[FTPSERVER-486] Timing Side Channel StringUtils

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 1.1.1
• Fix Version/s: 1.1.5
• Component/s: Core
• Labels: easyfix pull-request-available
• Environment: test on macOS High Sierra 10.13.4, but not relevant
• Flags: Important
• Assignee: Emmanuel Lécharny
• Reporter: Yannic Noller
• Created: 17/Apr/18 08:14
• Updated: 13/Mar/22 23:56

Description

Dear Apache FTPServer developers,

We have found a timing side-channel in class org.apache.ftpserver.util.StringUtils, method "public final static String pad(String src, char padChar, boolean rightPad, int totalLength)". This method leaks the necessary padding in a timing side channel, from which a potential attacker could obtain the length of the src String. In your project this method is used to add padding to a username, hence, a potential attacker could obtain the length of a given username, which might be used for further attacks.
Do you agree with our findings?

We found this class in the latest version of your git repo: https://git-wip-us.apache.org/repos/asf?p=mina-ftpserver.git;a=summary

As a secure fix we would recommend to use a variant of the equals method, which does iterate the complete strings in the case of the same string lengths, independent from whether they do match or not:

public final static String pad_safe(String src, char padChar, boolean rightPad, int totalLength) {

int srcLength = src.length();
if (srcLength >= totalLength)

{ return src; }

int padLength = totalLength - srcLength;
StringBuilder sb = new StringBuilder(padLength);
for (int i = 0; i < totalLength; ++i) {
if (i < padLength)

{ sb.append(padChar); }

else

{ sb.append(""); }

}

if (rightPad)

{ return src + sb.toString(); }

else

{ return sb.toString() + src; }

}

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:
[e-mail]

Best regards,
Yannic Noller