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The ungetc() function does not set the error indicator even when it fails, so it is not possible to check for errors reliably unless it is known that the argument is not equal to EOF.

The C Standard 7.31.3.10 paragraph 3 [ISO/IEC 9899:20112024] states that "one

 )ne wide character of pushback is guaranteed...

," so this should not be an issue if, at most, one character is ever pushed back before reading again. (See FIO13-C. Never push back anything other than one read character.)

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In this noncompliant code example, snprintf() is assumed to succeed. However, if the call fails (for example, because of insufficient memory, as described in GNU libc bug 441945), the subsequent call to log_message() has undefined behavior 174 because the character buffer is uninitialized and need not be null-terminated.

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Failing to detect error conditions can lead to unpredictable results, including abnormal program termination and denial-of-service attacks or, in some situations, could even allow an attacker to run arbitrary code.

Automated Detection

Related Vulnerabilities

The vulnerability in Adobe Flash [VU#159523] arises because Flash neglects to check the return value from calloc(). Even when calloc() returns a null pointer, Flash writes to an offset from the return value. Dereferencing a null pointer usually results in a program crash, but dereferencing an offset from a null pointer allows an exploit to succeed without crashing the program.

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