RE: Best way of preventing denial of service attacks by way of secure client-initiated renegotiation
Ok, great thanks. -Original Message- From: Matt Caswell Sent: Monday 15 April 2019 14:45 To: tim.j.culh...@gmail.com; openssl-users@openssl.org Subject: Re: Best way of preventing denial of service attacks by way of secure client-initiated renegotiation On 15/04/2019 14:41, tim.j.culh...@gmail.com wrote: > Hi Matt, > > Many thanks for your informative reply. > > So it seems the best approach is to upgrade to a version of OpenSSL > supporting the SSL_OP_NO_RENGOTIATION option. > > If this option is enabled will it still allow server-initiated secure > renegotiations if TLS 1.3 is being used? > > The docs suggests that only client side renegotiation requests are disabled > in TLS 1.3. Renegotiation does not exist as a concept in TLSv1.3 so this option has no impact in TLSv1.3. Matt > > Tim > > > -Original Message- > From: openssl-users On Behalf Of > Matt Caswell > Sent: Monday 15 April 2019 13:44 > To: openssl-users@openssl.org > Subject: Re: Best way of preventing denial of service attacks by way > of secure client-initiated renegotiation > > > > On 15/04/2019 09:35, tim.j.culh...@gmail.com wrote: >> I'm not sure if this means renegotiation has failed? Either way the >> connection remains open. Presumably if a client issued a large >> number of renegotiations like this the server could become overwhelmed. > > No - renegotiation was successful. > >> Note that I got the same results if I remove the >> -legacy_renegotiation option, so I don't think this has any impact? > > The legacy_renegotiation option does something different to what you > think it does. This option allows "insecure" renegotiation as opposed > to the later (and > default) "secure" renegotiation. This dates back to 2009 when a flaw in the > TLS protocol for renegotiation was discovered. > >> >> So, I suppose I firstly need to know if the results from testssl.sh >> and from my own investigations point to a potential security risk by >> way of a DoS attack? > > Over the years there have been many attacks against renegotiation. They've > all been fixed, however since this is a common attack vector and many > applications don't need this feature it is often recommended that it is > disabled. > > >> If so, what is the best way to prevent this. > > The best way is to upgrade to a recent version of OpenSSL and use the > SSL_OP_NO_RENGOTIATION option for this purpose (available from 1.1.0h and > above). > > If you *must* use OpenSSL 1.0.2 then there is a way to do it but it is > undocumented and unfortunately this method is no longer possible in OpenSSL > 1.1.0+ due to the opacity changes. > > You can mark a particular SSL object (call it "s") so that it should not do > renegotiation like this: > > s->s3->flags |= SSL3_FLAGS_NO_RENEGOTIATE_CIPHERS; > > >> From what I've read online it isn't possible to disable >> client-initiated secure renegotiation in openssl. >> Indeed, it could be argued that there are circumstances when it is >> perfectly valid for a client to renegotiate a connection especially >> if it is a long-running connection. >> >> The only way I could find of limiting such an attack was to track >> the number of renegotiation requests over a time and if we get a high >> number in a short period then close the connection. >> I believe this can be done in a callback function set up via a call to: >> >> SSL_CTX_set_info_callback > > I'd recommend against this approach. A number of applications took this route > due to a lack of a good alternative. However it can have unexpected > consequences if you later upgrade to OpenSSL 1.1.1 and start using TLSv1.3 > (where a number of legitimate interactions happen post-handshake that can be > mistaken for renegotiations). > > Matt >
Re: Best way of preventing denial of service attacks by way of secure client-initiated renegotiation
