libcurl HTTP/3 session resumption EarlyData info leak
CVE-2026-9545 Published on July 3, 2026

exposing HTTP/3 early data
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uses a proper HTTP/3 server for the initial transfers, and when it makes a second transfer to the same site it has been replaced by the attacker's impostor machine - without a valid certificate.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.

NVD

Vulnerability Analysis

CVE-2026-9545 can be exploited with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. An automatable proof of concept (POC) exploit exists. The potential impact of an exploit of this vulnerability is considered to have a high impact on confidentiality, with no impact on integrity and availability.

Attack Vector:
NETWORK
Attack Complexity:
LOW
Privileges Required:
NONE
User Interaction:
NONE
Scope:
UNCHANGED
Confidentiality Impact:
HIGH
Integrity Impact:
NONE
Availability Impact:
NONE

Weakness Type

What is an Information Disclosure Vulnerability?

The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information.

CVE-2026-9545 has been classified to as an Information Disclosure vulnerability or weakness.


Products Associated with CVE-2026-9545

You can be notified by email with stack.watch whenever vulnerabilities like CVE-2026-9545 are published in these products:

Canonical Ubuntu Linux
 
Haxx Curl
 
Red Hat Http Server
 

Affected Versions

curl: curl: curl: