CVE-2026-9545 exposing HTTP/3 early data
libcurl may leak sensitive data via HTTP/3 early data if a malicious server replaces a trusted one during a cached session.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-9545?
CVE-2026-9545 is a high-severity vulnerability in Haxx Curl, published on 3 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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uses a proper HTTP/3 server for the initial transfers, and when it makes a second transfer to the same site it has been replaced by the attacker's impostor machine - without a valid certificate.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.
Weakness: CWE-200
Which versions are affected by CVE-2026-9545?
It affects 1 product.
Products include Haxx Curl.
How do I fix CVE-2026-9545?
Apply the vendor's patch.
Is CVE-2026-9545 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-9545 exists.
Other vulnerabilities in Haxx Curl
Sources
Also known as ALPINE-CVE-2026-9545, BELL-CVE-2026-9545, CURL-CVE-2026-9545, DEBIAN-CVE-2026-9545, ECHO-5717-dc6a-4550, GHSA-6v72-wfcj-jv53 and 8 more.