Today, let’s take a closer look at CVE-2021-46848, a vulnerability found in *GNU Libtasn1* (before version 4.19.). This flaw is an off-by-one array size check—specifically, with a constant called ETYPE_OK—and affects the DER encoding process with the function asn1_encode_simple_der. In this article, we’ll break down the vulnerability in simple terms, walk through the dangerous code, link to authoritative sources, and explain how this bug can lead to real-world security problems.

What is Libtasn1?

GNU Libtasn1 is a library used in many security applications for parsing and manipulating ASN.1 (Abstract Syntax Notation One) objects, particularly related to X.509 certificates and cryptographic protocols. It’s widely used, so a bug here has broad implications.

What is an Off-by-one Bug?

An off-by-one (OBO) bug happens when code incorrectly accesses arrays or buffers by one element past the intended boundary—either too many, or too few. These bugs are notorious for causing buffer overflows, which can let attackers overwrite important memory and potentially execute arbitrary code.

Where is the Problem?

In Libtasn1, the constant ETYPE_OK is intended as a NaN (not-a-number) sentinel in an integer array named etype. The problem arises when code checks the size of this array, and then accesses one element too far:

for (i = ; i <= ETYPE_OK; i++) {
    // ... process etype[i]; ...
}

But the array’s declared size is just ETYPE_OK, so the loop ends up accessing etype[ETYPE_OK], which is *one element out-of-bounds*. If an attacker can influence what's stored here, they might be able to overwrite nearby memory.

Vulnerable Function: asn1_encode_simple_der

This function is used for DER encoding, a standard binary format encoding for ASN.1. Here’s a trimmed version to show the flaw:

int asn1_encode_simple_der(...) {
    int etype[ETYPE_OK];  // <-- array size
    // ... set up etype ...
    for (i = ; i <= ETYPE_OK; i++) {
        /* process etype[i]... 
           The loop should be i < ETYPE_OK, not i <= ETYPE_OK! */
    }
}

Key Mistake:
Using <= ETYPE_OK instead of < ETYPE_OK in the loop exposes one memory element past the end.

Exploitation Details

This bug can lead to an out-of-bounds write—if the code writes something to etype[ETYPE_OK], it will overwrite data just after the array. In security, this kind of bug is a classic stepping stone toward unexpected behaviors, including the possibility of:

- Process crashes (DoS): Overwriting memory could corrupt control structures and crash the process.

Information leaks: The overwritten data could be sensitive.

- Remote code execution: With careful exploitation, an attacker might get arbitrary code to run (though in this specific case, exploitability is limited by how the library is used).

Here’s a minimalist “exploit” demonstrating an overflow

#include <stdio.h>
#define ETYPE_OK 4

void buggy() {
    int arr[ETYPE_OK];
    for (int i = ; i <= ETYPE_OK; i++) {
        arr[i] = xdeadbeef; // i = ..4, but arr has space for ..3
    }
    printf("Overflow written!\n");
}

int main() {
    buggy();
    return ;
}

This code writes past the buffer. In a real-world GNU Libtasn1 scenario, an attacker might supply crafted ASN.1 data to trigger this code path.

Patched in: Libtasn1 4.19.

The fix is trivial—change all <= ETYPE_OK to < ETYPE_OK in the loops.

Patch Example

- for (i = ; i <= ETYPE_OK; i++) {
+ for (i = ; i < ETYPE_OK; i++) {

Reference to Patch:
- Upstream Patch Commit

Potentially escalate to code execution under specific (rare) circumstances

Key Point: Even if this doesn’t look “critical” at first glance, *bugs in parsing libraries for cryptography are always dangerous*. They’re at the front lines of networked applications and usually process untrusted input.

Recommendations

- Upgrade Now: If you maintain any application using Libtasn1, make sure you’re running at least 4.19. or later.
- Audit Usage: Since this bug existed for a while, review your systems for potential abuse or anomalous crashes tied to ASN.1 parsing.

References

- MITRE CVE-2021-46848 Entry
- Libtasn1 Official Website
- Upstream Patch Commit
- Debian Security Advisory

Conclusion

CVE-2021-46848 in GNU Libtasn1 is a clear example of how even small off-by-one mistakes can cause big security headaches. By understanding, patching, and learning from these bugs, we can keep our systems safer in a world that’s increasingly reliant on secure, correct code.

*Stay secure! And remember: always check your array bounds!*

Timeline

Published on: 10/24/2022 14:15:00 UTC
Last modified on: 10/24/2022 17:00:00 UTC