Vulnerabilities > CVE-2016-9244 - Information Exposure vulnerability in F5 products
Attack vector
NETWORK Attack complexity
LOW Privileges required
NONE Confidentiality impact
HIGH Integrity impact
NONE Availability impact
NONE Summary
A BIG-IP virtual server configured with a Client SSL profile that has the non-default Session Tickets option enabled may leak up to 31 bytes of uninitialized memory. A remote attacker may exploit this vulnerability to obtain Secure Sockets Layer (SSL) session IDs from other sessions. It is possible that other data from uninitialized memory may be returned as well.
Vulnerable Configurations
Common Weakness Enumeration (CWE)
Common Attack Pattern Enumeration and Classification (CAPEC)
- Subverting Environment Variable Values The attacker directly or indirectly modifies environment variables used by or controlling the target software. The attacker's goal is to cause the target software to deviate from its expected operation in a manner that benefits the attacker.
- Footprinting An attacker engages in probing and exploration activity to identify constituents and properties of the target. Footprinting is a general term to describe a variety of information gathering techniques, often used by attackers in preparation for some attack. It consists of using tools to learn as much as possible about the composition, configuration, and security mechanisms of the targeted application, system or network. Information that might be collected during a footprinting effort could include open ports, applications and their versions, network topology, and similar information. While footprinting is not intended to be damaging (although certain activities, such as network scans, can sometimes cause disruptions to vulnerable applications inadvertently) it may often pave the way for more damaging attacks.
- Exploiting Trust in Client (aka Make the Client Invisible) An attack of this type exploits a programs' vulnerabilities in client/server communication channel authentication and data integrity. It leverages the implicit trust a server places in the client, or more importantly, that which the server believes is the client. An attacker executes this type of attack by placing themselves in the communication channel between client and server such that communication directly to the server is possible where the server believes it is communicating only with a valid client. There are numerous variations of this type of attack.
- Browser Fingerprinting An attacker carefully crafts small snippets of Java Script to efficiently detect the type of browser the potential victim is using. Many web-based attacks need prior knowledge of the web browser including the version of browser to ensure successful exploitation of a vulnerability. Having this knowledge allows an attacker to target the victim with attacks that specifically exploit known or zero day weaknesses in the type and version of the browser used by the victim. Automating this process via Java Script as a part of the same delivery system used to exploit the browser is considered more efficient as the attacker can supply a browser fingerprinting method and integrate it with exploit code, all contained in Java Script and in response to the same web page request by the browser.
- Session Credential Falsification through Prediction This attack targets predictable session ID in order to gain privileges. The attacker can predict the session ID used during a transaction to perform spoofing and session hijacking.
Exploit-Db
description F5 BIG-IP 11.6 SSL Virtual Server - 'Ticketbleed' Memory Disclosure. CVE-2016-9244. Remote exploit for Hardware platform id EDB-ID:44446 last seen 2018-05-24 modified 2017-02-14 published 2017-02-14 reporter Exploit-DB source https://www.exploit-db.com/download/44446/ title F5 BIG-IP 11.6 SSL Virtual Server - 'Ticketbleed' Memory Disclosure description F5 BIG-IP SSL Virtual Server - Memory Disclosure. CVE-2016-9244. Remote exploit for Hardware platform file exploits/hardware/remote/41298.txt id EDB-ID:41298 last seen 2017-02-11 modified 2017-02-10 platform hardware port published 2017-02-10 reporter Exploit-DB source https://www.exploit-db.com/download/41298/ title F5 BIG-IP SSL Virtual Server - Memory Disclosure type remote
Nessus
