exposing HTTP/3 early data

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Summary

CVECVE-2026-9545
StatePUBLISHED
Assignercurl
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-07-03 07:16:25 UTC
Updated2026-07-07 15:03:56 UTC
DescriptionIn 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uses a proper HTTP/3 server for the initial transfers, and when it makes a second transfer to the same site it has been replaced by the attacker's impostor machine - without a valid certificate.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.

Risk And Classification

Primary CVSS: v3.1 7.5 HIGH from ADP

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

EPSS: 0.001340000 probability, percentile 0.032110000 (date 2026-07-06)

Problem Types: NVD-CWE-noinfo | CWE-200 Exposure of Sensitive Information to an Unauthorized Actor

VersionSourceTypeScoreSeverityVector
3.1ADPDECLARED7.5HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
3.1134c704f-9b21-4f2e-91b3-4a467353bcc0Secondary7.5HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Haxx Curl All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Curl Curl affected 8.20.0 8.20.0 semver Not specified
CNA Curl Curl affected 8.19.0 8.19.0 semver Not specified
CNA Curl Curl affected 8.18.0 8.18.0 semver Not specified
CNA Curl Curl affected 8.17.0 8.17.0 semver Not specified
CNA Curl Curl affected 8.16.0 8.16.0 semver Not specified
CNA Curl Curl affected 8.15.0 8.15.0 semver Not specified
CNA Curl Curl affected 8.14.1 8.14.1 semver Not specified
CNA Curl Curl affected 8.14.0 8.14.0 semver Not specified
CNA Curl Curl affected 8.13.0 8.13.0 semver Not specified
CNA Curl Curl affected 8.12.1 8.12.1 semver Not specified
CNA Curl Curl affected 8.12.0 8.12.0 semver Not specified
CNA Curl Curl affected 8.11.1 8.11.1 semver Not specified
CNA Curl Curl affected 8.11.0 8.11.0 semver Not specified

References

ReferenceSourceLinkTags
curl.se/docs/CVE-2026-9545.html 2499f714-1537-4658-8207-48ae4bb9eae9 curl.se Patch, Vendor Advisory
curl.se/docs/CVE-2026-9545.json 2499f714-1537-4658-8207-48ae4bb9eae9 curl.se Vendor Advisory
hackerone.com/reports/3752888 134c704f-9b21-4f2e-91b3-4a467353bcc0 hackerone.com Exploit, Issue Tracking, Third Party Advisory
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: Eunsoo Kim (Autonomous Code Security team at Microsoft) (en)

CNA: Stefan Eissing (en)

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