# CVE-2026-9545: curl information exposure via HTTP/3 early data transmission

Source: https://junglewise.ai/threats/cve-2026-9545-curl-information-exposure-via-http-3-early-data-transmission  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-03. Updated: 2026-07-03  
License: CC BY 4.0 with a link back

CVE-2026-9545 · Severity: info · Published 2026-07-03

curl is a widely used tool and library for transferring data over various network protocols. A security flaw in its HTTP/3 implementation could allow an attacker to intercept sensitive information, such as login credentials or private data, by impersonating a legitimate server. This occurs because the software may prematurely send data to a malicious server before verifying its security certificate during a resumed connection.

## Quick answers

**Is CVE-2026-9545 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-9545 being exploited in the wild.

**How severe is CVE-2026-9545?** Informational severity, published 3 July 2026.

**Which products and versions are affected?** curl curl 8.11.0 to 8.20.0.

**Is there a fix for CVE-2026-9545?** A patch was published on 2026-06-24: Fixed in curl version 8.21.0. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-9545 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-9545?** Check whether anything you run is named in the advisory. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A vulnerability exists in libcurl's HTTP/3 implementation (specifically the ngtcp2 + nghttp3 backend) related to TLS 1.3 early data (0-RTT). When a client resumes a cached SSL session with early data enabled (CURLSSLOPT_EARLYDATA), libcurl may transmit the request payload on a new connection before completing the certificate verification process. If an attacker performs a machine-in-the-middle attack or replaces the legitimate server with an impostor lacking a valid certificate, sensitive request bytes could be leaked to the unauthorized actor. The issue is fixed in version 8.21.0 by ensuring certificate validation occurs before early data transmission.

## Affected products

- curl curl 8.11.0 to 8.20.0

## Timeline

- 2026-05-19: disclosed: Reported to the curl project via HackerOne.
- 2026-06-24: patched: Fixed in curl version 8.21.0.
- 2026-06-24: advisory: Project curl security advisory published.
- 2026-07-03: other: CVE record published in NVD.

## References

- [https://curl.se/docs/CVE-2026-9545.html](https://curl.se/docs/CVE-2026-9545.html)
- [https://curl.se/docs/CVE-2026-9545.json](https://curl.se/docs/CVE-2026-9545.json)
- [https://hackerone.com/reports/3752888](https://hackerone.com/reports/3752888)