Format string vulnerabilities in C programs have been studied extensively in recent years. The focus has been on the execution of arbitrary code, although other effects are possible. It has now to the attention of the security community that numerous Perl code segments suffer from the same format string vulnerabilities as C programs.
For many other programming languages, format string vulnerabilities are also possible. Given the lack of attention to other languages, it is highly likely that a large number of applications have these issues
even when they have been audited for other types of vulnerabilities.
For any language that supports format strings, applications that are written in that language could be subject to format string vulnerabilities. The impact is specific to the behaviors that are supported by the format strings, and their interaction with language internals.
In recent days, Jack Louis of Dyad Security reported on a format string issue in a Webmin application that is written in Perl (CVE-2005-3912). He further showed how a problem in the Perl interpreter itself could allow modification of memory and code execution in vulnerable apps (CVE-2005-3962).
This paper focuses on format string vulnerabilities at the application level. It must be emphasized that even if the interpreter problem is fixed, the impact of format string vulnerabilities will only be
reduced, not completely eliminated.
Excluding problems within the interpreter itself, Perl application format string vulnerabilities can allow denials of service (primarily memory or disk consumption), information leaks, and modification of program variables in ways that may have security implications.
In particular, the sprintf() and printf() functions in Perl can be abused if an attacker can control the contents of the format string.
Since similar functions are used in C, it is possible that these functions will be used more frequently by C programmers who are new to Perl.
It should be noted that Perl's taint checker does not catch some variants of format string attacks. The behavior differs in Perl 5.004 and 5.6.1 (5.6.1 can identify additional dangerous inputs). However, modifying the taint checker itself may not be feasible or even appropriate.
2. Relevant History and Credits:
Jean-loup Gailly independently discovered and reported a format string problem in a Perl application on September 26, 2002 . Arjan de Vet included format strings and the taint checker in a presentation at YAPC:Europe in August 2001 . Steve Christey mentioned the possibility of format strings in PHP applications on April 3, 2003  and format strings in interpreted languages during a lightning round talk on vulnerability research gaps at CanSecWest in April 2004 .
Jack Louis reported on a format string problem in a Perl application on November 29, 2005 , following it with a description of an integer wrap within Perl itself that was exploitable via format strings on December 1, 2005 . The most notable early research on format strings in C was performed by Tim Newsham in September 2000 .
3. Attack and Impact Details:
Following are some of the more dangerous specifiers, along with their implications.
1) Memory or disk consumption
The "%s" specifier, and others that allow field widths to be defined, can be used to consume a large amount of CPU, memory, and/or disk, e.g. "%99999s". ("%999999999s" is sufficient to consume a gigabyte of memory or disk, but it has been reported that it can also cause the Perl program to crash.)
2) Modification of variables
Using the "%n" specifier, the attacker can modify the values of certain variables that are provided as arguments to print/sprintf, possibly altering program behavior in ways that have security implications. The variable is modified to a number, generally a 0.
The implications of this problem depend on how the program uses the variables.
Consider the following pseudo-code for an authentication routine:
If $input is "%10s", then str is formatted with up to 10 spaces of padding and $a is not modified; but if $input is "%n", then $a is changed to 0, and the attacker effectively bypasses the check for authentication.
3) Argument shifting
The "%p" specifier formats a pointer for the next argument to be processed in the call to *printf. This effectively misdirects or shifts all remaining arguments to different format specifiers than the programmer intended. The impact depends on the specific bug.
Argument shifting could also be used to bypass cleansing operations for other vulnerabilities, by shifting uncleansed values into variables that contain cleansed values.
4) Altering intended outputs
Any format specifier can alter the intended format of structured output. This in turn could corrupt files or enable the exploitation of vulnerabilities in other applications that process such output. For example, the '%p' specifier, which prints out a pointer value, could be used to generate integer values that exceed the expected range of inputs.
