Skip to content


Browse files Browse the repository at this point in the history
Fix DTLS replay protection
The DTLS implementation provides some protection against replay attacks
in accordance with RFC6347 section

A sliding "window" of valid record sequence numbers is maintained with
the "right" hand edge of the window set to the highest sequence number we
have received so far. Records that arrive that are off the "left" hand
edge of the window are rejected. Records within the window are checked
against a list of records received so far. If we already received it then
we also reject the new record.

If we have not already received the record, or the sequence number is off
the right hand edge of the window then we verify the MAC of the record.
If MAC verification fails then we discard the record. Otherwise we mark
the record as received. If the sequence number was off the right hand edge
of the window, then we slide the window along so that the right hand edge
is in line with the newly received sequence number.

Records may arrive for future epochs, i.e. a record from after a CCS being
sent, can arrive before the CCS does if the packets get re-ordered. As we
have not yet received the CCS we are not yet in a position to decrypt or
validate the MAC of those records. OpenSSL places those records on an
unprocessed records queue. It additionally updates the window immediately,
even though we have not yet verified the MAC. This will only occur if
currently in a handshake/renegotiation.

This could be exploited by an attacker by sending a record for the next
epoch (which does not have to decrypt or have a valid MAC), with a very
large sequence number. This means the right hand edge of the window is
moved very far to the right, and all subsequent legitimate packets are
dropped causing a denial of service.

A similar effect can be achieved during the initial handshake. In this
case there is no MAC key negotiated yet. Therefore an attacker can send a
message for the current epoch with a very large sequence number. The code
will process the record as normal. If the hanshake message sequence number
(as opposed to the record sequence number that we have been talking about
so far) is in the future then the injected message is bufferred to be
handled later, but the window is still updated. Therefore all subsequent
legitimate handshake records are dropped. This aspect is not considered a
security issue because there are many ways for an attacker to disrupt the
initial handshake and prevent it from completing successfully (e.g.
injection of a handshake message will cause the Finished MAC to fail and
the handshake to be aborted). This issue comes about as a result of trying
to do replay protection, but having no integrity mechanism in place yet.
Does it even make sense to have replay protection in epoch 0? That
issue isn't addressed here though.

This addressed an OCAP Audit issue.


Reviewed-by: Richard Levitte <>
  • Loading branch information
mattcaswell committed Aug 19, 2016
1 parent 30ea36e commit 3884b47
Show file tree
Hide file tree
Showing 3 changed files with 52 additions and 13 deletions.
60 changes: 48 additions & 12 deletions ssl/d1_pkt.c
Expand Up @@ -194,7 +194,7 @@ static int dtls1_record_needs_buffering(SSL *s, SSL3_RECORD *rr,
static int dtls1_buffer_record(SSL *s, record_pqueue *q,
unsigned char *priority);
static int dtls1_process_record(SSL *s);
static int dtls1_process_record(SSL *s, DTLS1_BITMAP *bitmap);

/* copy buffered record into SSL structure */
static int dtls1_copy_record(SSL *s, pitem *item)
Expand Down Expand Up @@ -320,13 +320,18 @@ static int dtls1_process_buffered_records(SSL *s)
pitem *item;
DTLS1_BITMAP *bitmap;
unsigned int is_next_epoch;
int replayok = 1;

item = pqueue_peek(s->d1->unprocessed_rcds.q);
if (item) {
/* Check if epoch is current. */
if (s->d1->unprocessed_rcds.epoch != s->d1->r_epoch)
return (1); /* Nothing to do. */
return 1; /* Nothing to do. */

rr = &s->s3->rrec;
rb = &s->s3->rbuf;

