Common Attack Pattern Enumeration and Classification

Common Attack Pattern Enumeration and Classification

67

Very High

The format string argument of the Syslog function can be tainted with user supplid data.

High

• Injection

If the source code of the application is available, an attacker can use static analysis tools to spot a syslog vulnerability (a simple grep may also work).

If the source code is not available, automated tools such as Fuzzer and advanced Web Scanner can be used. If the tool supplied data reaches the syslog's format string argument, the application under scrutiny may have unexpected behavior.

If the source code is not available, a more complexe technique involve the use of library and system call tracer combined with the use of binary auditing tool such as IDA Pro. Reverse Engineering technique can be used to find format string vulnerability in the syslog function call. For instance it is possible to get the address of the buffer that is later used as the format string when reading data

The code should be reviewed for misuse of the Syslog function call. Manual or automated code review can be used. The reviewer needs to ensure that all format string functions are passed a static string which cannot be controlled by the user and that the proper number of arguments are always sent to that function as well. If at all possible, do not use the %n operator in format strings. The following code shows a correct usage of Syslog(): ... syslog(LOG_ERR, "%s", cmdBuf); ... The following code shows a vulnerable usage of Syslog(): ... syslog(LOG_ERR, cmdBuf); // the buffer cmdBuff is taking user supplied data. ...

• Run Arbitrary Code
• Denial of Service
• Privilege Escalation
• Data Modification

Format String Vulnerabilities

When you get right down to it, format strings vulnerabilities are relatively simple in nature. An API call that takes a format string (i.e., %s) can be exploited when the format string argument is controlled by a remote attacker. Unfortunately, the problem exists mainly because of laziness on the part of the programmer. However, the problem is so simple that it can be detected automatically using simple code scanners. Thus, once the format string vulnerability was publicized in the late 1990s, it was rapidly hunted down and eliminated in most software.

Here is a trivial function that suffers from a format string problem:

void some_func(char *c)
{
   printf(c);
}

Untrusted user supplied data is the injection vector.

The maliciously crafted data can read from the stack if passed to the Syslog function as a format String.

The signature of the syslog function is as following : "void syslog(int priority, const char *format, ...);" A vulnerable usage would be : "syslog(LOG_ERR, cmdBuf);" where cmdBuf is a user controlled data which leads to a format string vulnerability. The activation zone is the format String argument which accepts user supplied data.

The impacts of this attack can be execution of arbitrary code. Execution of arbitary code can lead to many problems such as corruption of data, unauthorized access, etc.

Choose a language which is not subject to this flaw.

Do not use the Syslog() in your implementation.

Use manual or automated code review to spot potential format string vulnerability in functions such as Syslog(), Vsyslog(), snprintf(), etc.

• Reluctance to Trust

• Verify that input is of a limited size.
• If the message is coming from an outside source, check for %s type parameters and ensure that bounds will not be overwritten.
• Don't use text from an outside source as a format string.

Penetration

Exploitation

G. Hoglund and G. McGraw. Exploiting Software: How to Break Code. Addison-Wesley, February 2004.

CWE – Buffer Errors

Exploiting Format String Vulnerabilities, scut / team teso, http://doc.bughunter.net/format-string/exploit-fs.html

Halvar Flake, “Auditing binaries for security vulnerabilities”, http://www.blackhat.com/presentations/bh-europe- 00/HalvarFlake/HalvarFlake.ppt

Fortify Taxonomy of Vulnerabilities : http://vulncat.fortifysoftware.com/1/FS.html - Fortify Software (www.fortifysoftware.com)

Syslog man page : http://www.rt.com/man/syslog.3.html