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Access of Memory Location Before Start of Buffer

Incomplete Base
Structure: Simple
Description

The product reads or writes to a buffer using an index or pointer that references a memory location prior to the beginning of the buffer.

Extended Description

This typically occurs when a pointer or its index is decremented to a position before the buffer, when pointer arithmetic results in a position before the beginning of the valid memory location, or when a negative index is used.
Common Consequences 3
Scope: Confidentiality

Impact: Read Memory

For an out-of-bounds read, the attacker may have access to sensitive information. If the sensitive information contains system details, such as the current buffer's position in memory, this knowledge can be used to craft further attacks, possibly with more severe consequences.

Scope: IntegrityAvailability

Impact: Modify MemoryDoS: Crash, Exit, or Restart

Out of bounds memory access will very likely result in the corruption of relevant memory, and perhaps instructions, possibly leading to a crash.

Scope: Integrity

Impact: Modify MemoryExecute Unauthorized Code or Commands

If the corrupted memory can be effectively controlled, it may be possible to execute arbitrary code. If the corrupted memory is data rather than instructions, the system will continue to function with improper changes, possibly in violation of an implicit or explicit policy.
Detection Methods 1
FuzzingHigh
Fuzz testing (fuzzing) is a powerful technique for generating large numbers of diverse inputs - either randomly or algorithmically - and dynamically invoking the code with those inputs. Even with random inputs, it is often capable of generating unexpected results such as crashes, memory corruption, or resource consumption. Fuzzing effectively produces repeatable test cases that clearly indicate bugs, which helps developers to diagnose the issues.
Demonstrative Examples 3

ID : DX-87

In the following C/C++ example, a utility function is used to trim trailing whitespace from a character string. The function copies the input string to a local character string and uses a while statement to remove the trailing whitespace by moving backward through the string and overwriting whitespace with a NUL character.

Code Example:

Bad
C
c

// copy input string to a temporary string* char message[length+1]; int index; for (index = 0; index < length; index++) { ``` message[index] = strMessage[index]; } message[index] = '\0';

c

// return string without trailing whitespace* retMessage = message; return retMessage;}

However, this function can cause a buffer underwrite if the input character string contains all whitespace. On some systems the while statement will move backwards past the beginning of a character string and will call the isspace() function on an address outside of the bounds of the local buffer.

ID : DX-90

The following example asks a user for an offset into an array to select an item.

Code Example:

Bad
C
c
The programmer allows the user to specify which element in the list to select, however an attacker can provide an out-of-bounds offset, resulting in a buffer over-read (Buffer Over-read).

ID : DX-88

The following is an example of code that may result in a buffer underwrite. This code is attempting to replace the substring "Replace Me" in destBuf with the string stored in srcBuf. It does so by using the function strstr(), which returns a pointer to the found substring in destBuf. Using pointer arithmetic, the starting index of the substring is found.

Code Example:

Bad
C
c
In the case where the substring is not found in destBuf, strstr() will return NULL, causing the pointer arithmetic to be undefined, potentially setting the value of idx to a negative number. If idx is negative, this will result in a buffer underwrite of destBuf.
Observed Examples 7
CVE-2002-2227Unchecked length of SSLv2 challenge value leads to buffer underflow.
CVE-2007-4580Buffer underflow from a small size value with a large buffer (length parameter inconsistency, Improper Handling of Length Parameter Inconsistency)
CVE-2007-1584Buffer underflow from an all-whitespace string, which causes a counter to be decremented before the buffer while looking for a non-whitespace character.
CVE-2007-0886Buffer underflow resultant from encoded data that triggers an integer overflow.
CVE-2006-6171Product sets an incorrect buffer size limit, leading to "off-by-two" buffer underflow.
CVE-2006-4024Negative value is used in a memcpy() operation, leading to buffer underflow.
CVE-2004-2620Buffer underflow due to mishandled special characters
Applicable Platforms
Languages:
C : OftenC++ : Often
Functional Areas
  1. Memory Management
Affected Resources
  1. Memory
Related Weaknesses
ChildOf: Improper Restriction of Operations within the Bounds of a Memory Buffer (CWE-119)
ChildOf: Improper Restriction of Operations within the Bounds of a Memory Buffer (CWE-119)
ChildOf: Improper Restriction of Operations within the Bounds of a Memory Buffer (CWE-119)
Taxonomy Mapping
  • CERT C Secure Coding