Home > Articles > Security > Software Security

This chapter is from the book

This chapter is from the book

2.2 Common String Manipulation Errors

Programming with C-style strings, in C or C++, is error prone. The four most common errors are unbounded string copies, off-by-one errors, null termination errors, and string truncation.

Unbounded String Copies

Unbounded string copies occur when data is copied from an unbounded source to a fixed length character array (for example, when reading from standard input into a fixed length buffer). In Figure 2–1, the program reads characters from standard input using the gets() function (on line 4) into a fixed-length character array until a newline character is read or an end-of-file (EOF) condition is encountered.

Reading data from unbounded sources creates an interesting problem for a programmer. Because it is not possible to know beforehand how many characters a user will supply, it is not possible to pre-allocate an array of sufficient length. A common solution is to statically allocate an array that is much larger than needed, as shown in Figure 2–1. In this example, the programmer is only expecting the user to enter 8 characters so it is reasonable to assume that the 80-character length will not be exceeded. With friendly users, this approach works well. But with malicious users, a fixed-length character array can be easily exceeded.

It is also easy to make errors when copying and concatenating strings because the standard strcpy() and strcat() functions perform unbounded copy operations. In Figure 2–2, the command-line argument in argv[1] is copied into the fixed-length static array name (line 3). The static string " = " is concatenated after argv[1] in name (line 4). A second command-line argument (argv[2]) is concatenated after the static text (line 5). Can you tell which of

1. void main(void) {
2. char Password[80]; 
3. puts("Enter 8 character password:"); 
4.  gets(Password); ... 
5. }

Figure 2–1. Reading unbounded stream from standard input

1. int main(int argc, char *argv[]) {
2. char name[2048]; 
3. strcpy(name, argv[1]); 
4. strcat(name, " = "); 
5.  strcat(name, argv[2]); ... 
6. }

Figure 2–2. Unbounded string copy and concatenation

these string copy and concatenation operations may write outside the bounds of the statically allocated character array? The answer, of course, is all of them.

A simple solution is to test the length of the input using strlen() and dynamically allocate the memory, as shown in Figure 2–3. The call to malloc() on line 2 ensures that sufficient space is allocated to hold the command line argument argv[1] and a trailing null byte. The strdup() function can also be used on Single UNIX Specification, Version 2 compliant systems. The strdup() function accepts a pointer to a string and returns a pointer to a duplicate string. The strdup() function allocates memory for the duplicate string. This memory can be reclaimed by passing the return pointer to free().

Unbounded string copies are not limited to the C programming language. For example, if a user inputs more than 11 characters into the C++ program shown in Figure 2–4, it will result in an out-of-bounds write.

The standard object cin is an instantiation of the istream class. The istream class provides member functions to assist in reading and interpreting

 1. int main(int argc, char *argv[]) {
 2.  char *buff = (char *)malloc(strlen(argv[1])+1);
 3.  if (buff != NULL) {
 4.  strcpy(buff, argv[1]);
 5.  printf("argv[1] = %s.\n", buff);
 6.  }
 7.  else {
  /* Couldn't get the memory - recover */
 8.  }
 9.  return 0; 
10. } 

Figure 2–3. Dynamic allocation

1. #include <iostream.h>
2. int main() {
3. char buf[12]; 
4. cin >> buf; 
5. cout << "echo: " << buf << endl; 
6. }

Figure 2–4. Extracting characters from cin into a character array

input from a stream buffer. All formatted input is performed using the extraction operator operator>>. C++ also defines external operator>> overloaded functions that are global functions and not members of istream, including:

istream& operator>> (istream& is, char* str); 

This operator extracts characters and stores them in successive locations starting at the location pointed to by str. Extraction ends when the next element is either a valid white space or a null character, or if the EOF is reached. A null character is automatically appended after the extracted characters.

The extraction operation can be limited to a specified number of characters (thereby avoiding the possibility of out-of-bounds write) if the field width inherited member (ios_base::width) is set to a value greater than 0. In this case, the extraction ends one character before the count of characters extracted reaches the value of field width leaving space for the ending null character. After a call to this extraction operation the value of the field width is reset to 0.

Figure 2–5 contains a corrected version of the Figure 2–4 program that sets the field width member to the length of the character array.

