What’s Vulnerable › Vulnerabilities

CVE-2026-4873: connection reuse ignores TLS requirement

• Severity: medium (CVSS 5.9 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-05-13 · updated 2026-09-15
• Weakness: CWE-319

Description

A vulnerability exists where a connection requiring TLS incorrectly reuses an existing unencrypted connection from the same connection pool. If an initial transfer is made in clear-text (via IMAP, SMTP, or POP3), a subsequent request to that same host bypasses the TLS requirement and instead transmit data unencrypted.

Affected products

• curl: from 7.20.0 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-2026-4873.html
• https://curl.se/docs/CVE-2026-4873.json
• https://hackerone.com/reports/3621851
• http://www.openwall.com/lists/oss-security/2026/04/29/7

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