On 15/04/2019 14:41, tim.j.culh...@gmail.com wrote: > Hi Matt, > > Many thanks for your informative reply. > > So it seems the best approach is to upgrade to a version of OpenSSL > supporting the SSL_OP_NO_RENGOTIATION option. > > If this option is enabled will it still allow server-initiated secure > renegotiations if TLS 1.3 is being used? > > The docs suggests that only client side renegotiation requests are disabled > in TLS 1.3. Renegotiation does not exist as a concept in TLSv1.3 so this option has no impact in TLSv1.3. Matt > > Tim > > > -Original Message- > From: openssl-users On Behalf Of Matt > Caswell > Sent: Monday 15 April 2019 13:44 > To: openssl-users@openssl.org > Subject: Re: Best way of preventing denial of service attacks by way of > secure client-initiated renegotiation > > > > On 15/04/2019 09:35, tim.j.culh...@gmail.com wrote: >> I'm not sure if this means renegotiation has failed? Either way the >> connection remains open. Presumably if a client issued a large number >> of renegotiations like this the server could become overwhelmed. > > No - renegotiation was successful. > >> Note that I got the same results if I remove the -legacy_renegotiation >> option, so I don't think this has any impact? > > The legacy_renegotiation option does something different to what you think it > does. This option allows "insecure" renegotiation as opposed to the later (and > default) "secure" renegotiation. This dates back to 2009 when a flaw in the > TLS protocol for renegotiation was discovered. > >> >> So, I suppose I firstly need to know if the results from testssl.sh >> and from my own investigations point to a potential security risk by >> way of a DoS attack? > > Over the years there have been many attacks against renegotiation. They've > all been fixed, however since this is a common attack vector and many > applications don't need this feature it is often recommended that it is > disabled. > > >> If so, what is the best way to prevent this. > > The best way is to upgrade to a recent version of OpenSSL and use the > SSL_OP_NO_RENGOTIATION option for this purpose (available from 1.1.0h and > above). > > If you *must* use OpenSSL 1.0.2 then there is a way to do it but it is > undocumented and unfortunately this method is no longer possible in OpenSSL > 1.1.0+ due to the opacity changes. > > You can mark a particular SSL object (call it "s") so that it should not do > renegotiation like this: > > s->s3->flags |= SSL3_FLAGS_NO_RENEGOTIATE_CIPHERS; > > >> From what I've read online it isn't possible to disable >> client-initiated secure renegotiation in openssl. >> Indeed, it could be argued that there are circumstances when it is >> perfectly valid for a client to renegotiate a connection especially >> if it is a long-running connection. >> >> The only way I could find of limiting such an attack was to track the >> number of renegotiation requests over a time and if we get a high >> number in a short period then close the connection. >> I believe this can be done in a callback function set up via a call to: >> >> SSL_CTX_set_info_callback > > I'd recommend against this approach. A number of applications took this route > due to a lack of a good alternative. However it can have unexpected > consequences if you later upgrade to OpenSSL 1.1.1 and start using TLSv1.3 > (where a number of legitimate interactions happen post-handshake that can be > mistaken for renegotiations). > > Matt >
RE: Best way of preventing denial of service attacks by way of secure client-initiated renegotiation
Hi Matt, Many thanks for your informative reply. So it seems the best approach is to upgrade to a version of OpenSSL supporting the SSL_OP_NO_RENGOTIATION option. If this option is enabled will it still allow server-initiated secure renegotiations if TLS 1.3 is being used? The docs suggests that only client side renegotiation requests are disabled in TLS 1.3. Tim -Original Message- From: openssl-users On Behalf Of Matt Caswell Sent: Monday 15 April 2019 13:44 To: openssl-users@openssl.org Subject: Re: Best way of preventing denial of service attacks by way of secure client-initiated renegotiation On 15/04/2019 09:35, tim.j.culh...@gmail.com wrote: > I'm not sure if this means renegotiation has failed? Either way the > connection remains open. Presumably if a client issued a large number > of renegotiations like this the server could become overwhelmed. No - renegotiation was successful. > Note that I got the same results if I remove the -legacy_renegotiation > option, so I don't think this has any impact? The legacy_renegotiation option does something different to what you think it does. This option allows "insecure" renegotiation as opposed to the later (and default) "secure" renegotiation. This dates back to 2009 when a flaw in the TLS protocol for renegotiation was discovered. > > So, I suppose I firstly need to know if the results from testssl.sh > and from my own investigations point to a potential security risk by > way of a DoS attack? Over