CWE-786: Access of Memory Location Before Start of Buffer
Official CWE-786 CWE context with Glexia analysis, remediation guidance, related CVEs, and ATT&CK context.
Glexia's Take
CWE-786: Access of Memory Location Before Start of Buffer
Access of Memory Location Before Start of Buffer represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.
Executive Impact
- Confidentiality: 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.
- 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: Modify Memory,Execute 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.
Developer Pattern
CWE-786 is the kind of defect developers can usually prevent with explicit validation, safer framework defaults, and tests that exercise hostile input or unsafe state transitions.
Confidence
high confidence from CWE-786, 4.20.
Official CWE Definition
CWE-786: Access of Memory Location Before Start of Buffer
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.
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.
Developer And Remediation Guidance
How teams prevent and detect this weakness
Causes
- 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.
- The following example asks a user for an offset into an array to select an item. 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 (CWE-126).
- 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.
Remediation
- Use safe APIs
- Centralize the control
- Add regression tests
- Review logs and telemetry for attempted abuse
Detection
- Fuzzing: 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.
- Automated Dynamic Analysis: Use tools that are integrated during compilation to insert runtime error-checking mechanisms related to memory safety errors, such as AddressSanitizer (ASan) for C/C++ [REF-1518].
Mappings
Related CVEs, CWEs, and ATT&CK context
Related CWEs
- CWE-124: Buffer Underwrite ('Buffer Underflow')
- CWE-127: Buffer Under-read
- CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer
- CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer
- CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer
ATT&CK Relevance
ATT&CK relevance is shown only when reviewed or responsibly inferred.