[Bug] Format-string injection in FinSH prompt via directory name

[XueDugu]

RT-Thread Version

master at commit 25295501c0cc7181d6a541a867fdf7214879ddf8, also present in released tags that I checked, including: v5.2.2, v5.2.1, v5.2.0, v5.1.0, v5.0.2, v4.1.1, v4.0.4, v4.0.0, v3.1.5, and v3.0.4.

Hardware Type/Architectures

Any BSP / architecture with RT_USING_FINSH, DFS_USING_POSIX, and DFS_USING_WORKDIR enabled.

Develop Toolchain

GCC

Describe the bug

Summary

There is a format-string vulnerability in the FinSH/MSH prompt printing path.

finsh_get_prompt() appends the current working directory to the prompt buffer via getcwd(), and the result is later passed directly to rt_kprintf() as the format string in components/finsh/shell.c.

The vulnerable call is:

• components/finsh/shell.c:545 → rt_kprintf(FINSH_PROMPT);

The same unsafe pattern also appears later in the same file when the prompt is printed again after command handling.

This means the shell prompt becomes attacker-controlled when the working directory name contains format specifiers. A local user can create or enter a directory such as %08x%08x%08x, and the next prompt print will interpret that directory name as a format string instead of plain text.

Source-to-sink chain:

• Source: user-controlled directory name from mkdir argv[1] / cd argv[1]
• Propagation: getcwd() in finsh_get_prompt()
• Sink: rt_kprintf(FINSH_PROMPT)

Impact:

• Information disclosure / stack data leak via %x, %p, %s
• Possible crash / denial of service
• In builds where %n support is enabled, this may also allow memory write behavior

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Steps to Reproduce

1. Build RT-Thread with RT_USING_FINSH, DFS_USING_POSIX, and DFS_USING_WORKDIR enabled.
2. Boot into the MSH shell.
3. Create a directory whose name contains format specifiers:

   mkdir %08x%08x%08x

4. Change into that directory:

   cd %08x%08x%08x

5. Observe the next shell prompt.
6. The %08x sequences are interpreted by rt_kprintf() and stack values may be printed instead of the literal directory name.

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Expected Behavior

The prompt should print the working directory literally and must not treat it as a format string.

A safe fix is to use a constant format string:

rt_kprintf("%s", FINSH_PROMPT);

This should be applied to every prompt-printing location in components/finsh/shell.c.

Kindly let me know if you intend to request a CVE ID upon confirmation of the vulnerability.

Other additional context

No response