The Patching Friars' AlmanacA calendar of CVE saints

Patron of June 19

Trivial AuthHoly card for trivial-authentication, generated with AI (Claude, Anthropic) for the Lunario dei Frati Patchatori.CC BY-NC-SA 4.0 SSH Trivial Auth INTRA, NON ROGO
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Saint Trivial Authentication, Who Let You In Without Asking

Patron of SSH clients, login prompts repainted by the server, and people who type the password before looking at who is asking.

Life

In June 2021 it emerged that PuTTY and other SSH clients carried on with a session even though they had never sent a substantive authentication response. So a hostile server could wave the user through and then show a fake password prompt, capturing the credentials at leisure. The name describes the misplaced trust: an authentication too easy to be true.

Prayer

O Saint Trivial Authentication, who opens the door before checking the key, teach our clients to distrust servers that are too kind. Amen.

How to celebrate

Celebrated by updating PuTTY and the other clients, and remembering that a password prompt can come from one who should not be asking.

A door that opens by itself is a trap that comforts you.

Technical record

Disclosed on
June 19 2021
CVE
CVE-2021-36367CVE-2021-36368CVE-2021-36369
CVSS
8.1

PuTTY through 0.75 proceeds with establishing an SSH session even if it has never sent a substantive authentication response. This makes it easier for an attacker-controlled SSH server to present a later spoofed authentication prompt (that the attacker can use to capture credential data, and use that data for purposes that are undesired by the client user).

Sources: docs.ssh-mitm.at · github.com · bugzilla.mindrot.org

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