In late 2022, a new vulnerability was found in PicoC Version 3.2.2, a small C interpreter used for scripting and learning purposes. This bug, formally registered as CVE-2022-44321, relates to a heap buffer overflow originating in the LexSkipComment function inside *lex.c*. Understanding the details behind this vulnerability can help developers write safer code and help InfoSec professionals assess the risk in projects using PicoC.

What is PicoC?

PicoC is a lightweight, portable C interpreter. Designed for resource-limited environments (microcontrollers, embedded systems, etc.) or educational use, it allows executing C code on the fly rather than compiling it. While PicoC isn't as widely used as GCC or Clang, many projects use it for scripting, bootloaders, or rapid prototyping.

Where’s The Bug? In-Depth Explanation

The vulnerability exists in PicoC 3.2.2's LexSkipComment() function, found in the *lex.c* source file. This function is called when PicoC processes comments in source code. Comments in C are wrapped between /* and */, and during tokenization, PicoC must find and skip over them.

A key problem is that LexSkipComment does not properly check bounds while scanning for the closing */, which may lead to reading—or even writing—past the end of a heap-allocated buffer.

Here’s (a simplified) snippet from lex.c, showing the problem area

// abridged for clarity

void LexSkipComment(struct LexState *Lexer) {
    char *Pos = Lexer->Pos;
    while ((Pos[] != '*') || (Pos[1] != '/')) {
        Pos++;
    }
    Lexer->Pos = Pos + 2; // move past the comment end marker
}

The Problem

There are no bounds checks within the while-loop. If input does not end with */ (for example, the comment is unfinished), Pos will keep incrementing until it moves outside the bounds of the original buffer in heap memory. This can:

Cause a segmentation fault (crash)

- Open up the possibility for exploiting heap-based overflows, depending on the memory allocator and what’s nearby in memory

When called from LexScanGetToken, this problem can be triggered by simply providing a malicious input.

Attack Scenario

Consider a scenario where a remote attacker can submit arbitrary C source for interpretation by PicoC (e.g., via a web service, or in a plug-in system). By sending a crafted comment that never closes, they can exploit the heap buffer overflow.

Save this as *crash_comment.c*

/* this comment never ends

Then run

picoc crash_comment.c

PicoC will crash with a segmentation fault. In a more dangerous setup, an attacker may layout heap memory to overwrite adjacent structures. For more controlled exploitation (like arbitrary code execution), heap grooming and knowledge of the heap would be necessary.

Example of Trigger Code

// This line never closes the comment:
char demo = 1;
/* Maliciously unclosed comment
int x = 123;

Denial of Service: At minimum (and by default), this is a DoS (crashing the interpreter)

- Heap Corruption: Under certain heap/allocator conditions, this can lead to heap metadata corruption or code execution
- Remote Code Execution: If PicoC is embedded in an application with higher privileges or accessible remotely, the attacker could escalate this to RCE

Mitigation and Fix

The patch (as shown in the original GitHub commit) adds bounds checking to ensure the end of buffer is not crossed:

void LexSkipComment(struct LexState *Lexer) {
    char *Pos = Lexer->Pos;

    while (Pos < Lexer->End && ((Pos[] != '*') || (Pos[1] != '/'))) {
        Pos++;
    }

    if (Pos >= Lexer->End) {
        // Handle error: unterminated comment
        ProgramFail(Lexer->Parser, "Unterminated comment");
    } else {
        Lexer->Pos = Pos + 2; // Move past comment end marker
    }
}

Upgrade Now!

If you’re using PicoC anywhere in your application, or if you distribute PicoC-based tools, upgrade to the latest version immediately. If you maintain embedded hardware, cross-check your firmware and scripting subsystems.

References and Sources

- CVE-2022-44321 entry at NIST NVD
- GitHub issue/patch discussion
- PicoC main repository
- Full fix in commit

Conclusion

*CVE-2022-44321 is a classic example of how a simple logic bug—lack of buffer boundary checking while parsing comments—can lead to a security vulnerability in otherwise simple or trusted code. Even small interpreters like PicoC can be a stepping stone for attackers in embedded and scripting environments. Always review string and buffer handling–especially when parsing data from users.*

Timeline

Published on: 11/08/2022 15:15:00 UTC
Last modified on: 11/08/2022 21:55:00 UTC