Using libcurl, when a custom `Host:` header is first set for an HTTP request and a second request is subsequently done using the same *easy handle* but without the custom `Host:` header set, the second request would use stale information and pass on cookies meant for the first host in the second request. Leak them.

Metrics

No CVSS v4.0

Attack Vector Network

Attack Complexity High

Privileges Required None

Scope Unchanged

Confidentiality Impact Low

Integrity Impact None

Availability Impact None

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

Key SSVC decision points have not yet been added.

Affected Vendors & Products

Vendors Products
Curl
  • Libcurl

No data.

No data.

References
Link Providers
https://curl.se/docs/CVE-2026-6276.html cve-icon cve-icon
https://curl.se/docs/CVE-2026-6276.json cve-icon
https://hackerone.com/reports/3671818 cve-icon
https://nvd.nist.gov/vuln/detail/CVE-2026-6276 cve-icon
https://www.cve.org/CVERecord?id=CVE-2026-6276 cve-icon
History

Wed, 13 May 2026 09:15:00 +0000

Type Values Removed Values Added
Description A flaw was found in libcurl. This vulnerability allows for information disclosure when a custom `Host:` header is used in an initial HTTP request, and a subsequent request reuses the same connection without specifying a new `Host:` header. This can lead to libcurl incorrectly sending cookies intended for the first host to the second host, resulting in a cookie leak. This issue is categorized as an Origin Validation Error (CWE-346). Exploitation typically requires specific debugging configurations. Using libcurl, when a custom `Host:` header is first set for an HTTP request and a second request is subsequently done using the same *easy handle* but without the custom `Host:` header set, the second request would use stale information and pass on cookies meant for the first host in the second request. Leak them.
Title curl: libcurl: Information disclosure due to cookie leak when reusing connections with custom Host headers stale custom cookie host causes cookie leak
References

Fri, 01 May 2026 01:45:00 +0000

Type Values Removed Values Added
First Time appeared Curl
Curl libcurl
Vendors & Products Curl
Curl libcurl

Fri, 01 May 2026 00:15:00 +0000

Type Values Removed Values Added
Description A flaw was found in libcurl. This vulnerability allows for information disclosure when a custom `Host:` header is used in an initial HTTP request, and a subsequent request reuses the same connection without specifying a new `Host:` header. This can lead to libcurl incorrectly sending cookies intended for the first host to the second host, resulting in a cookie leak. This issue is categorized as an Origin Validation Error (CWE-346). Exploitation typically requires specific debugging configurations.
Title curl: libcurl: Information disclosure due to cookie leak when reusing connections with custom Host headers
Weaknesses CWE-346
References
Metrics threat_severity

None

 cvssV3_1

{'score': 3.7, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N'}

threat_severity

Low
cve-icon MITRE

Status: PUBLISHED

Assigner: curl

Published:

Updated: 2026-05-13T09:05:37.539Z

Reserved: 2026-04-14T14:01:54.772Z

Link: CVE-2026-6276

cve-icon Vulnrichment

No data.

cve-icon NVD

No data.

cve-icon Redhat

Severity : Low

Publid Date: 2026-04-29T00:00:00Z

Links: CVE-2026-6276 - Bugzilla