Vulnerability Description
In this scenario, libcurl first uses a proper HTTP/3 server for the initial transfers, and when it makes a second transfer to the same site it has been replaced by the attacker's impostor machine - without a valid certificate. When libcurl returns to the hostname the second time with a cached SSL session (`CURLOPT_SSL_SESSIONID_CACHE` is not disabled) and early data enabled (the `CURLSSLOPT_EARLYDATA` bit is set in `CURLOPT_SSL_OPTIONS`), libcurl might send off the second request's bytes on that new connection *before* enforcing the certificate verification failure. Potentially leaking sensitive information.
CVSS Score
HIGH
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Haxx | Curl | >= 8.11.0, < 8.21.0 |
References
- https://curl.se/docs/CVE-2026-9545.htmlPatchVendor Advisory
- https://curl.se/docs/CVE-2026-9545.jsonVendor Advisory
- https://hackerone.com/reports/3752888ExploitIssue TrackingThird Party Advisory
- https://hackerone.com/reports/3752888ExploitIssue TrackingThird Party Advisory
FAQ
What is CVE-2026-9545?
CVE-2026-9545 is a vulnerability with a CVSS score of 7.5 (HIGH). In this scenario, libcurl first uses a proper HTTP/3 server for the initial transfers, and when it makes a second transfer to the same site it has been replaced by the attacker's impostor machine - wi...
How severe is CVE-2026-9545?
CVE-2026-9545 has been rated HIGH with a CVSS base score of 7.5/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-9545?
Check the references section above for vendor advisories and patch information. Affected products include: Haxx Curl.