CodeQL queries 1.25
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Name: Possibly wrong buffer size in string copy

Description: Calling 'strncpy' with the size of the source buffer as the third argument may result in a buffer overflow.

ID: cpp/bad-strncpy-size

Kind: problem

Severity: warning

Precision: medium

Query: StrncpyFlippedArgs.ql
 * @name Possibly wrong buffer size in string copy
 * @description Calling 'strncpy' with the size of the source buffer
 *              as the third argument may result in a buffer overflow.
 * @kind problem
 * @problem.severity warning
 * @precision medium
 * @id cpp/bad-strncpy-size
 * @tags reliability
 *       correctness
 *       security
 *       external/cwe/cwe-676
 *       external/cwe/cwe-119
 *       external/cwe/cwe-251

import cpp
import Buffer
private import semmle.code.cpp.valuenumbering.GlobalValueNumbering

predicate isSizePlus(Expr e, BufferSizeExpr baseSize, int plus) {
  // baseSize
  e = baseSize and plus = 0
  exists(AddExpr ae, Expr operand1, Expr operand2, int plusSub |
    // baseSize + n or n + baseSize
    ae = e and
    operand1 = ae.getAnOperand() and
    operand2 = ae.getAnOperand() and
    operand1 != operand2 and
    isSizePlus(operand1, baseSize, plusSub) and
    plus = plusSub + operand2.getValue().toInt()
  exists(SubExpr se, int plusSub |
    // baseSize - n
    se = e and
    isSizePlus(se.getLeftOperand(), baseSize, plusSub) and
    plus = plusSub - se.getRightOperand().getValue().toInt()

predicate strncpyFunction(Function f, int argDest, int argSrc, int argLimit) {
  exists(string name | name = f.getName() |
      name = "strcpy_s" or // strcpy_s(dst, max_amount, src)
      name = "wcscpy_s" or // wcscpy_s(dst, max_amount, src)
      name = "_mbscpy_s" // _mbscpy_s(dst, max_amount, src)
    ) and
    argDest = 0 and
    argSrc = 2 and
    argLimit = 1
      name = "strncpy" or // strncpy(dst, src, max_amount)
      name = "strncpy_l" or // strncpy_l(dst, src, max_amount, locale)
      name = "wcsncpy" or // wcsncpy(dst, src, max_amount)
      name = "_wcsncpy_l" or // _wcsncpy_l(dst, src, max_amount, locale)
      name = "_mbsncpy" or // _mbsncpy(dst, src, max_amount)
      name = "_mbsncpy_l" // _mbsncpy_l(dst, src, max_amount, locale)
    ) and
    argDest = 0 and
    argSrc = 1 and
    argLimit = 2

string nthString(int num) {
  num = 0 and
  result = "first"
  num = 1 and
  result = "second"
  num = 2 and
  result = "third"

 * Gets the size of the expression, if it is initialized
 * with a fixed size array.
int arrayExprFixedSize(Expr e) {
  result = e.getUnspecifiedType().(ArrayType).getSize()
  result = e.(NewArrayExpr).getAllocatedType().(ArrayType).getSize()
  exists(SsaDefinition def, LocalVariable v |
    not e.getUnspecifiedType() instanceof ArrayType and
    e = def.getAUse(v) and
    result = arrayExprFixedSize(def.getDefiningValue(v))

  Function f, FunctionCall fc, int argDest, int argSrc, int argLimit, int charSize, Access copyDest,
  Access copySource, string name, string nth
  f = fc.getTarget() and
  strncpyFunction(f, argDest, argSrc, argLimit) and
  copyDest = fc.getArgument(argDest) and
  copySource = fc.getArgument(argSrc) and
  // Some of the functions operate on a larger char type, like `wchar_t`, so we
  // need to take this into account in the fixed size case.
  charSize = f.getParameter(argDest).getUnspecifiedType().(PointerType).getBaseType().getSize() and
    if exists(fc.getArgument(argLimit).getValue().toInt())
      // Fixed sized case
      exists(int size |
        size = arrayExprFixedSize(copyDest) and
        size < charSize * fc.getArgument(argLimit).getValue().toInt() and
        size != 0 // if the array has zero size, something special is going on
      exists(Access takenSizeOf, BufferSizeExpr sizeExpr, int plus |
        // Variable sized case
        sizeExpr = fc.getArgument(argLimit).getAChild*() and
        isSizePlus(fc.getArgument(argLimit), sizeExpr, plus) and
        plus >= 0 and
        takenSizeOf = sizeExpr.getArg() and
        globalValueNumber(copySource) = globalValueNumber(takenSizeOf) and // e.g. strncpy(x, y, strlen(y))
        globalValueNumber(copyDest) != globalValueNumber(takenSizeOf) // e.g. strncpy(y, y, strlen(y))
  ) and
  name = fc.getTarget().getName() and
  nth = nthString(argLimit)
select fc,
  "Potentially unsafe call to " + name + "; " + nth + " argument should be size of destination."

The standard library function strncpy copies a source string to a destination buffer. The third argument defines the maximum number of characters to copy and should be less than or equal to the size of the destination buffer. Calls of the form strncpy(dest, src, strlen(src)) or strncpy(dest, src, sizeof(src)) incorrectly set the third argument to the size of the source buffer. Executing a call of this type may cause a buffer overflow. Buffer overflows can lead to anything from a segmentation fault to a security vulnerability.


Check the highlighted function calls carefully, and ensure that the size parameter is derived from the size of the destination buffer, not the source buffer.


strncpy(dest, src, sizeof(src)); //wrong: size of dest should be used
strncpy(dest, src, strlen(src)); //wrong: size of dest should be used

  • strncpy.
  • I. Gerg. An Overview and Example of the Buffer-Overflow Exploit. IANewsletter vol 7 no 4. 2005.
  • M. Donaldson. Inside the Buffer Overflow Attack: Mechanism, Method & Prevention. SANS Institute InfoSec Reading Room. 2002.
  • Common Weakness Enumeration: CWE-676.
  • Common Weakness Enumeration: CWE-119.
  • Common Weakness Enumeration: CWE-251.