CWE-121 Variant Draft High likelihood

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…

Definition

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.

Auswirkungen in der Praxis

Real-world CVEs caused by CWE-121

CVE-2021-35395

Stack-based buffer overflows in SFK for wifi chipset used for IoT/embedded devices, as exploited in the wild per CISA KEV.

Wie Angreifer es ausnutzen

Angreiferpfad Schritt für Schritt

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).

Verwundbares Codebeispiel

Vulnerable C

While buffer overflow examples can be rather complex, it is possible to have very simple, yet still exploitable, stack-based buffer overflows:

Verwundbar C

#define BUFSIZE 256
  int main(int argc, char **argv) {
  	char buf[BUFSIZE];
  	strcpy(buf, argv[1]);
  }

Sicheres Codebeispiel

Secure pseudo

Sicher 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.

Präventions-Checkliste

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].

Erkennungssignale

How to detect CWE-121

Fuzzing High

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 Static Analysis High

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 Auto-Fix

Plexicus erkennt CWE-121 automatisch und öffnet in unter 60 Sekunden einen Fix-PR.

Codex Remedium scannt jeden Commit, identifiziert genau diese Schwachstelle und liefert einen reviewer-ready Pull Request mit dem Patch. Keine Tickets. Keine Hand-offs.

Demo anfordern Plexicus kostenlos testen

Häufig gestellte Fragen

Frequently asked questions

Was ist CWE-121?

Wie gravierend ist CWE-121?

MITRE stuft die Exploit-Wahrscheinlichkeit als hoch ein — diese Schwachstelle wird aktiv in freier Wildbahn ausgenutzt und sollte priorisiert behoben werden.

Welche Sprachen oder Plattformen sind von CWE-121 betroffen?

MITRE lists the following affected platforms: C, C++.

Wie kann ich CWE-121 verhindern?

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…

Wie erkennt und behebt Plexicus CWE-121?

Die SAST-Engine von Plexicus erkennt die Datenfluss-Signatur von CWE-121 bei jedem Commit. Bei einem Treffer öffnet unser Codex-Remedium-Agent einen Fix-PR mit korrigiertem Code, Tests und einer einzeiligen Zusammenfassung für den Reviewer.

Wo erfahre ich mehr über CWE-121?

MITRE veröffentlicht die kanonische Definition unter https://cwe.mitre.org/data/definitions/121.html. Für ergänzende Hinweise kannst du auch die OWASP- und NIST-Dokumentation heranziehen.

Verwandte Schwachstellen

Weaknesses related to CWE-121

CWE-788 Parent

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 Sibling

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 Sibling

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…

Ressourcen

Schluss mit dem Bezahlen pro Entwickler.
Schließ den Kreislauf.

Plexicus ist die KI-native ASPM, die scannt, filtert, fixt, pentestet und erklärt — autonom. Unbegrenzte Entwickler, unbegrenzte Repos, Fair-Use-KI-Aktionen. Echter kostenloser Tarif, €269/mo jährlich, wenn du bereit bist.

Kostenlos starten Demo buchen