A vulnerability, which was classified as critical, was found in LibTomCrypt up to 1.18.2. This affects the function der_decode_utf8_string of the file der_decode_utf8_string.c of the component UTF-8 Handler. The manipulation with an unknown input leads to a out-of-bounds vulnerability. CWE is classifying the issue as CWE-125. The product reads data past the end, or before the beginning, of the intended buffer. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

In LibTomCrypt through 1.18.2, the der_decode_utf8_string function (in der_decode_utf8_string.c) does not properly detect certain invalid UTF-8 sequences. This allows context-dependent attackers to cause a denial of service (out-of-bounds read and crash) or read information from other memory locations via carefully crafted DER-encoded data.

The weakness was published 10/03/2019 by Luigi Coniglio (werew) as confirmed bug report (GitHub Repository). It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2019-17362 since 10/09/2019. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. Technical details and a public exploit are known. The reason for this vulnerability is this part of code:

/* proceed to decode */
for (y = 0; x < inlen; ) {
  /* get first byte */
  tmp = in[x++];

  /* count number of bytes */
  for (z = 0; (tmp & 0x80) && (z <= 4); z++, tmp = (tmp << 1) & 0xFF);

  if (z > 4 || (x + (z - 1) > inlen)) {
	 return CRYPT_INVALID_PACKET;
  }

  /* decode, grab upper bits */
  tmp >>= z;

  /* grab remaining bytes */
  if (z > 1) { --z; }
  while (z-- != 0) {
	 if ((in[x] & 0xC0) != 0x80) {
		return CRYPT_INVALID_PACKET;
	 }
	 tmp = (tmp << 6) | ((wchar_t)in[x++] & 0x3F);
  }

  if (y < *outlen) {
	 out[y] = tmp;
  }
  y++;
}

A public exploit has been developed by Luigi Coniglio (werew) and been published immediately after the advisory. The exploit is shared for download at github.com. It is declared as proof-of-concept. The code used by the exploit is:

char *poc1 = "\x30\x04"                  // Start DER-sequence of length 4
             "\x0c\x02\xbf\xbf"          // Start UTF8 string of actual length 2 and evaluated length 3
             "\xaa"                      // One byte padding
             "\x30\x84\xff\xff\xff\xff"; // Start DER-sequence of length 0xffffffff (will very likely cause a crash)

Applying a patch is able to eliminate this problem. The bugfix is ready for download at github.com.

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Productinfo

Name

• LibTomCrypt

Version

• 1.18.0
• 1.18.1
• 1.18.2

CPE 2.3info

• 🔍
• 🔍
• 🔍

CPE 2.2info

• 🔍
• 🔍
• 🔍

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Exploitinginfo

Threat Intelligenceinfo

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Entryinfo

Submitinfo

Accepted

• Submit #92: Out-of-bounds read in LibTomCrypt 1.18.2 and earlier versions (by werew)

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