Improper Certificate Validation vulnerability in hackney_ssl in benoitc hackney allows an on-path…

Why it is Yellow

• CVSS 9.1 critical
• Remotely exploitable without logging in

References

• https://cna.erlef.org/cves/CVE-2026-115693.html
• https://github.com/benoitc/hackney/commit/915eb5737e9c489d8fc22eec8cdae30e9b027be4
• https://github.com/benoitc/hackney/commit/d8a9afcf5c3849904274e2eba1df56c63d1a1a0c
• https://github.com/benoitc/hackney/pull/978
• https://github.com/benoitc/hackney/releases/tag/4.9.0
• https://osv.dev/vulnerability/EEF-CVE-2026-115693

Original description

Improper Certificate Validation vulnerability in hackney_ssl in benoitc hackney allows an on-path attacker to impersonate a server pinned to a private certificate authority via a certificate chain that ends in a publicly trusted root. hackney adds {partial_chain, fun hackney_ssl:partial_chain/1} to every verify_peer configuration. The callback returns any certificate in the peer chain whose public key matches a root in the bundled certifi store as the trust anchor, and OTP honours that callback over the configured cacerts. hackney_ssl:merge_ssl_opts/3 kept the callback when the caller replaced the trust store with cacerts or cacertfile in ssl_options, so a connection pinned to a private CA still accepted any chain that carried a publicly trusted root. The hostname check still applies, so the attacker needs a publicly trusted certificate for the pinned hostname. This issue affects hackney: from 1.24.0 before 4.9.0.