CWE-124: Buffer Underwrite ('Buffer Underflow')

Code Examples & CVEs

Demonstrative Examples

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.

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.

BadC

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;                      }

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.

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.

BadC

int main() {                      ...                      char *result = strstr(destBuf, "Replace Me");                      int idx = result - destBuf;                      strcpy(&destBuf[idx], srcBuf);                      ...}

Observed CVE Examples (8)

CVE-2021-24018

buffer underwrite in firmware verification routine allows code execution via a crafted firmware image

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CVE-2002-2227

Unchecked length of SSLv2 challenge value leads to buffer underflow.

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CVE-2007-4580

Buffer underflow from a small size value with a large buffer (length parameter inconsistency, CWE-130)

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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.

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CVE-2007-0886

Buffer underflow resultant from encoded data that triggers an integer overflow.

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CVE-2006-6171

Product sets an incorrect buffer size limit, leading to "off-by-two" buffer underflow.

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CVE-2006-4024

Negative value is used in a memcpy() operation, leading to buffer underflow.

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CVE-2004-2620

Buffer underflow due to mishandled special characters

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Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-124 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-124, Buffer Underwrite ('Buffer Underflow'), throughout the product lifecycle. Derive review questions, static or dynamic checks, and negative tests from the CWE's causal behavior; define the components and lifecycle stages each check covers; and retain findings with enough evidence to distinguish the root cause from symptoms and impacts. Track escapes and false negatives, then improve the verification plan after every confirmed occurrence.

NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and Technology

Address Buffer Underwrite ('Buffer Underflow') during Requirements

MITRE associates mitigation with Requirements, and Implementation; documented detection approaches include Automated Static Analysis, and Automated Dynamic Analysis; recorded impacts include Modify Memory, DoS: Crash, Exit, or Restart, Execute Unauthorized Code or Commands, Bypass Protection Mechanism, and Other. Use these source-defined anchors to turn CWE-124 into implementation, review, and verification checks for the affected component.

CWE-124: Buffer Underwrite ('Buffer Underflow') — MITRE CWE

Triage CWE-124 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-124, Buffer Underwrite ('Buffer Underflow'), has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.

Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security Agency

Apply precise root-cause mapping to CWE-124

Apply MITRE's full root-cause mapping guidance when using CWE-124, Buffer Underwrite ('Buffer Underflow'). Separate weakness language from attacker prerequisites and technical impact, check the entry's abstraction and vulnerability-mapping notes, and prefer the most specific Base or Variant supported by the evidence. Record the rejected alternatives and require an independent review before the mapping is used for remediation trends or program metrics.

CVE to CWE Root Cause Mapping Guidance — MITRE CWE

Validate CWE-124 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-124, Buffer Underwrite ('Buffer Underflow'). Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.

CVE to CWE Root Cause Mapping Quick Tips — MITRE CWE

Frequently Asked Questions

What is CWE-124: Buffer Underwrite ('Buffer Underflow')?+

CWE-124: Buffer Underwrite ('Buffer Underflow') is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product writes to a buffer using an index or pointer that references a memory location prior to the beginning of the buffer.

What are the security consequences of Buffer Underwrite ('Buffer Underflow')?+

If exploited, CWE-124 (Buffer Underwrite ('Buffer Underflow')) it can compromise Integrity, Availability, Confidentiality, Access Control and Other, leading to outcomes such as Modify Memory, DoS: Crash, Exit, or Restart, Execute Unauthorized Code or Commands, Bypass Protection Mechanism and Other.

How do you prevent or mitigate Buffer Underwrite ('Buffer Underflow')?+

Recommended mitigations for CWE-124 include: Choose a language that is not susceptible to these issues. All calculated values that are used as index or for pointer arithmetic should be validated to ensure that they are within an expected range.

Which programming languages are affected by Buffer Underwrite ('Buffer Underflow')?+

CWE-124 commonly affects C, C++ and Memory-Unsafe. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What are real-world examples of Buffer Underwrite ('Buffer Underflow')?+

MITRE documents real CVEs mapped to CWE-124, including CVE-2021-24018, CVE-2002-2227, CVE-2007-4580, CVE-2007-1584 and CVE-2007-0886. You can look up the full details of each CVE, including CVSS scores and remediation guidance, on our CVE Lookup tool.

What is the difference between a CWE and a CVE?+

A CWE (Common Weakness Enumeration) like CWE-124 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.

CWE-124: Buffer Underwrite ('Buffer Underflow')

Technical Details

Applicable To

Security Consequences

Scope

Impact

Scope

Impact

Scope

Impact

Mitigation Strategies

Phase

Description

Phase

Description

Detection Methods

Code Examples & CVEs

Demonstrative Examples

Observed CVE Examples (8)

CWE Relationships

Parent Weaknesses

Source-backed guidance

Verify controls for CWE-124 with SSDF evidence

Address Buffer Underwrite ('Buffer Underflow') during Requirements

Triage CWE-124 against known exploitation evidence

Apply precise root-cause mapping to CWE-124

Validate CWE-124 with root-cause mapping checks

Frequently Asked Questions

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