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Copying data to a buffer that is not large enough to hold that data results in a buffer overflow. Buffer overflows occur frequently when manipulating strings [Seacord 2013b]. To prevent such errors, either limit copies through truncation or, preferably, ensure that the destination is of sufficient size to hold the character data to be copied and the null-termination character. (see See STR03-C. Do not inadvertently truncate a string.).

When strings live on the heap, this rule is a specific instance of MEM35-C. Allocate sufficient memory for an object. Because strings are represented as arrays of characters, this rule is related to both ARR30-C. Do not form or use out-of-bounds pointers or array subscripts and ARR38-C. Guarantee that library functions do not form invalid pointers.

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The gets_s() function reads, at most, one less than the number of characters specified from the stream pointed to by stdin into an array.

The C Standard, Annex K 3.5.4.1 paragraph 4 [ISO/IEC 9899:20112024], states:

No additional characters are read after a new-line character (which is discarded) or after end-of-file. The discarded new-line character does not count towards number of characters read. A null character is written immediately after the last character read into the array.

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The getline() function is similar to the fgets() function but can dynamically allocate memory for the input buffer. If passed a null pointer, getline() dynamically allocates a buffer of sufficient size to hold the input. If passed a pointer to dynamically allocated storage that is too small to hold the contents of the string, the getline() function resizes the buffer, using realloc(), rather than truncating the input. If successful, the getline() function returns the number of characters read, which can be used to determine if the input has any null characters before the newline. The getline() function works only with dynamically allocated buffers. Allocated memory must be explicitly deallocated by the caller to avoid memory leaks. (see See MEM31-C. Free dynamically allocated memory when no longer needed.).

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
void func(void) {
  int ch;
  size_t buffer_size = 32;
  char *buffer = malloc(buffer_size);
 
  if (!buffer) {
    /* Handle error */
    return;
  }

  if ((ssize_t size = getline(&buffer, &buffer_size, stdin))
        == -1) {
    /* Handle error */
  } else {
    char *p = strchr(buffer, '\n');
    if (p) {
      *p = '\0';
    } else {
      /* Newline not found; flush stdin to end of line */
      while ((ch = getchar()) != '\n' && ch != EOF)
	    ;
	  if (ch == EOF && !feof(stdin) && !ferror(stdin)) {
         /* Character resembles EOF; handle error */
      }
    }
  }
  free (buffer);
}

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In this compliant solution, characters are no longer copied to buf once index == BUFFERSIZE - 1, leaving room to null-terminate the string. The loop continues to read characters until the end of the line, the end of the file, or an error is encountered. When chars_read > index truncated == true, the input string has been truncated.

#include <stdio.h>
 
enum { BUFFERSIZE = 32 };
 
void func(void) {
  char buf[BUFFERSIZE];
  int ch;
  size_t index = 0;
  size_t chars_readbool truncated = 0false;
 
  while ((ch = getchar()) != '\n' && ch != EOF) {
    if (index < sizeof(buf) - 1) {
      buf[index++] = (char)ch;
    } else {
    chars_read++  truncated = true;
    }
  }
  buf[index] = '\0';  /* Terminate string */
  if (ch == EOF) {
    /* Handle EOF or error */
  }
  if (chars_read > indextruncated) {
    /* Handle truncation */
  }
}

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The strcpy_s() function provides additional safeguards, including accepting the size of the destination buffer as an additional argument. (see See STR07-C. Use the bounds-checking interfaces for string manipulation.).

#define __STDC_WANT_LIB_EXT1__ 1
#include <stdlib.h>
#include <string.h>
 
int main(int argc, char *argv[]) {
  /* Ensure argv[0] is not null */
  const char *const name = (argc && argv[0]) ? argv[0] : "";
  char *prog_name;
  size_t prog_size;

  prog_size = strlen(name) + 1;
  prog_name = (char *)malloc(prog_size);

  if (prog_name != NULL) {
    if (strcpy_s(prog_name, prog_size, name)) {
      /* Handle  error */
    }
  } else {
    /* Handle error */
  }
  /* ... */
  free(prog_name);
  return 0;
}

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According to the C Standard, 7.22.4.6 [ISO/IEC 9899:2011]:

The getenv function searches an environment list, provided by the host environment, for a string that matches the string pointed to by name. The set of environment names and the method for altering the environment list are implementation defined.

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In this noncompliant code example, name refers to an external string; it could have originated from user input, from the file system, or from the network. The program constructs a file name from the string in preparation for opening the file.

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#include <stdio.h>
 
void func(const char *name) {
  char filename[128];
  sprintf(filename, "%.123s.txt", name);
}

Compliant Solution (snprintf())

A more general solution is to use the snprintf() functionYou can also use * to indicate that the precision should be provided as a variadic argument:

#include <stdio.h>
 
void func(const char *name) {
  char filename[128];
  snprintfsprintf(filename, "%.*s.txt", sizeof(filename), "%s.txt" - 5, name);
}

Risk Assessment

Copying string data to a buffer that is too small to hold that data results in a buffer overflow. Attackers can exploit this condition to execute arbitrary code with the permissions of the vulnerable process.

Automated Detection

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Tool

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Version

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Checker

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Description

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LANG.MEM.BO
LANG.MEM.TO
MISC.MEM.NTERM
BADFUNC.BO.*

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Buffer overrun
Type overrun
No space for null terminator
A collection of warning classes that report uses of library functions prone to internal buffer overflows

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Compass/ROSE

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Can detect violations of the rule. However, it is unable to handle cases involving strcpy_s() or manual string copies such as the one in the first example

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STRING_OVERFLOW
STRING_SIZE
SECURE_CODING

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Fully implemented

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Fortify SCA

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5.0

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...

