Time-of-check time-of-use (TOCTOU) race condition

Description

Usage of the `readlink` function call hints at a potential Time Of Check Time Of Use (TOCTOU) vulnerability. An attacker may be able to modify the file being specified by the `readlink` function prior to the `readlink` function being called. Additionally, care must be taken that the buffer provided is large enough to hold the contents of the file.

Examples

Insecure Code

readlink(path, buffer, sizeof(buffer))

Secure Code

if (lstat(path, &stat_buf) == 0) { fd = open(path, O_RDONLY); if (fd != -1) { if (fstat(fd, &stat_buf_fd) == 0 && stat_buf.st_dev == stat_buf_fd.st_dev && stat_buf.st_ino == stat_buf_fd.st_ino) { // safe to read and operate on the file's contents } } }

Remediation

Use `lstat` prior to opening the file and confirm the attributes are correct. Then use `open` to get a file descriptor to this file. Call `fstat` on the `open` file descriptor to confirm that `st_dev` and `st_ino` are equal between the two.

Rule Details

Field	Value
ID	CODE-0609
Category	Generic
Severity	MEDIUM
CWE	CWE-367
Confidence	HIGH
Impact	MEDIUM
Likelihood	MEDIUM
Exploitability	MODERATE
Tags	TOCTOU, race condition
OWASP	A5:2017-Broken Access Control, A01:2021-Broken Access Control

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