$index = GetUserInput();
if (($index > 32) || ($index < 0))
print STDERR "Error: Index must be between 0 and 32.";
($sec,$min,$hour,$mday,$mon,$year) = gmtime(time);
printf DATABASE, "$index %4d/%02d/%02d %02d:%02d:%02d\n",
$year+1900, $mon+1, $mday;
If $index is "1", then the result might be:
1 2002/10/01 06:58:42
But if $index is "%p", the error condition is not detected (since the string evaluates to 0), and the result would be:
130690 10/01/06 58:42:00
Here, not only does the 'index' value exceed the maximum of 32, but all the other values are wrong! This is because the %p was used to format a pointer to the $year+1900 expression. All the other arguments were then misdirected, and applied to the wrong format specifier. Thus the month value is formatted as the year, the seconds value is formatted as the minute, etc.
5) Bypassing cleansing operations
Cleansing operations that remove spaces could be tricked by using "%2s" or other format specifiers that generate spaces. Programs may try to remove spaces when passing arguments to commands, or formatting data.
Here's one example:
while ($file = readdir(DIR))
if ($file =~ /\s/)
print STDERR "Warning: '$file' has spaces, replacing with _\n";
$file =~ s/\s/_/g;
if ($file =~ /^-(fiprR)+$/)
print STDERR "Warning: '$file' matches switches for /bin/cp!\n";
# skip this one.
$backup = sprintf("$file.bak"); # C programmers might do this
system("/bin/cp $file $backup"); # but this *is* just an example
If $file is set to "-R%2ssubdir", then the check for "dangerous switches" would fail, and the resulting system call would be:
system("/bin/cp -R subdir subdir.bak");
6) Other Attack Scenarios
Some feasible attack scenarios involve Perl programs that generate log messages or reports:
- File names containing format specifiers could alter which files are processed
- IP addresses whose DNS reverse lookup includes format strings could be returned as the result of gethostbyname().
Log files could be filled easily using "%999s" style strings.
The possibility of CRLF injection was theorized, but a casual investigation was not successful.
4. Some Discussion on Format Strings and the Taint Checker: In 5.004:
The taint checker apparently does not flag filenames as tainted (e.g. as obtained from the readdir() function). Presumably, other types of "indirect input" may not be tainted. However, it does identify more direct sources of input such as stdin and environment variables.
Filenames are tainted, and the taint checker terminates the program. While the program is safe from exploitation through dangerous calls, there is still a denial of service, which could be a problem with critical code that is expected to fully complete its task, such as a log processing program (although the programmer should take the possibility of failure into account while running in taint mode anyway!)
Note that the taint checker does not exit until a *printf-tainted variable is passed to a dangerous call such as system(). So, if the program is not tested with specifiers such as '%n' (which modifies an argument to *printf), then the taint check may not be discovered.
Attacks such as resource consumption and data format modification will still work; however, changing the taint checker to exit as soon as the printf/sprintf is encountered could break existing programs.
This is a factor though: "testing" sprintf/printf with normal file names won't directly trigger the taint checker, unless %n is actually included in the filename; so, if the programmer tests the Perl code, but does not include the '%n' option, they won't necessarily find the taint error. However, a later input with '%n' could cause the program to halt unexpectedly due to the taint error.
Note: the taint checker doesn't complain when system() is called with arguments in the following fashion:
Statement From Perl Language Developer:
These issues do not represent a substantial security hole in perl itself. Future versions of perl may extend tainting checks to format strings, or just to certain aspects of formats (such as %n).
5. Real-World Vulnerable Program Examples:
In 2002, at least 3 different Perl programs were found vulnerable to format string attacks:
3) WASD OpenVMS web server
ftplogcheck is a program used for processing wu-ftpd logs and generating statistics.
It is not part of the wu-ftp distribution.
One portion of ftplogcheck report lists which files were uploaded to the server by the "anonymous" user.
The code is:
printf REPORT "$time $host $filesize $filename $name\n";
If the wu-ftp server is configured to allow uploads from anonymous users, then attackers can upload files whose names contain malicious format strings, which are then fed into the $filename variable.
In this case, the attacker could consume memory or disk space by causing an extremely large report to be generated (if $filename is "%999999s") or misrepresent the name of the file that has been uploaded (if $filename is "word1%1sword2", which would generate the string "word1 word2").
