CVE-2026-9545 curl vulnerability
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uses a proper HTTP/3 server for the initial transfers, and when it makes a second transfer to the same site it has been replaced by the attacker's impostor machine - without a valid certificate.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.
Quick answer
haxx curl should be reviewed and updated if it matches the affected versions. The recommended fix is to apply the vendor-supported patched version or the mitigation steps below, then retest the public website with Fixnx.
Who is affected
Affected versions
- from 8.11.0 before 8.21.0
Fixed versions
- 8.21.0
How to fix it
CVE-2026-9545 affects curl or libcurl. It may expose information. Upgrade curl and libcurl to 8.21.0, or apply the vendor patch and rebuild affected apps.
- List every system, container, and app that uses curl or libcurl.
- Check the curl and libcurl versions on each system.
- Upgrade curl and libcurl to 8.21.0 or a distro backport that includes the fix.
- Rebuild apps and containers that statically link libcurl.
- Restart services that load libcurl after the update.
- Avoid risky options or workflows from the advisory until patched.
- Review proxy, TLS, SSH, and HTTP logs for unexpected data leaks.
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Verify the fix
- Run curl --version or package checks and confirm 8.21.0 or a patched vendor build.
- Confirm rebuilt apps link to the patched libcurl.
- Retest affected SCP, SFTP, proxy, TLS, or HTTP flows as relevant.
- Confirm logs no longer show the unsafe behavior.
- Record package versions and rebuild evidence.
Trusted references
FAQ
What is affected by CVE-2026-9545?
haxx curl versions listed as affected should be reviewed: from 8.11.0 before 8.21.0.
What should I fix first?
Start with internet-facing sites, admin panels, login flows, plugins, themes, modules, packages, and systems that process user-controlled input or sensitive data.
How do I confirm the fix worked?
Apply the patched version or mitigation, clear caches where relevant, retest the affected workflow, and run a new Fixnx scan to verify public website exposure signals.
How are Fixnx security risk categories chosen?
Fixnx keeps one canonical risk page and assigns only broad, relevant categories such as ecosystem, technology area, or vulnerability class.