🔒 Add ScramAuthenticator#max_iterations (backports #654)#656
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nevans merged 4 commits intov0.5-stablefrom Apr 22, 2026
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🔒 Add ScramAuthenticator#max_iterations (backports #654)#656nevans merged 4 commits intov0.5-stablefrom
ScramAuthenticator#max_iterations (backports #654)#656nevans merged 4 commits intov0.5-stablefrom
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`SASL::ScramAlgorithm` and `SASL::ScramAuthenticator` use `salted_password` _at least_ twice: once to compute `client_key` and once to compute `server_key`. It is actually used more than that, since `client_key` and `server_key` are also used multiple times each. Computing `salted_password` is _intentionally_ computationally expensive, so it should be cached. Although `client_key` and `server_key` are far less computationally expensive, they _are_ used multiple times, so they are memoized too. Ultimately, we _could_ memoize most of the methods in `ScramAlgorithm`, but I've decided to keep it simple by only memoizing these three.
`max_iterations` enforces a maximum allowed iteration count. A higher number iterations will raise an error. As noted in RFC5802 §9: >>> A hostile server can perform a computational denial-of-service attack on clients by sending a big iteration count value. Note that `OpenSSL::KDF.pbkdf2_hmac` is implemented by a blocking C function, and cannot be interrupted by +Timeout+ or `Thread.raise`. And (as of v4.0 of the +openssl+ gem) it keeps the Global VM lock, so other ruby threads will not be able to run either. The default value is `2³¹ - 1`, the maximum signed 32-bit integer. This is large enough for the computation to take several minutes, and insufficient protection against hostile servers. _To prevent a denial of service attack,_ this must be set to a safe value, based on user's hardware and version of OpenSSL. _It is the user's responsibility_ to enforce minimum and maximum iteration counts that are appropriate for their security context.
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ScramAuthenticator#max_iterationsScramAuthenticator#max_iterations (backports #654)
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This backports #654 to
v0.5-stable.