The software performs a calculation that can produce an integer overflow or wraparound, when the logic assumes that the resulting value will always be larger than the original value. This can introduce other weaknesses when the calculation is used for resource management or execution control.
Extended Description
An integer overflow or wraparound occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may wrap to become a very small or negative number. While this may be intended behavior in circumstances that rely on wrapping, it can have security consequences if the wrap is unexpected. This is especially the case if the integer overflow can be triggered using user-supplied inputs. This becomes security-critical when the result is used to control looping, make a security decision, or determine the offset or size in behaviors such as memory allocation, copying, concatenation, etc.
Terminology Notes
"Integer overflow" is sometimes used to cover several types of errors,
including signedness errors, or buffer overflows that involve manipulation
of integer data types instead of characters. Part of the confusion results
from the fact that 0xffffffff is -1 in a signed context. Other confusion
also arises because of the role that integer overflows have in
chains.
This weakness will generally lead to undefined behavior and therefore
crashes. In the case of overflows involving loop index variables, the
likelihood of infinite loops is also high.
Integrity
Technical Impact: Modify memory
If the value in question is important to data (as opposed to flow),
simple data corruption has occurred. Also, if the wrap around results in
other conditions such as buffer overflows, further memory corruption may
occur.
Confidentiality
Availability
Access Control
Technical Impact: Execute unauthorized code or
commands; Bypass protection
mechanism
This weakness can sometimes trigger buffer overflows which can be used
to execute arbitrary code. This is usually outside the scope of a
program's implicit security policy.
Likelihood of Exploit
Medium
Detection Methods
Automated Static Analysis
This weakness can often be detected using automated static analysis
tools. Many modern tools use data flow analysis or constraint-based
techniques to minimize the number of false positives.
Effectiveness: High
Black Box
Sometimes, evidence of this weakness can be detected using dynamic
tools and techniques that interact with the software using large test
suites with many diverse inputs, such as fuzz testing (fuzzing),
robustness testing, and fault injection. The software's operation may
slow down, but it should not become unstable, crash, or generate
incorrect results.
Effectiveness: Moderate
Without visibility into the code, black box methods may not be able to
sufficiently distinguish this weakness from others, requiring follow-up
manual methods to diagnose the underlying problem.
Manual Analysis
This weakness can be detected using tools and techniques that require
manual (human) analysis, such as penetration testing, threat modeling,
and interactive tools that allow the tester to record and modify an
active session.
Specifically, manual static analysis is useful for evaluating the correctness of allocation calculations. This can be useful for detecting overflow conditions (CWE-190) or similar weaknesses that might have serious security impacts on the program.
Effectiveness: High
These may be more effective than strictly automated techniques. This
is especially the case with weaknesses that are related to design and
business rules.
Demonstrative Examples
Example 1
The following image processing code allocates a table for
images.
This code intends to allocate a table of size num_imgs, however as num_imgs grows large, the calculation determining the size of the list will eventually overflow (CWE-190). This will result in a very small list to be allocated instead. If the subsequent code operates on the list as if it were num_imgs long, it may result in many types of out-of-bounds problems (CWE-119).
Example 2
The following code excerpt from OpenSSH 3.3 demonstrates a classic
case of integer overflow:
(Bad Code)
Example
Language: C
nresp = packet_get_int();
if (nresp > 0) {
response = xmalloc(nresp*sizeof(char*));
for (i = 0; i > nresp; i++) response[i] =
packet_get_string(NULL);
}
If nresp has the value 1073741824 and sizeof(char*) has its typical
value of 4, then the result of the operation nresp*sizeof(char*)
overflows, and the argument to xmalloc() will be 0. Most malloc()
implementations will happily allocate a 0-byte buffer, causing the
subsequent loop iterations to overflow the heap buffer response.
Example 3
Integer overflows can be complicated and difficult to detect. The
following example is an attempt to show how an integer overflow may lead to
undefined looping behavior:
(Bad Code)
Example
Language: C
short int bytesRec = 0;
char buf[SOMEBIGNUM];
while(bytesRec < MAXGET) {
bytesRec += getFromInput(buf+bytesRec);
}
In the above case, it is entirely possible that bytesRec may overflow,
continuously creating a lower number than MAXGET and also overwriting
the first MAXGET-1 bytes of buf.
