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https://issues.apache.org/jira/browse/LANG-806?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13439377#comment-13439377
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Henri Yandell commented on LANG-806:
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Please feel free to take a look :)
> RandomStringUtils can enter infinite loop if chosen char does not meet
> letter/digit requirements
> ------------------------------------------------------------------------------------------------
>
> Key: LANG-806
> URL: https://issues.apache.org/jira/browse/LANG-806
> Project: Commons Lang
> Issue Type: Bug
> Affects Versions: 2.6, 3.1
> Reporter: Sebb
> Assignee: Sebb
>
> An infinite loop can result if the selection process never returns a char
> that passes the validation test.
> This can occur if the subset specified by the start and end characters does
> not contain any valid characters.
> For example:
> RandomStringUtils.random(3, 5, 10, true, true); // 1
> RandomStringUtils.random(3, 56192, 56319, false, false); // 2
> There's also the case where only surrogates are allowed, but the buffer is
> not an even number of characters, for example:
> RandomStringUtils.random(3, 56320, 57343, false, false); // 3
> The second example is easy to detect, but in general it does not seem easy to
> determine in advance if the subset contains any valid characters - except by
> evaluating all the possible char values. This would be expensive if the
> subset range is large.
> One possibility is to count the total number of loops (or retries), and throw
> an error if it exceeds a given value. Or count the number of consecutive
> retries.
> In both cases the threshold value must be set high enough to allow for the
> cases where the allowable char range contains only a small proportion of
> valid characters.
> In the case of digits only, the default allowable range is currently set to
> digits + letters, so the proportion of valid chars is 10/90 i.e. approx 11%.
> A minimum proportion of 1% or 0.1% would be necessary to reduce the number of
> false positives.
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