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https://issues.apache.org/jira/browse/FTPSERVER-485?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16446040#comment-16446040
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Jonathan Valliere commented on FTPSERVER-485:
---------------------------------------------
If you return instantly if the lengths do not match then the attacker can
deduce the password length by increasing the length until it takes longer.
Consuming enough cycles equal to the input password length will cause the time
before return equal to a multiple of the input length which does not leak any
information.
> Timing Side Channel PasswordEncryptor
> -------------------------------------
>
> Key: FTPSERVER-485
> URL: https://issues.apache.org/jira/browse/FTPSERVER-485
> Project: FtpServer
> Issue Type: Bug
> Components: Core
> Affects Versions: 1.1.1
> Environment: tested on macOS High Sierra 10.13.4, but it is not
> relevant
> Reporter: Yannic Noller
> Assignee: Jonathan Valliere
> Priority: Major
> Labels: easyfix, pull-request-available
> Fix For: 1.1.2
>
> Original Estimate: 24h
> Remaining Estimate: 24h
>
> Dear Apache FTPServer developers,
> We have found a timing side-channel in class
> org.apache.ftpserver.usermanager.ClearTextPasswordEncryptor, method "public
> boolean matches(String passwordToCheck, String storedPassword)". This is due
> to the use of String.equals for comparison which returns as soon as a
> character does not match. This represents a timing side channel, which could
> be used by a potential attacker to obtain knowledge about the hidden secret
> password.
> Do you agree with our findings?
> A similar issue is present in method "matches" from classes
> org.apache.ftpserver.usermanager.Md5PasswordEncryptor and
> org.apache.ftpserver.usermanager.SaltedPasswordEncryptor.
> We found these classes in the latest version of your git repo:
> https://git-wip-us.apache.org/repos/asf?p=mina-ftpserver.git;a=summary
> The problem can be fixed easily by using the following safe version for
> String comparison in all three methods:
> public boolean isEqual_safe(String a, String b) {
> char a_value[] = a.toCharArray();
> char b_value[] = b.toCharArray();
> boolean unused;
> boolean matches = true;
> for (int i = 0; i < a_value.length; i++) {
> if (i < b_value.length) {
> if (a_value[i] != b_value[i]) {
> matches = false;
> } else {
> unused = true;
> }
> } else {
> unused = false;
> unused = true;
> }
> }
> return matches;
> }
> Do you agree with our patch proposal?
> Please feel free to contact us for further clarification! You can reach us by
> the following email address:
> [email protected]
> Best regards,
> Yannic Noller
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