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.
CWE-121 Variante Borrador
Stack-based Buffer Overflow
A stack-based buffer overflow occurs when a program writes more data to a buffer located on the call stack than it can hold, corrupting adjacent memory and potentially hijacking the program's…
Definición
What is CWE-121?
A stack-based buffer overflow occurs when a program writes more data to a buffer located on the call stack than it can hold, corrupting adjacent memory and potentially hijacking the program's execution flow.
This vulnerability happens when functions like `strcpy`, `gets`, or `scanf` are used without proper bounds checking. Since the buffer is allocated as a local variable within a function, the overflow overwrites other critical data on the stack, such as saved register values, function parameters, and most importantly, the return address. By carefully crafting the excess data, an attacker can redirect execution to malicious code they've injected or to existing code that benefits their attack, leading to a complete compromise.
To prevent this, developers should always use secure, length-limited alternatives (like `strncpy` or `snprintf`) and perform explicit bounds checking before any write operation. Enabling compiler protections like stack canaries, address space layout randomization (ASLR), and non-executable stacks can also mitigate exploitation, but the primary defense is writing safe, validated code that never trusts unchecked input.
Impacto en el mundo real
Real-world CVEs caused by CWE-121
-
Stack-based buffer overflows in SFK for wifi chipset used for IoT/embedded devices, as exploited in the wild per CISA KEV.
Cómo lo explotan los atacantes
Ruta del atacante paso a paso
- 1
While buffer overflow examples can be rather complex, it is possible to have very simple, yet still exploitable, stack-based buffer overflows:
- 2
The buffer size is fixed, but there is no guarantee the string in argv[1] will not exceed this size and cause an overflow.
- 3
This example takes an IP address from a user, verifies that it is well formed and then looks up the hostname and copies it into a buffer.
- 4
This function allocates a buffer of 64 bytes to store the hostname, however there is no guarantee that the hostname will not be larger than 64 bytes. If an attacker specifies an address which resolves to a very large hostname, then the function may overwrite sensitive data or even relinquish control flow to the attacker.
- 5
Note that this example also contains an unchecked return value (CWE-252) that can lead to a NULL pointer dereference (CWE-476).
Ejemplo de código vulnerable
Vulnerable C
While buffer overflow examples can be rather complex, it is possible to have very simple, yet still exploitable, stack-based buffer overflows:
#define BUFSIZE 256
int main(int argc, char **argv) {
char buf[BUFSIZE];
strcpy(buf, argv[1]);
} Ejemplo de código seguro
Secure pseudo
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
const safe = validateAndEscape(input);
return executeWithGuards(safe);
} What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Lista de prevención
How to prevent CWE-121
- Operation / Build and Compilation Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking. D3-SFCV (Stack Frame Canary Validation) from D3FEND [REF-1334] discusses canary-based detection in detail.
- Architecture and Design Use an abstraction library to abstract away risky APIs. Not a complete solution.
- Implementation Implement and perform bounds checking on input.
- Implementation Do not use dangerous functions such as gets. Use safer, equivalent functions which check for boundary errors.
- Operation / Build and Compilation Run or compile the software using features or extensions that randomly arrange the positions of a program's executable and libraries in memory. Because this makes the addresses unpredictable, it can prevent an attacker from reliably jumping to exploitable code. Examples include Address Space Layout Randomization (ASLR) [REF-58] [REF-60] and Position-Independent Executables (PIE) [REF-64]. Imported modules may be similarly realigned if their default memory addresses conflict with other modules, in a process known as "rebasing" (for Windows) and "prelinking" (for Linux) [REF-1332] using randomly generated addresses. ASLR for libraries cannot be used in conjunction with prelink since it would require relocating the libraries at run-time, defeating the whole purpose of prelinking. For more information on these techniques see D3-SAOR (Segment Address Offset Randomization) from D3FEND [REF-1335].
Señales de detección
How to detect CWE-121
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
Plexicus detecta automáticamente CWE-121 y abre un PR de corrección en menos de 60 segundos.
Codex Remedium escanea cada commit, identifica esta debilidad concreta y entrega un pull request listo para revisión con el parche. Sin tickets. Sin traspasos.
Preguntas frecuentes ¿Qué es CWE-121?
¿Qué gravedad tiene CWE-121?
¿Qué lenguajes o plataformas se ven afectados por CWE-121?
¿Cómo puedo prevenir CWE-121?
¿Cómo detecta y corrige Plexicus CWE-121?
¿Dónde puedo aprender más sobre CWE-121?
Frequently asked questions
¿Qué es CWE-121?
A stack-based buffer overflow occurs when a program writes more data to a buffer located on the call stack than it can hold, corrupting adjacent memory and potentially hijacking the program's execution flow.
¿Qué gravedad tiene CWE-121?
MITRE califica la probabilidad de explotación como Alta — esta debilidad se explota activamente en la práctica y debe priorizarse para su remediación.
¿Qué lenguajes o plataformas se ven afectados por CWE-121?
MITRE lists the following affected platforms: C, C++.
¿Cómo puedo prevenir CWE-121?
Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking. D3-SFCV (Stack Frame Canary Validation) from D3FEND [REF-1334] discusses canary-based detection in detail. Use an abstraction library to abstract away risky APIs. Not a…
¿Cómo detecta y corrige Plexicus CWE-121?
El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-121 en cada commit. Cuando hay coincidencia, nuestro agente Codex Remedium abre un PR de corrección con el código corregido, las pruebas y un resumen de una línea para el revisor.
¿Dónde puedo aprender más sobre CWE-121?
MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/121.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.
Debilidades relacionadas
Weaknesses related to CWE-121
CWE-788 Padre
Access of Memory Location After End of Buffer
This vulnerability occurs when software attempts to read from or write to a memory buffer using an index or pointer that points past the…
CWE-122 Hermano
Heap-based Buffer Overflow
A heap-based buffer overflow occurs when a program writes more data to a memory buffer allocated in the heap than it can hold, corrupting…
CWE-126 Hermano
Buffer Over-read
This vulnerability occurs when a program reads data from a memory buffer using an index or pointer that points beyond the buffer's…
Recursos
Further reading
- MITRE — CWE-121 oficial https://cwe.mitre.org/data/definitions/121.html
- Smashing The Stack For Fun And Profit https://phrack.org/issues/49/14.html
- Writing Secure Code https://www.microsoftpressstore.com/store/writing-secure-code-9780735617223
- The CLASP Application Security Process https://cwe.mitre.org/documents/sources/TheCLASPApplicationSecurityProcess.pdf
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