Description
curl might erroneously pass on credentials for a first proxy to a second proxy. This can happen when the following conditions are true: 1. curl is setup to use specific different proxies for different URL schemes 2. the first proxy needs credentials 3. the second proxy uses no credentials 4. while using the first proxy (using say `http://`), curl is asked to follow a redirect to a URL using another scheme (say `https://`), accessed using a second, different, proxy
Affected products
haxx: curl
References5
- curl.se/docs/CVE-2026-6253.html PatchVendor Advisory
- curl.se/docs/CVE-2026-6253.json Vendor Advisory
- www.openwall.com/lists/oss-security/2026/04/29/11 Mailing ListPatchThird Party Advisory
- hackerone.com/reports/3669637 ExploitIssue TrackingThird Party Advisory
- hackerone.com/reports/3669637 ExploitIssue TrackingThird Party Advisory
Metrics
- CVSS 3.1
-
5.9
MEDIUM
Score source: CNA
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H - EPSS — exploit probability
- 1% percentile: 54 · as of 14 November 2026
- CWE
- CWE-522
Could this be exploited in your environment?
A patch closes the known hole. A penetration test shows what an attacker can still reach — with proof.