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Despite the seriousness of the vulnerability, Heartbleed is the result of a common programming error and an apparent lack of awareness of secure coding principles. Following is the vulnerable code:

int dtls1_process_heartbeat(SSL *s) {          
  unsigned char *p = &s->s3->rrec.data[0], *pl;
  unsigned short hbtype;
  unsigned int payload;
  unsigned int padding = 16; /* Use minimum padding */

  /* Read type and payload length first */
  hbtype = *p++;
  n2s(p, payload);
  pl = p;

  /* ... More code ... */

  if (hbtype == TLS1_HB_REQUEST) {
    unsigned char *buffer, *bp;
    int r;

    /* Allocate memory for the response, size is 1 byte
     * message type, plus 2 bytes payload length, plus
     * payload, plus padding
     */
    buffer = OPENSSL_malloc(1 + 2 + payload + padding);
    bp = buffer;

    /* Enter response type, length and copy payload */
    *bp++ = TLS1_HB_RESPONSE;
    s2n(payload, bp);
    memcpy(bp, pl, payload);

    /* ... More code ... */
  }
  /* ... More code ... */
}

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The p pointer, along with payload and p1, contain data from a packet. The code allocates a buffer sufficient to contain payload bytes, with some overhead, then copies payload bytes starting at p1 into this buffer and sends it to the client. Notably absent from this code are any checks that the payload integer variable extracted from the heartbeat packet corresponds to the size of the packet data. Because the client can specify an arbitrary value of payload, an attacker can cause the server to read and return the contents of memory beyond the end of the packet data, which violates INT04-C. Enforce limits on integer values originating from tainted sources. The resulting call to memcpy() can then copy the contents of memory past the end of the packet data and the packet itself, potentially exposing sensitive data to the attacker. This call to memcpy() violates ARR38-C. Guarantee that library functions do not form invalid pointers. A version of ARR38-C also appears in ISO/IEC TS 17961:2013, "Forming invalid pointers by library functions [libptr]." This rule would require a conforming analyzer to diagnose the Heartbleed vulnerability.

int dtls1_process_heartbeat(SSL *s) {          
  unsigned char *p = &s->s3->rrec.data[0], *pl;
  unsigned short hbtype;
  unsigned int payload;
  unsigned int padding = 16; /* Use minimum padding */

  /* ... More code ... */

  /* Read type and payload length first */
  if (1 + 2 + 16 > s->s3->rrec.length)
    return 0; /* silently discard */
  hbtype = *p++;
  n2s(p, payload);
  if (1 + 2 + payload + 16 > s->s3->rrec.length)
    return 0; /* silently discard per RFC 6520 */
  pl = p;

  /* ... More code ... */

  if (hbtype == TLS1_HB_REQUEST) {
    unsigned char *buffer, *bp;
    int r;

    /* Allocate memory for the response, size is 1 byte
     * message type, plus 2 bytes payload length, plus
     * payload, plus padding
     */
    buffer = OPENSSL_malloc(1 + 2 + payload + padding);
    bp = buffer;
    /* Enter response type, length and copy payload */
    *bp++ = TLS1_HB_RESPONSE;
    s2n(payload, bp);
    memcpy(bp, pl, payload);
    /* ... More code ... */
  }
  /* ... More code ... */
}

Failing to enforce the limits on integer values can result in a denial-of-service attack, unauthorized disclosure of information, or to run arbitrary code.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography

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