CWE-194: Unexpected Sign Extension

Code Examples & CVEs

Demonstrative Examples

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.

This code first exhibits an example of CWE-839, allowing "s" to be a negative number. When the negative short "s" is converted to an unsigned integer, it becomes an extremely large positive integer. When this converted integer is used by strncpy() it will lead to a buffer overflow (CWE-119).

BadC

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);                      }

Observed CVE Examples (6)

CVE-2018-10887

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

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CVE-1999-0234

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

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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.

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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.

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CVE-2006-1834

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

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CVE-2005-2753

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

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Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-194 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-194, Unexpected Sign Extension, 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 Unexpected Sign Extension during Implementation

MITRE associates mitigation with Implementation; documented detection approaches include Automated Static Analysis; recorded impacts include Read Memory, Modify Memory, and Other. Use these source-defined anchors to turn CWE-194 into implementation, review, and verification checks for the affected component.

CWE-194: Unexpected Sign Extension — MITRE CWE

Triage CWE-194 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-194, Unexpected Sign Extension, 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-194

Apply MITRE's full root-cause mapping guidance when using CWE-194, Unexpected Sign Extension. 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-194 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-194, Unexpected Sign Extension. 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-194: Unexpected Sign Extension?+

CWE-194: Unexpected Sign Extension is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product performs an operation on a number that causes it to be sign extended when it is transformed into a larger data type. When the original number is negative, this can produce unexpected values that lead to resultant weaknesses.

What are the security consequences of Unexpected Sign Extension?+

If exploited, CWE-194 (Unexpected Sign Extension) it can compromise Integrity, Confidentiality, Availability and Other, leading to outcomes such as Read Memory, Modify Memory and Other.

How do you prevent or mitigate Unexpected Sign Extension?+

Recommended mitigations for CWE-194 include: 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.

Which programming languages are affected by Unexpected Sign Extension?+

CWE-194 commonly affects C and C++. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What are real-world examples of Unexpected Sign Extension?+

MITRE documents real CVEs mapped to CWE-194, including CVE-2018-10887, CVE-1999-0234, CVE-2003-0161, CVE-2007-4988 and CVE-2006-1834. 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-194 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-194: Unexpected Sign Extension

Technical Details

Applicable To

Security Consequences

Scope

Impact

Mitigation Strategies

Phase

Description

Detection Methods

Code Examples & CVEs

Demonstrative Examples

Observed CVE Examples (6)

CWE Relationships

Parent Weaknesses

Source-backed guidance

Verify controls for CWE-194 with SSDF evidence

Address Unexpected Sign Extension during Implementation

Triage CWE-194 against known exploitation evidence

Apply precise root-cause mapping to CWE-194

Validate CWE-194 with root-cause mapping checks

Frequently Asked Questions

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