Hello everyone! Today, we will dive deep into a recent vulnerability that has been given the identifier CVE-2022-22826. This security issue is found in the Expat library, specifically in the xmlparse.c file, and can lead to an integer overflow. But before we jump into the details, let's learn a little about the background of this library.

Background: Expat (libexpat)

Expat, also known as libexpat, is a widely-used XML parsing library written in C. It's a popular choice for XML parsing because of its speed and ease of use. It's used in numerous software applications and programming languages, making it quite crucial to ensure its security.

Now that we have an idea of what Expat is let's get to the crux of the vulnerability.

Vulnerability: CVE-2022-22826

The vulnerability in question, CVE-2022-22826, is an integer overflow that has been observed in the function 'nextScaffoldPart' located in the xmlparse.c file. This issue affects versions of Expat before 2.4.3.

An integer overflow occurs when an operation attempts to store a value that exceeds the maximum limit of the data type, causing it to "wrap around" to a smaller value. In some cases, this can be exploited by attackers to cause unexpected behavior or even execute arbitrary code.

Here's a snippet of the vulnerable code from the file xmlparse.c

    int count; /* number of ordered types or groups */
    XML_Parser parser;
    struct SCAFFOLD *scaffold;
static struct SCAFFOLD *
nextScaffoldPart(XML_Parser parser) {
  memoryManager *const memMgr = &(parser->m_mem);
  CONTENT_MODEL * const model = &(parser->m_tempPool->m_openInternalEntities);;
  struct  CONTENT_SCAFFOLD * scaffold;
  if (model->count >= model->size) {
    struct SCAFFOLD *temp = REALLOC(&model->scaffold, model->size * 2 * sizeof(*temp)); /* ISSUE OCCURS HERE */
    if (temp) {
      model->scaffold = temp;
      model->size *= 2;
    } else {
      model->scaffold = NULL;
      return NULL;
  scaffold = &model->scaffold[model->count];
  return scaffold;

The vulnerability occurs when the model->size variable is multiplied by 2 and the size of the struct SCAFFOLD. If this multiplication results in an integer overflow, the memory allocation will be incorrect, possibly leading to memory corruption or other adverse effects.

Exploitation Details

Although this vulnerability on its own may not be directly exploitable for remote code execution, it could potentially be used in conjunction with other memory corruption issues or vulnerabilities to create more severe exploits. For example, an attacker could exploit this vulnerability to cause the program to crash or misbehave, or corrupt memory in a controlled way for further exploitation.

Remediation and References

The Expat development team has released a new version, 2.4.3, which fixes this vulnerability. It is strongly recommended to update your Expat library to the latest version to mitigate this issue. You can find the latest release here:

- Expat GitHub: https://github.com/libexpat/libexpat/releases/tag/R_2_4_3

- CVE-2022-22826 Advisory: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2022-22826
- Exploit Database: https://www.exploit-db.com/exploits/50448


In conclusion, CVE-2022-22826 is an integer overflow vulnerability found in the Expat library's xmlparse.c file, specifically concerning the 'nextScaffoldPart' function. Update your Expat library to version 2.4.3 to mitigate this issue. Due to the widespread use of this library, it is essential to keep it up to date and follow security best practices to minimize the chances of exploitation.


Published on: 01/10/2022 14:12:00 UTC
Last modified on: 06/14/2022 11:15:00 UTC