A command injection vulnerability in an administrative Web application in Java

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Example 533 A command injection vulnerability in an administrative Web application
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String btype = requestgetParameter("backuptype"); String cmd = new String("cmdexe /K \"c:\\util\\rmanDBbat " + btype + "&&c:\\utl\\cleanupbat\"") RuntimegetRuntime()exec(cmd);
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Once again, the problem is that the program does not do any validation on the backuptype parameter read from the user Typically, the Runtimeexec() method will not execute multiple commands, but in this case, the program rst runs the cmdexe shell in order to run multiple commands with a single call to Runtimeexec() When the shell is invoked, it will happily execute multiple commands separated by two ampersands If an attacker passes a string of the form "&& del c:\\dbms\\**", the application will execute this command along with the others speci ed by the program Because of the nature of the application, it runs with the privileges necessary to interact with the database, which means that whatever command the attacker injects will run with those privileges as well Example 534 shows the code from Example 533 corrected to limit the values that are allowed to appear in the btype parameter with a whitelist that accepts only the 26 characters in the English alphabet
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Example 534 This code uses a whitelist to prevent command injection
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final static int MAXNAME = 50; final static String FILE_REGEX = "[a-zA-Z]{1,"+MAXNAME+"}"; // vanilla chars in prefix final static Pattern BACKUP_PATTERN = Patterncompile(FILE_REGEX); public void validateBackupName(String backupname) { if(backupname == null || !BACKUP_PATTERNmatcher(backupname)matches()) { throw new ValidationException("illegal backupname"); } } String btype = validateBackupName(requestgetParameter("backuptype")); String cmd = new String("cmdexe /K \"c:\\util\\rmanDBbat " + btype + "&&c:\\utl\\cleanupbat\"") RuntimegetRuntime()exec(cmd);
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Log Forging For the same reason that they are a valuable resource for system administrators and developers, logs are a target for attackers If attackers can control a value that is written to the log, they might be able to fabricate events on the system by including entire falsi ed log entries in the input they provide If an attacker is allowed to inject entries into the logs due to a lack of proper input validation, interpretation of the log les might be hindered or misdirected, diminishing their value
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5 Handling Input
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In the most benign case, an attacker might be able to insert false entries into the log le by providing the application with input that includes special characters If the log le is processed automatically, the attacker can render the le unusable by corrupting its format A more subtle attack might involve skewing the log le statistics Forged or otherwise, corrupted log les can be used to cover an attacker s tracks or even to implicate another party in the commission of a malicious act If your log contains security-relevant information, attackers might turn the log into the boy who cried wolf: If they can ll the log so full of false alarms that no one pays attention anymore, no one will be paying attention when the real attack comes Hoglund and McGraw describe a signi cantly nastier outcome, whereby a maliciously crafted piece of input triggers a vulnerability in the program responsible for processing the log les and leads to a further exploit [Hoglund and McGraw, 2004] The web application code in Example 535 attempts to read an integer value from a request object If the value fails to parse as an integer, the input is logged with an error message indicating what happened
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Example 535 A log forging vulnerability caused by unvalidated input read from an HTTP request
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String val = requestgetParameter("val"); try { int value = IntegerparseInt(val); } catch (NumberFormatException) { loginfo("Failed to parse val = " + val); }
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If a user submits the string "twenty-one" for val, the following entry is logged:
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INFO: Failed to parse val=twenty-one
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However, if an attacker submits the string "twenty-one%0a%0aINFO:
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+User+logged+out%3dbadguy", the following entry is logged:
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INFO: Failed to parse val=twenty-one INFO: User logged out=badguy
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Clearly, attackers can use this same mechanism to insert arbitrary log entries If you plan to write your own output lter, the most critical character to keep out is typically the \n (newline), but the set of important characters depends entirely on the format of the log le and the tools that will be used to examine the log It s prudent to encode anything that isn t a printable ASCII character Good logging practices sometimes get left behind when developers turn their attention to debugging Example 536 shows a real-world example of one of the many errors related to debug logging in Tomcat (Thanks to Edward Lee for pointing out this issue) In the example, an attacker can write arbitrary data to the log through the orgapachecatalinavalvesRequestDumperValve class, which is meant to be a debugging aid A request enters the class through the invoke() method, and several user-controlled elements of the request are allowed to appear unmodi ed in the log le The comment at the top of the source says:
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Implementation of a Valve that logs interesting contents from the speci ed Request (before processing) and the corresponding Response (after processing) It is especially useful in debugging problems related to headers and cookies
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(A valve is a Tomcat component similar to a Servlet lter)
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