What’s Vulnerable › Vulnerabilities

CVE-2026-9546: sending old referer

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.7% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-07-03 · updated 2026-09-15
• Weakness: CWE-200

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.

Affected products

• curl: from 8.18.0 before 8.21.0 — fixed in 8.21.0

How to fix

Upgrade to a fixed version: curl 8.21.0. Check the vendor advisory for mitigations.

References

• https://curl.se/docs/CVE-2026-9546.html
• https://curl.se/docs/CVE-2026-9546.json
• https://hackerone.com/reports/3754343
• https://hackerone.com/reports/3754343

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