CVE-2020-18651: A Deep Dive into the Buffer Overflow Vulnerability in Exempi 2.5. and Earlier, Exploiting Audio Files with ID3v2 Frames

Vulnerabilities in software libraries can open doors for attackers, exposing sensitive information and leading to potential exploitation of user's systems. One such vulnerability, categorized as a Buffer Overflow bug, was found in Exempi 2.5. and earlier versions. This vulnerability, assigned the identifier CVE-2020-18651, allows attackers to cause a denial of service (DoS) by opening a specially crafted audio file with an ID3v2 frame. In this post, we will dissect the vulnerability, explore the ID3_Support::ID3v2Frame::getFrameValue function, and discuss potential exploitation methods.

Vulnerability Details

The vulnerability resides in the ID3_Support::ID3v2Frame::getFrameValue function in versions 2.5. and earlier of the exempi library. Before diving into the specific code snippet and function responsible for this vulnerability, let's look at the bigger picture.

Exempi is a C++ library that is used for parsing XMP metadata (Extensible Metadata Platform) from various file formats like PDF, JPEG, and audio files, developed and maintained by Adobe. ID3v2 frames are employed by MP3 files to store metadata like song title, artist, and album.

The vulnerability exists due to insufficient bounds-checking in the ID3_Support::ID3v2Frame::getFrameValue function of the Exempi library, which can be exploited by an attacker to trigger a buffer overflow. The consequences include denial of service or even potentially remote code execution if cleverly exploited.

Here's the relevant code snippet from Exempi

std::string ID3v2Frame::getFrameValue() const {
  std::string fieldValue;
  fieldValue.reserve(64);
  for (size_t i = ; i < _dataSize; ++i) {
    if (_data[i] != ) {
      fieldValue.push_back(_data[i]);
    }
  }
  return fieldValue;
}

original source: https://github.com/hfiguiere/exempi/blob/master/libid3/id3v2frame.cpp

In the above code, a new string (fieldValue) with a reserved size of 64 bytes is created. The for loop iterates through the data contained in the ID3v2 frame and appends nonzero data bytes to fieldValue. At no point is there any check to ensure that the fieldValue string does not exceed the initial 64-byte buffer, allowing for a buffer overflow scenario.

Exploitation

To exploit this vulnerability, an attacker could create a specifically crafted audio file containing a large ID3v2 frame that would trigger a buffer overflow when the getFrameValue() function attempts to process it. When unsuspecting users open this audio file, their system or application might crash or hang, causing a denial of service.

The attacker might also exploit this vulnerability to execute arbitrary code by carefully crafting the contents of the ID3v2 frame to overwrite essential memory addresses or take control of the application's execution flow.

Mitigation

This vulnerability was fixed in Exempi 2.5.1, which introduced proper bounds checking to the getFrameValue() function as follows:

std::string ID3v2Frame::getFrameValue() const {
  std::string fieldValue;
  fieldValue.reserve(64);
  for (size_t i = ; i < _dataSize; ++i) {
    if (_data[i] != ) {
      fieldValue.push_back(_data[i]);
      if (fieldValue.size() >= 64) {
        break;
      }
    }
  }
  return fieldValue;
}

In the updated code, a bounds check is added to ensure that the fieldValue string does not exceed 64 bytes, thus preventing the buffer overflow from occurring.

To protect against this vulnerability, users of Exempi should update to version 2.5.1 (or later) and verify that they are not using a vulnerable version. Furthermore, users should be cautious when opening audio files from untrusted sources to avoid attacks leveraging this or similar vulnerabilities.

Conclusion

CVE-2020-18651 is a severe buffer overflow vulnerability that affected exempi versions 2.5. and earlier. Thanks to diligent work by the developers and the prompt release of a patched version, Exempi users can prevent buffer overflow attacks by upgrading to version 2.5.1 or later. This vulnerability underscores the importance of thoroughly checking input data and memory management functions in software libraries to maintain secure and robust systems.

Timeline

Published on: 08/22/2023 19:15:00 UTC
Last modified on: 09/26/2023 01:15:00 UTC