CWE-194: Unexpected Sign Extension

Definition

What is CWE-194?

This vulnerability occurs when a signed number from a smaller data type is moved or cast to a larger type, causing its sign bit to be incorrectly extended. If the original value is negative, this sign extension can fill the new, higher-order bits with '1's, leading to unexpectedly large positive values and causing logic errors, buffer overflows, or security bypasses.

Sign extension is a standard behavior in programming languages like C and C++ when promoting a signed integer (e.g., a signed 8-bit `char`) to a larger signed type (e.g., a 32-bit `int`). The problem arises when developers don't account for this automatic behavior, especially when treating the resulting value as an unsigned number or using it for operations like memory allocation, array indexing, or length validation. A classic example is reading a byte value of `0xFF` (-1 as a signed char) into an unsigned integer, which becomes `0xFFFFFFFF` (a very large positive number), potentially leading to out-of-bounds access. To prevent this, developers must be explicit about data types during conversions. Always consider if the source data should be treated as signed or unsigned before widening it. Use explicit casts to the intended target type, and when working with raw byte data or protocol parsing, prefer unsigned types for counts and indices. Performing range checks on the source value before the conversion or using bit masks (e.g., `new_value = old_value & 0xFF`) can effectively strip unwanted sign-extended bits and ensure the resulting value matches the intended logic.

Auswirkungen in der Praxis

Real-world CVEs caused by CWE-194

CVE-2018-10887

Chain: unexpected sign extension (CWE-194) leads to integer overflow (CWE-190), causing an out-of-bounds read (CWE-125)
CVE-1999-0234

Sign extension error produces -1 value that is treated as a command separator, enabling OS command injection.
CVE-2003-0161

Product uses "char" type for input character. When char is implemented as a signed type, ASCII value 0xFF (255), a sign extension produces a -1 value that is treated as a program-specific separator value, effectively disabling a length check and leading to a buffer overflow. This is also a multiple interpretation error.
CVE-2007-4988

chain: signed short width value in image processor is sign extended during conversion to unsigned int, which leads to integer overflow and heap-based buffer overflow.
CVE-2006-1834

chain: signedness error allows bypass of a length check; later sign extension makes exploitation easier.
CVE-2005-2753

Sign extension when manipulating Pascal-style strings leads to integer overflow and improper memory copy.

Wie Angreifer es ausnutzen

Angreiferpfad Schritt für Schritt

1
Identifiziere einen Codepfad, der nicht vertrauenswürdige Eingaben ohne Validierung verarbeitet.
2
Erzeuge eine Payload, die das unsichere Verhalten auslöst — Injection, Traversal, Overflow oder Logik-Missbrauch.
3
Liefere die Payload über einen normalen Request aus und beobachte die Reaktion der Anwendung.
4
Iteriere, bis die Antwort Daten preisgibt, Angreifer-Code ausführt oder Berechtigungen eskaliert.

Verwundbares Codebeispiel

Vulnerable C

The following code reads a maximum size and performs a sanity check on that size. It then performs a strncpy, assuming it will not exceed the boundaries of the array. While the use of "short s" is forced in this particular example, short int's are frequently used within real-world code, such as code that processes structured data.

Verwundbar C

int GetUntrustedInt () {
  	return(0x0000FFFF);
  }
  void main (int argc, char **argv) {
  		char path[256];
  		char *input;
  		int i;
  		short s;
  		unsigned int sz;
  		i = GetUntrustedInt();
  		s = i;
  		/* s is -1 so it passes the safety check - CWE-697 */
  		if (s > 256) {
  			DiePainfully("go away!\n");
  		}
  		/* s is sign-extended and saved in sz */
  		sz = s;
  		/* output: i=65535, s=-1, sz=4294967295 - your mileage may vary */
  		printf("i=%d, s=%d, sz=%u\n", i, s, sz);
  		input = GetUserInput("Enter pathname:");
  		/* strncpy interprets s as unsigned int, so it's treated as MAX_INT
  		(CWE-195), enabling buffer overflow (CWE-119) */
  		strncpy(path, input, s);
  		path[255] = '\0'; /* don't want CWE-170 */
  		printf("Path is: %s\n", path);
  }

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-194

Implementation Avoid using signed variables if you don't need to represent negative values. When negative values are needed, perform validation after you save those values to larger data types, or before passing them to functions that are expecting unsigned values.

Erkennungssignale

How to detect CWE-194

SAST High

Führe statische Analyse (SAST) auf der Codebasis aus und suche im Datenfluss nach dem unsicheren Muster.

DAST Moderate

Führe dynamische Application-Security-Tests gegen den Live-Endpoint aus.

Runtime Moderate

Beobachte Runtime-Logs auf ungewöhnliche Exception-Traces, fehlerhafte Eingaben oder Versuche, Autorisierung zu umgehen.

Code review Moderate

Code Review: Markiere jeden neuen Code, der Eingaben von dieser Oberfläche ohne validierte Framework-Helper verarbeitet.

CWE-194

Don't catalog this weakness. Prove it's reachable.

Plexicus turns CWE catalogs into evidence: every CWE-pattern is matched against your real code graph, reach is proven on a sandbox clone, and verified findings ship as reviewed PRs.

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Häufig gestellte Fragen

Frequently asked questions

Was ist CWE-194?

This vulnerability occurs when a signed number from a smaller data type is moved or cast to a larger type, causing its sign bit to be incorrectly extended. If the original value is negative, this sign extension can fill the new, higher-order bits with '1's, leading to unexpectedly large positive values and causing logic errors, buffer overflows, or security bypasses.

Wie gravierend ist CWE-194?

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-194 betroffen?

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

Wie kann ich CWE-194 verhindern?

Avoid using signed variables if you don't need to represent negative values. When negative values are needed, perform validation after you save those values to larger data types, or before passing them to functions that are expecting unsigned values.

Wie erkennt und behebt Plexicus CWE-194?

Die SAST-Engine von Plexicus erkennt die Datenfluss-Signatur von CWE-194 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-194?

MITRE veröffentlicht die kanonische Definition unter https://cwe.mitre.org/data/definitions/194.html. Für ergänzende Hinweise kannst du auch die OWASP- und NIST-Dokumentation heranziehen.

Verwandte Schwachstellen

Weaknesses related to CWE-194

CWE-681 Parent

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Unexpected Sign Extension

What is CWE-194?

Real-world CVEs caused by CWE-194

Angreiferpfad Schritt für Schritt

Vulnerable C

Secure pseudo

How to prevent CWE-194

How to detect CWE-194

Don't catalog this weakness. Prove it's reachable.

Frequently asked questions

Weaknesses related to CWE-194

Incorrect Conversion between Numeric Types

Integer Coercion Error

Signed to Unsigned Conversion Error

Unsigned to Signed Conversion Error

Numeric Truncation Error

Further reading

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Sample evidence handover

Server-Side Request Forgery in `webhooks/receiver`