if (rb->left > 0) {
Expand All @@ -343,11 +348,41 @@ static int dtls1_process_buffered_records(SSL *s)
/* Process all the records. */
while (pqueue_peek(s->d1->unprocessed_rcds.q)) {
if (!dtls1_process_record(s))
return (0);
bitmap = dtls1_get_bitmap(s, rr, &is_next_epoch);
if (bitmap == NULL) {
* Should not happen. This will only ever be NULL when the
* current record is from a different epoch. But that cannot
* be the case because we already checked the epoch above
return 0;
/* Only do replay check if no SCTP bio */
if (!BIO_dgram_is_sctp(SSL_get_rbio(s)))
* Check whether this is a repeat, or aged record. We did this
* check once already when we first received the record - but
* we might have updated the window since then due to
* records we subsequently processed.
replayok = dtls1_record_replay_check(s, bitmap);

if (!replayok || !dtls1_process_record(s, bitmap)) {
/* dump this record */
rr->length = 0;
s->packet_length = 0;

if (dtls1_buffer_record(s, &(s->d1->processed_rcds),
s->s3->rrec.seq_num) < 0)
return -1;
return 0;

Expand All @@ -358,7 +393,7 @@ static int dtls1_process_buffered_records(SSL *s)
s->d1->processed_rcds.epoch = s->d1->r_epoch;
s->d1->unprocessed_rcds.epoch = s->d1->r_epoch + 1;

return (1);
return 1;

#if 0
Expand Down Expand Up @@ -405,7 +440,7 @@ static int dtls1_get_buffered_record(SSL *s)


static int dtls1_process_record(SSL *s)
static int dtls1_process_record(SSL *s, DTLS1_BITMAP *bitmap)
int i, al;
int enc_err;
Expand Down Expand Up @@ -565,6 +600,10 @@ static int dtls1_process_record(SSL *s)

/* we have pulled in a full packet so zero things */
s->packet_length = 0;

/* Mark receipt of record. */
dtls1_record_bitmap_update(s, bitmap);

return (1);

Expand Down Expand Up @@ -600,7 +639,7 @@ int dtls1_get_record(SSL *s)
* The epoch may have changed. If so, process all the pending records.
* This is a non-blocking operation.
if (dtls1_process_buffered_records(s) < 0)
if (!dtls1_process_buffered_records(s))
return -1;

/* if we're renegotiating, then there may be buffered records */
Expand Down Expand Up @@ -735,20 +774,17 @@ int dtls1_get_record(SSL *s)
if (dtls1_buffer_record
(s, &(s->d1->unprocessed_rcds), rr->seq_num) < 0)
return -1;
/* Mark receipt of record. */
dtls1_record_bitmap_update(s, bitmap);
rr->length = 0;
s->packet_length = 0;
goto again;

if (!dtls1_process_record(s)) {
if (!dtls1_process_record(s, bitmap)) {
rr->length = 0;
s->packet_length = 0; /* dump this record */
goto again; /* get another record */
dtls1_record_bitmap_update(s, bitmap); /* Mark receipt of record. */

return (1);

Expand Down
1 change: 1 addition & 0 deletions ssl/ssl.h
Expand Up @@ -2615,6 +2615,7 @@ void ERR_load_SSL_strings(void);
# define SSL_F_DTLS1_HEARTBEAT 305
# define SSL_F_DTLS1_READ_BYTES 258
Expand Down
4 changes: 3 additions & 1 deletion ssl/ssl_err.c
@@ -1,6 +1,6 @@
/* ssl/ssl_err.c */
/* ====================================================================
* Copyright (c) 1999-2015 The OpenSSL Project. All rights reserved.
* Copyright (c) 1999-2016 The OpenSSL Project. All rights reserved.
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
Expand Down Expand Up @@ -93,6 +93,8 @@ static ERR_STRING_DATA SSL_str_functs[] = {
{ERR_FUNC(SSL_F_DTLS1_HEARTBEAT), "dtls1_heartbeat"},
{ERR_FUNC(SSL_F_DTLS1_OUTPUT_CERT_CHAIN), "dtls1_output_cert_chain"},
Expand Down

0 comments on commit 3884b47

Please sign in to comment.