CVE-2026-9545Haxx Curl
NVDCVSS scored by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Vulnerability Summary
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transfers, and when it makes a second transfer to the same site it has been
replaced by the attacker's impostor machine - without a valid certificate.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.
transfers, and when it makes a second transfer to the same site it has been
replaced by the attacker's impostor machine - without a valid certificate.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 v3.1 Base Metrics — Score 7.5 (HIGH)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:NAttack VectorNetwork
Attack ComplexityLow
Privileges RequiredNone
User InteractionNone
ScopeUnchanged
ConfidentialityHigh
IntegrityNone
AvailabilityNone
Affected & Patched Versions
- Haxx Curl >= 8.11.0 and < 8.21.0
- Haxx Curl 8.21.0
What to do
Prioritize this fix. Exploit code is public, so attacks can follow quickly.
- Check whether you run Haxx Curl and which version.
- Update to a patched version listed above, then confirm the new version.
- Review logs and exposed services for signs of earlier compromise.
Timeline
- Jul 3, 2026Published
- Jul 7, 2026Record last modified
Score, severity and exploitation changes are tracked from the day Watchtower started logging them.
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External References
Data last checked: Nov 15, 2026 03:31 UTC