This script processes NoCeM notices, which can be used by the server to process third-party, PGP-signed article cancellation notices.
In do_nocem(), a call is made to sprintf() after inserting the values of the $nid and $issuer variables into the format string:
The value of $nid is obtained from a "notice-id" news article header.
It is not sanity-checked; therefore, malicious format strings can be inserted into this sprintf() call. The $issuer variable is obtained from an "issuer" header, but this value must be allowed by the perl-nocem control file. It may be possible to use a wildcard character and match any issuer.
The $nr variable contains the total number of articles to be canceled, and the $diff variable attempts to measure the amount of time required to cancel the articles, generally 0.01 due to an apparent bug.
According to the developer, the scope of this attack is limited: "the message is printed only after the nocem notice has been PGP-verified, so the attacker must be one of the trusted cancellers."
Assume that 10 articles are to be canceled ($nr = 10) and $diff is 0.01.
With a $nid (Notice-ID header) of "NID" and a $issuer (Issuer header) of "ISSUER@example.com", the log message output would be:
processed notice NID by ISSUER@example.com (10 ids, 0.01000 s, 1000.0/s)
With a Notice-ID of "%9999999s", a large amount of memory and/or log file space is consumed:
Modification of the $diff variable:
With a notice-id of "%n", perl-nocem changes the $diff variable to 17 (the length of the "processed notice " substring), as opposed to its original value (typically 0.01). This changes the error message to misrepresent how long it took to cancel the articles:
processed notice by ISSUER@example.com (10 ids, 1000.00000 s, 0.0/s)
(notice the double-space in "notice by" where the notice-id would be).
Note that if perl-nocem had used a format string that began with the "$nid" variable (e.g. "$nid notice processed" instead of "processed notice $nid"), then the $diff variable would have been set to 0, and the "$nr / $diff" expression would have caused the program to exit with a division-by-zero error.
Other output modifications:
With a notice-id of "%p", the resulting log message would be like:
processed notice 130498 by [ISSUER] (10 ids, 0.50000 s, 0.0/s)
where the "130498" is an incorrect notice id.
[This is] not easily exploitable, the message is printed only after the nocem notice has been PGP-verified, so the attacker must be one of the trusted cancellers.
Jean-loup Gailly suggested the presence of a format string issue in the WASD OpenVMS web server .
A sample program, PerlRTE_example1.pl, contained the following vulnerable code:
where the $name variable can be altered by an attacker to contain format strings (e.g. through a query string).
6. Avoiding Format String Vulnerabilities During Development:
When writing Perl programs, follow these guidelines.
1) Use constant strings for formatting.
2) Do not feed Perl variables directly into format strings, e.g. "$bad %10s" or $bad . " %10s"
3) Where possible, avoid using printf and sprintf
4) If absolutely necessary, consider quoting the "%" specifier before including a user-controlled input into a format string.
5) Run your program with taint checking enabled, which can help protect against many of the problems identified here.
Notes on Detecting Vulnerabilities in Source Code:
Detection of suspicious code is slightly more difficult than it is for C code. Constant strings can contain Perl entities such as variables or references, which are inserted into the string before it is passed
$fmt = <USER_INPUT>;
printf("THIS IS A POTENTIALLY VULNERABLE $fmt FORMAT STRING\n");
7. Suggestions for Further Research:
This paper is not an exhaustive work, so further research is needed.
Software developers and vulnerability researchers are encouraged to actively search for format string issues in all programming languages, not just C and Perl.
Suggested research topics include:
- for each programming language, identify and publicize all built in or common library functions that use format strings.
- extend source and binary code analysis tools to look for improper use of these functions
- audit individual applications that have been previously deemed free of obvious vulnerabilities, with a focus on format strings
- study the implications of interactions between a high-level language and the language it is implemented in. For example, there may be format string analogues to the problems of the null byte in Perl and PHP programs and their interaction with the underlying C code.