the years there have been many attacks against renegotiation. They've all been fixed, however since this is a common attack vector and many applications don't need this feature it is often recommended that it is disabled. > If so, what is the best way to prevent this. The best way is to upgrade to a recent version of OpenSSL and use the SSL_OP_NO_RENGOTIATION option for this purpose (available from 1.1.0h and above). If you *must* use OpenSSL 1.0.2 then there is a way to do it but it is undocumented and unfortunately this method is no longer possible in OpenSSL 1.1.0+ due to the opacity changes. You can mark a particular SSL object (call it "s") so that it should not do renegotiation like this: s->s3->flags |= SSL3_FLAGS_NO_RENEGOTIATE_CIPHERS; > From what I've read online it isn't possible to disable > client-initiated secure renegotiation in openssl. > Indeed, it could be argued that there are circumstances when it is > perfectly valid for a client to renegotiate a connection especially > if it is a long-running connection. > > The only way I could find of limiting such an attack was to track the > number of renegotiation requests over a time and if we get a high > number in a short period then close the connection. > I believe this can be done in a callback function set up via a call to: > > SSL_CTX_set_info_callback I'd recommend against this approach. A number of applications took this route due to a lack of a good alternative. However it can have unexpected consequences if you later upgrade to OpenSSL 1.1.1 and start using TLSv1.3 (where a number of legitimate interactions happen post-handshake that can be mistaken for renegotiations). Matt
Re: Best way of preventing denial of service attacks by way of secure client-initiated renegotiation
On 15/04/2019 09:35, tim.j.culh...@gmail.com wrote: > I'm not sure if this means renegotiation has failed? Either way the > connection remains open. Presumably if a client issued a large number of > renegotiations like this the server could become overwhelmed. No - renegotiation was successful. > Note that I got the same results if I remove the -legacy_renegotiation > option, so I don't think this has any impact? The legacy_renegotiation option does something different to what you think it does. This option allows "insecure" renegotiation as opposed to the later (and default) "secure" renegotiation. This dates back to 2009 when a flaw in the TLS protocol for renegotiation was discovered. > > So, I suppose I firstly need to know if the results from testssl.sh and from > my own investigations point to a potential security risk by way of a DoS > attack? Over the years there have been many attacks against renegotiation. They've all been fixed, however since this is a common attack vector and many applications don't need this feature it is often recommended that it is disabled. > If so, what is the best way to prevent this. The best way is to upgrade to a recent version of OpenSSL and use the SSL_OP_NO_RENGOTIATION option for this purpose (available from 1.1.0h and above). If you *must* use OpenSSL 1.0.2 then there is a way to do it but it is undocumented and unfortunately this method is no longer possible in OpenSSL 1.1.0+ due to the opacity changes. You can mark a particular SSL object (call it "s") so that it should not do renegotiation like this: s->s3->flags |= SSL3_FLAGS_NO_RENEGOTIATE_CIPHERS; > From what I've read online it isn't possible to disable client-initiated > secure renegotiation in openssl. > Indeed, it could be argued that there are circumstances when it is perfectly > valid for a client to renegotiate a connection especially if it is a > long-running connection. > > The only way I could find of limiting such an attack was to track the > number of renegotiation requests over a time and if we get a high number in > a short period then close the connection. > I believe this can be done in a callback function set up via a call to: > > SSL_CTX_set_info_callback I'd recommend against this approach. A number of applications took this route due to a lack of a good alternative. However it can have unexpected consequences if you later upgrade to OpenSSL 1.1.1 and start using TLSv1.3 (where a number of legitimate interactions happen post-handshake that can be mistaken for renegotiations). Matt
Best way of preventing denial of service attacks by way of secure client-initiated renegotiation
