# CVE-2026-9545: exposing HTTP/3 early data

CVE-2026-9545 is fixed in 13 products on 1 platform, first by curl 8.18.0-1ubuntu2.2, released June 26, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-9545
- Priority: High
- CVSS: 7.5 (Important)
- EPSS: 0.0041
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jun 24, 2026
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-9545

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uses a proper HTTP/3 server for the initial transfers, and when it makes a second transfer to the same site it has been replaced by the attacker's impostor machine - without a valid certificate.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

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | curl Debian 14 | 8.21.0~rc2-1 | [DEBIAN-CVE-2026-9545](https://patchblox.com/security/data/fix/linux/debian-cve-2026-9545.md) | Jul 24, 2026 |
| Linux | curl Ubuntu 25.10 (questing) | 8.14.1-2ubuntu1.4 | [curl 8.14.1-2ubuntu1.4](https://patchblox.com/security/data/fix/linux/ubuntu-questing-curl-8.14.1-2ubuntu1.4.md) | Jul 3, 2026 |
| Linux | curl Ubuntu 26.04.1 LTS (resolute) | 8.18.0-1ubuntu2.2 | [curl 8.18.0-1ubuntu2.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-curl-8.18.0-1ubuntu2.2.md) | Jun 26, 2026 |
| Linux | curl amzn 2023 | 8.21.0-5.amzn2023.0.1 | [ALAS2023-2026-2132](https://patchblox.com/security/data/fix/linux/alas2023-2026-2132.md) | Sep 14, 2026 |
| Linux | curl opensuse-leap 16.0 | 8.14.1-160000.8.1 | [OPENSUSE-SU-2026:21272-1](https://patchblox.com/security/data/fix/linux/opensuse-su-2026-21272-1.md) | Jul 8, 2026 |
| Linux | curl sles 15.6 | 8.14.1-150600.4.46.1 | [SUSE-SU-2026:2926-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-2926-1.md) | Jul 13, 2026 |
| Linux | curl sles 15.7 | 8.14.1-150700.7.20.1 | [SUSE-SU-2026:2925-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-2925-1.md) | Jul 13, 2026 |
| Linux | curl sles 16.0 | 8.14.1-160000.8.1 | [SUSE-SU-2026:22582-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-22582-1.md) | Jul 8, 2026 |
| Linux | curl-debuginfo amzn 2023 | 8.21.0-5.amzn2023.0.1 | [ALAS2023-2026-2132](https://patchblox.com/security/data/fix/linux/alas2023-2026-2132.md) | Sep 14, 2026 |
| Linux | curl-debugsource amzn 2023 | 8.21.0-5.amzn2023.0.1 | [ALAS2023-2026-2132](https://patchblox.com/security/data/fix/linux/alas2023-2026-2132.md) | Sep 14, 2026 |
| Linux | curl-mini sles 16.0 | 8.14.1-160000.8.1 | [SUSE-SU-2026:22582-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-22582-1.md) | Jul 8, 2026 |
| Linux | curl-minimal amzn 2023 | 8.21.0-5.amzn2023.0.1 | [ALAS2023-2026-2132](https://patchblox.com/security/data/fix/linux/alas2023-2026-2132.md) | Sep 14, 2026 |
| Linux | curl-minimal-debuginfo amzn 2023 | 8.21.0-5.amzn2023.0.1 | [ALAS2023-2026-2132](https://patchblox.com/security/data/fix/linux/alas2023-2026-2132.md) | Sep 14, 2026 |
| Linux | libcurl amzn 2023 | 8.21.0-5.amzn2023.0.1 | [ALAS2023-2026-2132](https://patchblox.com/security/data/fix/linux/alas2023-2026-2132.md) | Sep 14, 2026 |
| Linux | libcurl-debuginfo amzn 2023 | 8.21.0-5.amzn2023.0.1 | [ALAS2023-2026-2132](https://patchblox.com/security/data/fix/linux/alas2023-2026-2132.md) | Sep 14, 2026 |
| Linux | libcurl-devel amzn 2023 | 8.21.0-5.amzn2023.0.1 | [ALAS2023-2026-2132](https://patchblox.com/security/data/fix/linux/alas2023-2026-2132.md) | Sep 14, 2026 |
| Linux | libcurl-minimal amzn 2023 | 8.21.0-5.amzn2023.0.1 | [ALAS2023-2026-2132](https://patchblox.com/security/data/fix/linux/alas2023-2026-2132.md) | Sep 14, 2026 |
| Linux | libcurl-minimal-debuginfo amzn 2023 | 8.21.0-5.amzn2023.0.1 | [ALAS2023-2026-2132](https://patchblox.com/security/data/fix/linux/alas2023-2026-2132.md) | Sep 14, 2026 |
| Linux | libcurl3t64-gnutls Ubuntu 25.10 (questing) | 8.14.1-2ubuntu1.4 | [libcurl3t64-gnutls 8.14.1-2ubuntu1.4](https://patchblox.com/security/data/fix/linux/ubuntu-questing-libcurl3t64-gnutls-8.14.1-2ubuntu1.4.md) | Jul 3, 2026 |
| Linux | libcurl3t64-gnutls Ubuntu 26.04.1 LTS (resolute) | 8.18.0-1ubuntu2.2 | [libcurl3t64-gnutls 8.18.0-1ubuntu2.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-libcurl3t64-gnutls-8.18.0-1ubuntu2.2.md) | Jun 26, 2026 |
| Linux | libcurl4t64 Ubuntu 25.10 (questing) | 8.14.1-2ubuntu1.4 | [libcurl4t64 8.14.1-2ubuntu1.4](https://patchblox.com/security/data/fix/linux/ubuntu-questing-libcurl4t64-8.14.1-2ubuntu1.4.md) | Jul 3, 2026 |
| Linux | libcurl4t64 Ubuntu 26.04.1 LTS (resolute) | 8.18.0-1ubuntu2.2 | [libcurl4t64 8.18.0-1ubuntu2.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-libcurl4t64-8.18.0-1ubuntu2.2.md) | Jun 26, 2026 |

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Data as of 2026-11-08T06:59:42Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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