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Klocwork

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NNTS.TAINTED
SV.STRBO.GETS
SV.USAGERULES.UNBOUNDED_STRING_COPY

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LDRA tool suite

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489 S, 109 D, 66 X, 70 X, 71 X

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Partially implemented

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Array access out of bounds

Buffer overflow from incorrect string format specifier

Destination buffer overflow in string manipulation

Invalid use of standard library string routine

Missing null in string array

Pointer access out of bounds

Tainted NULL or non-null-terminated string

Use of dangerous standard function

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Compliant Solution (snprintf())

A more general solution is to use the snprintf() function, which also truncates name if it will not fit in the filename.

#include <stdio.h>
 
void func(const char *name) {
  char filename[128];
  int result = snprintf(filename, sizeof(filename), "%s.txt", name);
  if (result != strlen(filename) {
    /* truncation occurred */
  }
}

Risk Assessment

Copying string data to a buffer that is too small to hold that data results in a buffer overflow. Attackers can exploit this condition to execute arbitrary code with the permissions of the vulnerable process.

Automated Detection

Array access out of bounds, Buffer overflow from incorrect string format specifier, Destination buffer overflow in string manipulation, Invalid use of standard library string routine, Missing null in string array, Pointer access out of bounds, Tainted NULL or non-null-terminated string, Use of dangerous standard function 

Related Vulnerabilities

CVE-2009-1252 results from a violation of this rule. The Network Time Protocol daemon (NTPd), before versions 4.2.4p7 and 4.2.5p74, contained calls to sprintf that allow an attacker to execute arbitrary code by overflowing a character array [xorl 2009].

CVE-2009-0587 results from a violation of this rule. Before version 2.24.5, Evolution Data Server performed unchecked arithmetic operations on the length of a user-input string and used the value to allocate space for a new buffer. An attacker could thereby execute arbitrary code by inputting a long string, resulting in incorrect allocation and buffer overflow [xorl 2009].

CVE-2021-3156 results from a violation of this rule in versions of Sudo before 1.9.5p2. Due to inconsistencies on whether backslashes are escaped, vulnerable versions of Sudo enabled a user to create a heap-based buffer overflow and exploit it to execute arbitrary code. [BC].

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-122 and STR31-C

STR31-C = Union( CWE-122, list) where list =

• Buffer overflows on strings in the stack or data segment

CWE-125 and STR31-C

Independent( ARR30-C, ARR38-C, EXP39-C, INT30-C)

STR31-C = Subset( Union( ARR30-C, ARR38-C))

STR32-C = Subset( ARR38-C)

Intersection( STR31-C, CWE-125) =

• Directly reading beyond the end of a string

STR31-C – CWE-125 =

• Directly writing beyond the end of a string

CWE-125 – STR31-C =

• Reading beyond a non-string array

• Reading beyond a string using library functions

CWE-676 and STR31-C

• Independent( ENV33-C, CON33-C, STR31-C, EXP33-C, MSC30-C, ERR34-C)

• STR31-C implies that several C string copy functions, like strcpy() are dangerous.

Intersection( CWE-676, STR31-C) =

• Buffer Overflow resulting from invocation of the following dangerous functions:

• gets(), fscanf(), strcpy(), sprintf()

STR31-C – CWE-676 =

• Buffer overflow that does not involve the dangerous functions listed above.

CWE-676 - STR31-C =

• Invocation of other dangerous functions

CWE-121 and STR31-C

STR31-C = Union( CWE-121, list) where list =

• Buffer overflows on strings in the heap or data segment

CWE-123 and STR31-C

Independent(ARR30-C, ARR38-C)

STR31-C = Subset( Union( ARR30-C, ARR38-C))

STR32-C = Subset( ARR38-C)

Intersection( CWE-123, STR31-C) =

• Buffer overflow that overwrites a (unrelated) pointer with untrusted data

STR31-C – CWE-123 =

• Buffer overflow that does not overwrite a (unrelated) pointer

CWE-123 – STR31-C =

• Arbitrary writes that do not involve buffer overflows

CWE-119 and STR31-C

Independent( ARR30-C, ARR38-C, ARR32-C, INT30-C, INT31-C, EXP39-C, EXP33-C, FIO37-C)

STR31-C = Subset( Union( ARR30-C, ARR38-C))

STR32-C = Subset( ARR38-C)

CWE-119 = Union( STR31-C, list) where list =

• Out-of-bounds reads or writes that are not created by string copy operations

CWE-193 and STR31-C

Intersection( CWE-193, STR31-C) = Ø

CWE-193 involves an integer computation error (typically off-by-one), which is often a precursor to (slight) buffer overflow. However the two errors occur in different operations and are thus unrelated.

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Splint

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Related Vulnerabilities

CVE-2009-1252 results from a violation of this rule. The Network Time Protocol daemon (NTPd), before versions 4.2.4p7 and 4.2.5p74, contained calls to sprintf that allow an attacker to execute arbitrary code by overflowing a character array [xorl 2009].

CVE-2009-0587 results from a violation of this rule. Before version 2.24.5, Evolution Data Server performed unchecked arithmetic operations on the length of a user-input string and used the value to allocate space for a new buffer. An attacker could thereby execute arbitrary code by inputting a long string, resulting in incorrect allocation and buffer overflow [xorl 2009].

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

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STR03-C. Do not inadvertently truncate a string
STR07-C. Use the bounds-checking interfaces for remediation of existing string manipulation code
MSC24-C. Do not use deprecated or obsolescent functions
MEM00-C. Allocate and free memory in the same module, at the same level of abstraction
FIO34-C. Distinguish between characters read from a file and EOF or WEOF

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Using a tainted value to write to an object using a formatted input or output function [taintformatio]
Tainted strings are passed to a string copying function [taintstrcpy]

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Bibliography

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