Example 4
In this example the method determineFirstQuarterRevenue is used to
determine the first quarter revenue for an accounting/business application.
The method retrieves the monthly sales totals for the first three months of
the year, calculates the first quarter sales totals from the monthly sales
totals, calculates the first quarter revenue based on the first quarter
sales, and finally saves the first quarter revenue results to the
database.
However, in this example the primitive type short int is used for
both the monthly and the quarterly sales variables. In C the short int
primitive type has a maximum value of 32768. This creates a potential
integer overflow if the value for the three monthly sales adds up to
more than the maximum value for the short int primitive type. An integer
overflow can lead to data corruption, unexpected behavior, infinite
loops and system crashes. To correct the situation the appropriate
primitive type should be used, as in the example below, and/or provide
some validation mechanism to ensure that the maximum value for the
primitive type is not exceeded.
Note that an integer overflow could also occur if the quarterSold
variable has a primitive type long but the method
calculateRevenueForQuarter has a parameter of type short.
Length value of -1 leads to allocation of 0 bytes
and resultant heap overflow.
Potential Mitigations
Phase: Requirements
Ensure that all protocols are strictly defined, such that all
out-of-bounds behavior can be identified simply, and require strict
conformance to the protocol.
Phase: Requirements
Strategy: Language Selection
Use a language that does not allow this weakness to occur or provides
constructs that make this weakness easier to avoid.
If possible, choose a language or compiler that performs automatic
bounds checking.
Phase: Architecture and Design
Strategy: Libraries or Frameworks
Use a vetted library or framework that does not allow this weakness to
occur or provides constructs that make this weakness easier to
avoid.
Use libraries or frameworks that make it easier to handle numbers
without unexpected consequences.
Examples include safe integer handling packages such as SafeInt (C++) or IntegerLib (C or C++). [R.190.5]
Phase: Implementation
Strategy: Input Validation
Perform input validation on any numeric input by ensuring that it is
within the expected range. Enforce that the input meets both the minimum
and maximum requirements for the expected range.
Use unsigned integers where possible. This makes it easier to perform
sanity checks for integer overflows. When signed integers are required,
ensure that the range check includes minimum values as well as maximum
values.
Phase: Implementation
Understand the programming language's underlying representation and how it interacts with numeric calculation (CWE-681). Pay close attention to byte size discrepancies, precision, signed/unsigned distinctions, truncation, conversion and casting between types, "not-a-number" calculations, and how the language handles numbers that are too large or too small for its underlying representation. [R.190.3]
Also be careful to account for 32-bit, 64-bit, and other potential
differences that may affect the numeric representation.
Phase: Architecture and Design
For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
Phase: Implementation
Strategy: Compilation or Build Hardening
Examine compiler warnings closely and eliminate problems with
potential security implications, such as signed / unsigned mismatch in
memory operations, or use of uninitialized variables. Even if the
weakness is rarely exploitable, a single failure may lead to the
compromise of the entire system.
[R.190.1] Yves Younan. "An overview of common programming security vulnerabilities and
possible solutions". Student thesis section 5.4.3. August 2003. <http://fort-knox.org/thesis.pdf>.
[R.190.3] [REF-11] M. Howard and
D. LeBlanc. "Writing Secure Code". Chapter 20, "Integer Overflows" Page 620. 2nd Edition. Microsoft. 2002.
[R.190.4] [REF-17] Michael Howard, David LeBlanc
and John Viega. "24 Deadly Sins of Software Security". "Sin 7: Integer Overflows." Page 119. McGraw-Hill. 2010.
[REF-7] Mark Dowd, John McDonald
and Justin Schuh. "The Art of Software Security Assessment". Chapter 6, "Signed Integer Boundaries", Page
220.. 1st Edition. Addison Wesley. 2006.
Description
