What’s Vulnerable › Vulnerabilities

CVE-2026-6253: proxy credentials leak over redirect-to proxy

• Severity: medium (CVSS 5.9 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.7% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-05-13 · updated 2026-09-15
• Weakness: CWE-522

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

• curl: from 7.14.1 before 8.20.0 — fixed in 8.20.0

How to fix

Upgrade to a fixed version: curl 8.20.0. Check the vendor advisory for mitigations.

References

• https://curl.se/docs/CVE-2026-6253.html
• https://curl.se/docs/CVE-2026-6253.json
• https://hackerone.com/reports/3669637
• http://www.openwall.com/lists/oss-security/2026/04/29/11
• https://hackerone.com/reports/3669637

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