# 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.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 3 Jul 2026
- **Updated:** 15 Sep 2026
- **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, JLSEC-2026-1220, MINI-827p-9g5j-wjxj, MINI-rp7p-6356-j7hc, ROOT-OS-ALPINE-320-CVE-2026-9545, ROOT-OS-ALPINE-321-CVE-2026-9545, ROOT-OS-ALPINE-322-CVE-2026-9545, ROOT-OS-DEBIAN-13-CVE-2026-9545, UBUNTU-CVE-2026-9545

## 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transfers, and when it makes a second transfer to the same site it has been
replaced by the attacker's impostor machine - without a valid certificate.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 (Exposure of Sensitive Information to an Unauthorized Actor)

## Which versions are affected by CVE-2026-9545?

It affects 1 product.


Products: 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

- [CVE-2026-19931](https://insights.sbomify.com/CVE-2026-19931/): Negotiate ambient user conn reuse (critical 9.8)
- [CVE-2026-10536](https://insights.sbomify.com/CVE-2026-10536/): HTTP/2 stream-dependency tree UAF (critical 9.8)
- [CVE-2026-11856](https://insights.sbomify.com/CVE-2026-11856/): cross-origin Digest auth state leak (critical 9.8)
- [CVE-2026-8925](https://insights.sbomify.com/CVE-2026-8925/): SASL double-free (critical 9.8)
- [CVE-2026-9079](https://insights.sbomify.com/CVE-2026-9079/): stale proxy password leak (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-9545 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-9545 exists.

Affected product: Haxx Curl. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-9545/ (Markdown: https://insights.sbomify.com/CVE-2026-9545/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-9545
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-9545
- OSV: https://osv.dev/vulnerability/CVE-2026-9545

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-9545/
Summaries are generated and may contain mistakes; the official description above is the record.