Hi all, A customer of ours was recently running security checks against our mail server. To do this they were running the testssl.sh script available at: https://testssl.sh/ The test tool reports a potential DoS thread as a result of client-initiated secure renegotiation as shown from the following line from the test tool's output: [1m Secure Client-Initiated Renegotiation [m[0;33mVULNERABLE (NOT ok)[m, potential DoS threat I have also reproduced their findings in-house. Our mail server is using version 1.0.2g of openssl and the client openssl version was 1.0.2j. In the testssl.sh script it connects to the ssl port using TLS1.2 and the legacy_renegotiation option. When I do a similar test in my test environment I get results like the below: Issue a connect request, pause for 1 second and then Send 'R' to start a renegotiation: (echo -n; sleep 1; echo R) | openssl s_client -CAfile NightlyBuild-CA.cert -crlf -tls1_2 -legacy_renegotiation -connect :465 Output is as follows: CONNECTED(0003) --- Certificate chain 0 s:/C=ie/CN=something/CN=devimpala13.es.cpth.ie i:/C=AU/ST=Some-State/O=Internet Widgits Pty Ltd --- Server certificate -BEGIN CERTIFICATE- MIIC0DCCAjmgAwIBAgIBATANBgkqhkiG9w0BAQQFADBFMQswCQYDVQQGEwJBVTET ... -END CERTIFICATE- subject=/C=ie/CN=something/CN=devimpala13.es.cpth.ie issuer=/C=AU/ST=Some-State/O=Internet Widgits Pty Ltd --- No client certificate CA names sent Client Certificate Types: RSA sign, DSA sign, ECDSA sign Requested Signature Algorithms: RSA+SHA512:DSA+SHA512:ECDSA+SHA512:RSA+SHA384:DSA+SHA384:ECDSA+SHA384:RSA+SH A256:DSA+SHA256:ECDSA+SHA256:RSA+SHA224:DSA+SHA224:ECDSA+SHA224:RSA+SHA1:DSA +SHA1:ECDSA+SHA1 Shared Requested Signature Algorithms: RSA+SHA512:DSA+SHA512:ECDSA+SHA512:RSA+SHA384:DSA+SHA384:ECDSA+SHA384:RSA+SH A256:DSA+SHA256:ECDSA+SHA256:RSA+SHA224:DSA+SHA224:ECDSA+SHA224:RSA+SHA1:DSA +SHA1:ECDSA+SHA1 Peer signing digest: SHA512 Server Temp Key: ECDH, P-256, 256 bits --- SSL handshake has read 1297 bytes and written 445 bytes --- New, TLSv1/SSLv3, Cipher is ECDHE-RSA-AES256-GCM-SHA384 Server public key is 1024 bit Secure Renegotiation IS supported Compression: NONE Expansion: NONE No ALPN negotiated SSL-Session: Protocol : TLSv1.2 Cipher: ECDHE-RSA-AES256-GCM-SHA384 Session-ID: A1E559EAFC571033874C52C715E8CE3452751FB80AD843F4E051E77664F20FFE Session-ID-ctx: Master-Key: BE06E684D71B9B3349DBB057C433BB0C0C44717EF2157EAA912A896F43DF8E6E912A69B8258E C9031CF2219F0F031C1B Key-Arg : None PSK identity: None PSK identity hint: None SRP username: None TLS session ticket lifetime hint: 300 (seconds) TLS session ticket: - 61 3a da 77 02 6c 7e 26-c1 98 84 ae 26 21 8f 1d a:.w.l~&&!.. ... Start Time: 1555315404 Timeout : 7200 (sec) Verify return code: 0 (ok) --- 220 devimpala13.es.cpth.ie ESMTP Service ready RENEGOTIATING depth=1 C = AU, ST = Some-State, O = Internet Widgits Pty Ltd verify return:1 depth=0 C = ie, CN = something, CN = devimpala13.es.cpth.ie verify return:1 DONE So, as you can see the initial connection is successful. The renegotiation begins and it returns: verify return:1 I'm not sure if this means renegotiation has failed? Either way the connection remains open. Presumably if a client issued a large number of renegotiations like this the server could become overwhelmed. Note that I got the same results if I remove the -legacy_renegotiation option, so I don't think this has any impact? So, I suppose I firstly need to know if the results from testssl.sh and from my own investigations point to a potential security risk by way of a DoS attack? If so, what is the best way to prevent this. >From what I've read online it isn't possible to disable client-initiated secure renegotiation in openssl. Indeed, it could be argued that there are circumstances when it is perfectly valid for a client to renegotiate a connection especially if it is a long-running connection. The only way I could find of limiting such an attack was to track the number of renegotiation requests over a time and if we get a high number in a short period then close the connection. I believe this can be done in a callback function set up via a call to: SSL_CTX_set_info_callback So, is the above information correct or is there a better way of preventing this sort of attack. Do we really need to allow a limited form of client-based secure renegotiation? If preventing an attack by way of monitoring the number of renegotiations over time is the only good approach, do you have any code examples in C as to how to implement this? Many thanks, Tim