1. #include <iostream.h>
2. int main() {
3. char buf[12]; 
4. cin.width(12); 
5. cin >> buf; 
6. cout << "echo: " << buf << endl; 
7. }

Figure 2–5. Extracting characters using the field width member

1. int main(int argc, char* argv[]) {
 2.   char source[10];
 3.   strcpy(source, "0123456789");
 4.   char *dest = (char *)malloc(strlen(source));
 5.   for (int i=1; i <= 11; i++) {
 6.   dest[i] = source[i];
 7.   }
 8.   dest[i] = '\0';
 9.   printf("dest = %s", dest); 
10. }

Figure 2–6. Common off-by-one defects

Off-by-One Errors

Another common problem with C-style strings are off-by-one errors. Off-by-one errors are similar to unbounded string copies in that they both involve writing outside the bounds of an array. The program shown in Figure 2–6 compiles and links cleanly under Microsoft Visual C++ 6.0 and runs without error on Windows 2000 but contains several off-by-one errors.3 Can you find all the off-by-one errors in this program?

Off-by-one errors in this simple ten-line program include the following:

  • The source character array (declared on line 2) is 10 bytes long, but strcpy() (line 3) copies 11 bytes, including a one-byte terminating null character.
  • The malloc() function (line 4) allocates memory on the heap of the length of the source string. However, the value returned by strlen() does not account for the null byte.
  • The index value i in the for loop (line 5) starts at 1, but the first position in a C array is indexed by 0.
  • The ending condition for the loop (line 5) is i <= 11 . This means the loop will iterate one more time than the programmer likely intended.
  • The assignment on line 8 also causes an out-of-bounds write.

Many of these mistakes are rookie errors, but experienced programmers may make them as well. It is easy to develop and deploy programs similar to this one that compile and run without error on most systems.

Null-Termination Errors

Another common problem with C-style strings is a failure to properly null terminate. In Figure 2–7, the static declarations for the three character arrays (a[], b[], and c[]) fail to allocate storage for the null-termination character. As a result, the strcpy() to a (line 5) writes a null character beyond the end of the array. Depending on how the compiler allocates storage, this null byte may be overwritten by the strcpy() on line 6. If this occurs, a now points to an array of 20 characters, while b points to an array of 10 characters. The strcpy() to c (line 7) fills c, causing the strcat() on line 8 to write well beyond the bounds of the array (particularly because the terminating null byte for b is overwritten by the strcpy() to c on line 7).

Null-termination errors, like the other string errors described in this chapter, are difficult to detect and can lie dormant in deployed code until a particular set of inputs causes a failure. The code in Figure 2–7 is also highly dependent on how the compiler allocates memory. When compiled on Windows XP, using Microsoft Visual C++ 2005 Beta 1, this program crashes while executing line 8. Interestingly, the same program in the same environment runs without error in the debugger.

 1. int main(int argc, char* argv[]) {
 2.   char a[16];
 3.   char b[16];
 4.   char c[32];
 5.   strcpy(a, "0123456789abcdef");
 6.   strcpy(b, "0123456789abcdef");
 7.   strcpy(c, a);
 8.   strcat(c, b);
 9.   printf("a = %s\n", a); 
10.   return 0; 
11. }

Figure 2–7. Null-termination defect

String Truncation

String truncation occurs when a destination character array is not large enough to hold the contents of a string. String truncation may occur while reading user input or copying a string and is often the result of a programmer trying to prevent a buffer overflow. While not as bas as a buffer overflow, string truncation results in a loss of data and, in some cases, can lead to software vulnerabilities. The code in Figure 2–5, for example, will truncate user input exceeding 11 characters.

String Errors without Functions

There are many standard string handling functions that are highly susceptible to error, including strcpy(), strcat(), gets(), streadd(), strecpy(), and strtrns(). Microsoft Visual Studio 2005, for example, has deprecated many of these functions as a result.

However, because C-style strings are character arrays, it is possible to perform an insecure string operation even without invoking a function. Figure 2–8 shows a sample C program that contains a defect resulting from a string copy operation but does not call any string library functions.

The defective program accepts a string argument (line 1), copies it to the buff character array (lines 5–9), and prints the contents of the buffer (line 10). The variable buff is declared as a fixed array of 128 characters (line 3). If the

 1. int main(int argc, char *argv[]) {
 2.   int i = 0;
 3.   char buff[128];
 4.   char *arg1 = argv[1];
 5.   while (arg1[i] != '\0' ) {
 6.   buff[i] = arg1[i]; 
7.   i++;
 8.   }
 9.   buff[i] = '\0'; 
10.   printf("buff = %s\n", buff); 
11. }

Figure 2–8. Defective string manipulation code

first argument to the program equals or exceeds 128 characters (remember the trailing null character), the program writes outside the bounds of the fixed-size array. Clearly, eliminating the use of dangerous functions does not guarantee your program is free from security flaws. In the following sections, you will see how these security flaws can lead to exploitable vulnerabilities.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020