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This code processes a 'heartbeat' packet from a client. As specified in RFC 6520, when the program receives a heartbeat packet, it must echo the packet's data back to the client. In addition to the data, the packet contains a length field that conventionally indicates the number of bytes in the packet data, but there is nothing to prevent a malicious packet from lying about its data length.
The p pointer, along with payload and p1 contain data from this a packet. The code allocates a buffer sufficient to contain payload bytes, with some overhead, and copies payload bytes starting at p1 into this buffer, and sends it to the client. Notably absent are any checks that payload actually indicates the correct size of the memory. Because an attacker can specify an arbitrary value for payload, she can cause this routine to read and return memory beyond the block allocated to p.
Compliant Solution (Heartbleed)
OpenSSL version 1.0.1g contains the following patch, which guarantees that payload is within a valid range:. The range is limited by the size of the input record.
| Code Block | ||||
|---|---|---|---|---|
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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 ... */
} |
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