CWE-124 缓冲区下溢

Buffer Underwrite ('Buffer Underflow')

结构: Simple

Abstraction: Base

状态: Incomplete

被利用可能性: Medium

基本描述

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

扩展描述

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.

相关缺陷

  • cwe_Nature: ChildOf cwe_CWE_ID: 786 cwe_View_ID: 1000 cwe_Ordinal: Primary

  • cwe_Nature: ChildOf cwe_CWE_ID: 786 cwe_View_ID: 699 cwe_Ordinal: Primary

  • cwe_Nature: ChildOf cwe_CWE_ID: 787 cwe_View_ID: 1000

  • cwe_Nature: ChildOf cwe_CWE_ID: 787 cwe_View_ID: 699

适用平台

Language: [{'cwe_Name': 'C', 'cwe_Prevalence': 'Undetermined'}, {'cwe_Name': 'C++', 'cwe_Prevalence': 'Undetermined'}]

常见的影响

范围 影响 注释
['Integrity', 'Availability'] ['Modify Memory', 'DoS: 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.
['Integrity', 'Confidentiality', 'Availability', 'Access Control', 'Other'] ['Execute Unauthorized Code or Commands', 'Modify Memory', 'Bypass Protection Mechanism', 'Other'] 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. The consequences would only be limited by how the affected data is used, such as an adjacent memory location that is used to specify whether the user has special privileges.
['Access Control', 'Other'] ['Bypass Protection Mechanism', 'Other'] When the consequence is arbitrary code execution, this can often be used to subvert any other security service.

可能的缓解方案

策略:

Requirements specification: The choice could be made to use a language that is not susceptible to these issues.

Implementation

策略:

Sanity checks should be performed on all calculated values used as index or for pointer arithmetic.

示例代码

例

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.

bad C

char trimTrailingWhitespace(char strMessage, int length) {
char retMessage;
char message = malloc(sizeof(char)*(length+1));

// 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';

// trim trailing whitespace
int len = index-1;
while (isspace(message[len])) {
message[len] = '\0';
len--;
}

// 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.

例

The following is an example of code that may result in a buffer underwrite, if find() returns a negative value to indicate that ch is not found in srcBuf:

bad C

int main() {
...
strncpy(destBuf, &srcBuf[find(srcBuf, ch)], 1024);
...
}

If the index to srcBuf is somehow under user control, this is an arbitrary write-what-where condition.

分析过的案例

标识 说明 链接
CVE-2002-2227 Unchecked length of SSLv2 challenge value leads to buffer underflow. http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2002-2227
CVE-2007-4580 Buffer underflow from a small size value with a large buffer (length parameter inconsistency, CWE-130) http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2007-4580
CVE-2007-1584 Buffer underflow from an all-whitespace string, which causes a counter to be decremented before the buffer while looking for a non-whitespace character. http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2007-1584
CVE-2007-0886 Buffer underflow resultant from encoded data that triggers an integer overflow. http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2007-0886
CVE-2006-6171 Product sets an incorrect buffer size limit, leading to "off-by-two" buffer underflow. http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2006-6171
CVE-2006-4024 Negative value is used in a memcpy() operation, leading to buffer underflow. http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2006-4024
CVE-2004-2620 Buffer underflow due to mishandled special characters http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2004-2620

Notes

Relationship This could be resultant from several errors, including a bad offset or an array index that decrements before the beginning of the buffer (see CWE-129). Research Gap Much attention has been paid to buffer overflows, but "underflows" sometimes exist in products that are relatively free of overflows, so it is likely that this variant has been under-studied.

分类映射

映射的分类名 ImNode ID Fit Mapped Node Name
PLOVER UNDER - Boundary beginning violation ('buffer underflow'?)
CLASP Buffer underwrite
Software Fault Patterns SFP8 Faulty Buffer Access

引用