CVE-2021-46848: A Closer Look at GNU Libtasn1 4.19. Vulnerability and How to Exploit It

GNU Libtasn1 is a library for implementing ASN.1 encoding and decoding. It is widely used in various applications and protocols, such as X.509 certificates and TLS. Before version 4.19., there was a vulnerability found in Libtasn1, designated as CVE-2021-46848. In this long-read article, we will dive deep into the vulnerability details, reference links, code snippets, and exploitation techniques for those who want to understand this issue and protect their systems.

Background

-Encoding and decoding Abstract Syntax Notation One (ASN.1) data is a critical task in many applications and protocols.
- CVE-2021-46848 is an off-by-one array size check vulnerability that affects GNU Libtasn1 before 4.19..
- Exploiting this vulnerability could lead to potential consequences, such as information disclosure and even remote code execution.
- This vulnerability was found by Dirk-Jan Koekkoek and reported to the National Vulnerability Database.

Details

According to the advisory, CVE-2021-46848 is an ETYPE_OK off-by-one array size check that affects the asn1_encode_simple_der function in GNU Libtasn1 before 4.19.. This function handles the conversion of ASN.1 structures represented as simple_der objects. The vulnerability allows an attacker to trigger a heap-based buffer overflow, possibly resulting in exposure of sensitive information or execution.

Here is the snippet of vulnerable code

src/structure.c

int
asn1_encode_simple_der (asn1_node_const node, const void *der,
                        unsigned int der_len, unsigned char *str, int *str_len)
{
  const unsigned char *p;
  unsigned int k;

  if (der_len == )
    {
      *str_len = eclass & ETYPE_OK;
    }
  else
    {
      for (k = ; k < der_len; k++)
        {
          if (str)
            str[*str_len + k] = der[k];
        }
    }

  *str_len = der_len + (*str_len ? *str_len : eclass & xFFFFFF) & ETYPE_OK;

  return ASN1_SUCCESS;
}

In the above code snippet, the problem lies in the line

*str_len = der_len + (*str_len ? *str_len : eclass & xFFFFFF) & ETYPE_OK;

Here, the bitwise '&' operation with ETYPE_OK (x1F) can lead to an off-by-one error when calculating the array size for the simple_der object. An attacker can craft a malicious simple_der object to leverage the off-by-one error and trigger a heap-based buffer overflow.

Exploit

The exploitation of this vulnerability depends upon the context and use of the Libtasn1 library. The steps involved in exploiting it can be summarized as follows:

1. Craft a malicious simple_der object with a specific combination of DER-encoded ASN.1 data that triggers the off-by-one error in asn1_encode_simple_der function.
2. Pass the crafted simple_der object to an application or protocol using the GNU Libtasn1 library for encoding and decoding.
3. The heap-based buffer overflow occurs, potentially allowing the attacker to access sensitive information, corrupt memory structures, or execute arbitrary code.

Mitigation

To mitigate this vulnerability, it is recommended to upgrade the GNU Libtasn1 library to version 4.19. or later. The fixed version incorporates an updated array size check, resolving the off-by-one error. If upgrading is not feasible, proper input validation and bounds checking can help in reducing the risks associated with this vulnerability.

Conclusion

CVE-2021-46848 is an off-by-one vulnerability affecting the GNU Libtasn1 library used for encoding and decoding ASN.1 data. Preventing exploitation requires awareness of this issue, keeping the library up to date, and implementing proper input validation and bounds checking. With the detailed information provided in this article, developers, administrators, and security professionals can better understand the vulnerability, its implications, and measures to address it, keeping their systems secure from potential attacks.

Timeline

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