[quicwg/base-drafts] Spoofed retry token attack on IP authentication (#2394)

[MikkelFJ <[e-mail]>]

If a Man on the Side attacker (A) is able to observe a Retry token from server (S) intended for a legitimate client (C), then (A) can either race (C) to use the token, or hope that the token can be reused within a limited time frame and use the token if if (C) has already done so.

(A) can now start a new apparently legitimate connection by spoofing the source address to that of (C) since it can observe packets sent to that address.

If (S) issued the Retry token in the hope of verifying that it speaks to a trusted machine on the network, then (A) might be able escalate privilege to that of (C).

The defence would be for (S) to not authenticate (C) based solely on its IP, but the Retry facility might lead some operators to believe that a Retry is a reasonably safe identity mechanism.

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