SEI CERT C Coding Standard

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changes.mady.by.user Liz Whiting

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Comment: Updated UB references from C11->C23

According to the C Standard, 7.4.1 paragraph 1 [ISO/IEC 9899:20112024],

The header <ctype.h> declares several functions useful for classifying and mapping characters. In all cases the argument is an int, the value of which shall be representable as an unsigned char or shall equal the value of the macro EOF. If the argument has any other value, the behavior is undefined.

See also undefined behavior 113112.

This rule is applicable only to code that runs on platforms where the char data type is defined to have the same range, representation, and behavior as signed char.

Following are the character classification functions that this rule addresses:

isalnum()

isalpha()

isascii()XSI

isblank()

iscntrl()

isdigit()

isgraph()

islower()

isprint()

ispunct()

isspace()

isupper()

isxdigit()

toascii()XSI

toupper()

tolower()

XSI denotes an X/Open System Interfaces Extension to ISO/IEC 9945—POSIX. These functions are not defined by the C Standard.

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Passing values to character handling functions that cannot be represented as an unsigned char to character handling functions is undefined behavior.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

STR37-C

Low

Unlikely

Low

P3

L3

Automated Detection

Tool

Version

Checker

Description

Astrée
Include Page
Astrée_V
Astrée_V
ctype-limitsPartially checked
Axivion Bauhaus Suite

Include Page
Axivion Bauhaus Suite_V
Axivion Bauhaus Suite_V

CertC-STR37Fully implemented
CodeSonar
Include Page
CodeSonar_V
CodeSonar_V
MISC.NEGCHARNegative character value
Compass/ROSE
 
 


Could detect violations of this rule by seeing if the argument to a character handling function (listed above) is not an unsigned char

ECLAIR

Include Page
ECLAIR_V
ECLAIR_V

CC2.STR37

Fully implemented

Helix QAC

Include Page
Helix QAC_V
Helix QAC_V

C4413, C4414

C++3051


Klocwork
Include Page
Klocwork_V
Klocwork_V
AUTOSAR.STDLIB.CCTYPE.UCHAR
MISRA.ETYPE.ASSIGN.2012


LDRA tool suite
Include Page
LDRA_V
LDRA_V
663 SFully implemented
PRQA QA-C Include PagePRQA QA-C_vPRQA QA-C_v4413,4414
Parasoft C/C++test

Include Page
Parasoft_V
Parasoft_V

CERT_C-STR37-aDo not pass incorrect values to ctype.h library functions
Polyspace Bug Finder

Include Page
Polyspace Bug Finder_V
Polyspace Bug Finder_V

CERT C: Rule STR37-C

Checks for invalid use of standard library integer routine (rule fully covered)

RuleChecker

Include Page
RuleChecker_V
RuleChecker_V

ctype-limitsPartially checked
TrustInSoft Analyzer

Include Page
TrustInSoft Analyzer_V
TrustInSoft Analyzer_V

valid_charPartially verified.
Fully implemented

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Key here (explains table format and definitions)

Taxonomy

Taxonomy item

Relationship

CERT C Secure Coding StandardSTR34-C. Cast characters to unsigned char before converting to larger integer sizesPrior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961Passing arguments to character-handling functions that are not representable as unsigned char [chrsgnext]
MITRE CWE
Prior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11CWE-704, Incorrect Type Conversion or Cast2017-06-14: CERT: Rule subset of CWE

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-686 and STR37-C

Intersection( CWE-686, STR37-C) = Ø

STR37-C is not about the type of the argument passed (which is signed int), but about the restrictions placed on the value in this type (must be 0-UCHAR_MAX or EOF). I interpret ‘argument type’ to be specific to the C language, so CWE-686

...

does not apply to incorrect argument values, just incorrect types (which is relatively rare in C, but still possible).

CWE-704 and STR37-C

STR37-C = Subset( STR34-C)

CWE-683 and STR37-C

Intersection( CWE-683, STR37-C) = Ø

STR37-C excludes mis-ordered function arguments (assuming they pass type-checking), because there is no easy way to reliably detect violations of CWE-683.

Bibliography

[ISO/IEC 9899:
2011
2024]7.4.1, "Character Handling <ctype.h>"
[Kettlewell 2002]Section 1.1, "<ctype.h> and Characters Types"

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