo | Insufficient Information to Classify Weakness | [email protected] |
| CWE-200 | Exposure of Sensitive Information to an Unauthorized Actor | curl |
Affected Products
| Product | Versions |
|---|---|
| haxx curl | >= 8.18.0, < 8.21.0 |
CPE
Remediation
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Change History
5 change records found show changes
| Date | Action | Recorded By |
|---|---|---|
| Sep 15, 2026 | CVE Modified | curl |
| Sep 15, 2026 | CVE Modified | CISA-ADP |
| Jul 7, 2026 | Initial Analysis | [email protected] |
| Jul 6, 2026 | CVE Modified | CISA-ADP |
| Jul 3, 2026 | New CVE Received | curl |