- further examination of the taint checker (see below)
Note: in the author's limited experience, format string vulnerabilities do not appear as frequently in Perl or PHP applications as they do in C programs. However, more focused efforts are needed before this suspicion can be confirmed.
8. Demonstration Programs:
These programs demonstrate some the problems described above.
1) Argument modification
# when run with taint checking (-T), this seems to properly barf about
# dependency errors (try a "clean" format string like "5s%s%s" vs. a
# dirty one with a "%n" in it).
************** End Sample Vulnerable Program **************
********* Sample 2 **********
# Create a directory that contains files with these names:
# This was gleaned from some real-world code, but the print was
# changed to printf.
# Change what filenames are processed via format strings in
# the filenames, such as a file named "%p%n"
# You can "erase" a filename by using '%s', and having this "blank"
# filename could throw off the argument count to system or exec calls,
# which could alter behavior. Consider a backup command like
# exec("/bin/cp", file1, file2) where file1 can be "blanked" out
# Similarly, you could "erase" portions of a filename with "%n" or
# "%s". The filename ABC.TXT would be equivalent to ABC%n.%nTXT
# You can create very long filenames by using '%999s' (for example).
while ($file = readdir(DIR))
print "Real filename: $file\n";
printf "Filename in format string: $file\n\n";
2) Misuse of format string in log processing, for which many Perl programs have been written. Could cause larger strings than expected to be written to files or sent to processes; code that depends on well-formatted input from the program may be subject to buffer overflow or other issues.
I've seen several programs that do something like this:
******** End Sample 2 ************
10. Disclosure History:
Jun 10, 2002 - Theorized issue; began discovery and investigation; search for potentially vulnerable programs initially unsuccessful
Sep 26, 2002 - Jean-loup Gailly (firstname.lastname@example.org) posts Perl format string problem in OpenVMS
Sep 28, 2002 - deeper investigation into format specifiers, other vulnerable programs
Sep 30, 2002 - more writing on security advisory; investigated whether taint checker did "the right thing"
Sep 30, 2002 - tried to find a way to report a security vuln to Perl developers (in case taint issue is a Perl bug, and to consult on possibility of buffer overflows).
Registered to site, only to be told by a web page to email my report to a certain address. Left out details in the email because I had no idea who would be viewing the report at that address. This turned out to be a good decision, as that post has been publicly archived.
Sep 30, 2002 - investigated taint checker issues, %p
Sep 30, 2002 - initial response from Perl contact (within 50 minutes) saying it was OK to post details to that address, gave an alternate POC just in case.
Oct 1, 2002 - provided Perl developer list with details
Oct 1, 2002 - notified CERT/CC
Oct 8, 2002 - sent followup inquiry to Perl developer list and primary Perl POC; haven't heard anything back, do they plan to modify the taint checker?
Oct 10, 2002 - asked a colleague to try contacting Perl developers
Oct 11, 2002 - response from email@example.com saying that message had not been forwarded to the mailing list. Replied to various points; suggested possible statement on taint checker.
Oct 17, 2002 - Statement modified and approved from firstname.lastname@example.org
Nov 1, 2002 - notified Mark.Daniel@wasd.vsm.com.au (WASD developer) http://wasd.vsm.com.au/ht_root/src/perl/readmore.html
Nov 1, 2002 - more investigation into perl-nocem
Nov 1, 2002 - notified perl-nocem author, Marco d'Itri (email@example.com)
Nov 3, 2002 - received acknowledgement from perl-nocem author
Nov 3, 2002 - received acknowledgement from WASD author, approval to release
Dec 5, 2002 - inquiry to perl-nocem author; are patches available?
Dec 5, 2002 - perl-nocem patches had been made
Dec 5, 2002 - investigation of ftplogcheck
Dec 19, 2002 - refined advisory, cleaned up demonstration code
Nov 29, 2005 - posted to DailyDave
Dec 1, 2005 - Jack Louis releases Perl integer wrap advisory
Dec 2, 2005 - further edits, table of contents, enhancement of argument shifting
Dec 2, 2005 - posted to Bugtraq, Full-Disclosure