The fgets()
function is typically used to read a newline-terminated line of input from a stream. The fgets()
function takes a size parameter for the destination buffer and copies, at most, size-1
characters from a stream to a string. Truncation errors can occur if the programmer blindly assumes that the last character in the destination string will be a newline.
Non-Compliant Code Example
This non-compliant code example is intended to be used to remove the trailing newline (\n
) from an input line.
char buf[BUFSIZ + 1]; if (fgets(buf, sizeof(buf), fp)) { if (*buf) { /* see FIO37-C */ buf[strlen(buf) - 1] = '\0'; } } else { /* Handle error condition */ }
However, if the last character in buf
is not a newline, this code overwrites an otherwise-valid character.
Compliant Solution
This compliant solution uses strchr()
to replace the newline character in the string (if it exists).
char buf[BUFSIZ + 1]; char *p; if (fgets(buf, sizeof(buf), fp)) { p = strchr(buf, '\n'); if (p) { *p = '\0'; } } else { /* handle error condition */ }
Risk Assessment
Assuming a newline character is read can result in data truncation.
Rule |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
---|---|---|---|---|---|
FIO36-C |
2 (medium) |
1 (unlikely) |
3 (low) |
P6 |
L2 |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
[[Lai 06]]
[[Seacord 05]] Chapter 2, "Strings"
[[ISO/IEC 9899-1999]] Section 7.19.7.2, "The fgets function"
FIO35-C. Use feof() and ferror() to detect end-of-file and file errors 09. Input Output (FIO) FIO37-C. Don't assume character data has been read