NASL family General NASL id F5_SESSION_ID_MEM_DISCLOSURE.NASL description Based on its response to a resumed TLS connection, the remote service appears to be affected by an information disclosure vulnerability, known as Ticketbeed, in the TLS Session Ticket implementation. The issue is due to the server incorrectly echoing back 32 bytes of memory, even if the Session ID was shorter. A remote attacker can exploit this vulnerability, by providing a 1-byte Session ID, to disclose up to 31 bytes of uninitialized memory which may contain sensitive information such as private keys, passwords, and other sensitive data. Note that this vulnerability is only exploitable if the non-default Session Tickets option enabled. last seen 2020-04-07 modified 2017-02-15 plugin id 97191 published 2017-02-15 reporter This script is Copyright (C) 2017-2020 and is owned by Tenable, Inc. or an Affiliate thereof. source https://www.tenable.com/plugins/nessus/97191 title F5 TLS Session Ticket Implementation Remote Memory Disclosure (Ticketbleed) (uncredentialed check) NASL family F5 Networks Local Security Checks NASL id F5_BIGIP_SOL05121675.NASL description A BIG-IP SSL virtual server with the non-default Session Tickets option enabled may leak up to 31 bytes of uninitialized memory, aka the Ticketbleed bug. (CVE-2016-9244) last seen 2020-06-01 modified 2020-06-02 plugin id 97091 published 2017-02-10 reporter This script is Copyright (C) 2017-2019 and is owned by Tenable, Inc. or an Affiliate thereof. source https://www.tenable.com/plugins/nessus/97091 title F5 Networks BIG-IP : F5 TLS vulnerability (K05121675) (Ticketbleed)
Seebug
| bulletinFamily | exploit |
| description | Ticketbleed (CVE-2016-9244) is a software vulnerability in the TLS stack of certain F5 products that allows a remote attacker to extract up to 31 bytes of uninitialized memory at a time, which can contain any kind of random sensitive information, like in Heartbleed. If you suspect you might be affected by this vulnerability, you can find details and mitigation instructions at [ticketbleed.com](https://ticketbleed.com) (including an online test) or in the [F5 K05121675 article](https://support.f5.com/csp/article/K05121675).  In this post we'll talk about how Ticketbleed was found, verified and reported. ## JIRA RG-XXX It all started with a bug report from a customer using Cloudflare [Railgun](https://www.cloudflare.com/website-optimization/railgun/). > **rg-listener <> origin requests fail with "local error: unexpected message"** > > A PCAP of the rg-listener <> origin traffic is attached and shows a TLS alert being triggered during the handshake. > > Worth noting the customer is using an F5 Load Balancer in front of the Railgun and the Origin Web Server: `visitor > edge > cache > rg-sender > F5 > rg-listener > F5 > origin web server` > > Matthew was unable to replicate by using a basic TLS.Dial in Go so this seems tricky so far. A bit of context on Railgun: Railgun speeds up requests between the Cloudflare edge and the origin web site by establishing a permanent optimized connection and performing delta compression on HTTP responses.  The Railgun connection uses a custom binary protocol over TLS, and the two endpoints are Go programs: one on the Cloudflare edge and one installed on the customer servers. This means that the whole connection goes through the Go TLS stack, crypto/tls. That connection failing with `local error: unexpected message` means that the customer’s side of the connection sent something that confused the Go TLS stack of the Railgun running on our side. Since the customer is running an F5 load balancer between their Railgun and ours, this points towards an **incompatibility between the Go TLS stack and the F5 one**. However, when my colleague Matthew tried to reproduce the issue by connecting to the load balancer with a simple Go `crypto/tls.Dial`, it succeeded. ## PCAP diving Since Matthew sits at a desk opposite of mine in the Cloudflare London office, he knew I've been working with the Go TLS stack for our TLS 1.3 implementation. We quickly ended up in a joint debugging session. Here's the PCAP we were staring at.  So, there's the ClientHello, right. The ServerHello, so far so good. And then immediately a ChangeCipherSpec. Oh. Ok. A ChangeCipherSpec is how TLS 1.2 says "let's switch to encrypted". The only way a ChangeCipherSpec can come this early in a 1.2 handshake, is if session resumption happened. And indeed, by focusing on the ClientHello we can see that the Railgun client sent a Session Ticket.  A Session Ticket carries some encrypted key material from a previous session to allow the server to resume that previous session immediately instead of negotiating a new one.  _To learn more about session resumption in TLS 1.2, watch the first part of [the Cloudflare Crypto Team TLS 1.3 talk](https://blog.cloudflare.com/tls-1-3-explained-by-the-cloudflare-crypto-team-at-33c3/), [read the transcript](https://blog.cloudflare.com/tls-1-3-overview-and-q-and-a/), or the ["TLS Session Resumption" post](https://blog.cloudflare.com/tls-session-resumption-full-speed-and-secure/) on the Cloudflare blog._ After that ChangeCipherSpec both Railgun and Wireshark get pretty confused (HelloVerifyRequest? Umh?). So we have reason to believe the issue is related to Session Tickets. In Go you have to explicitly enable Session Tickets on the client side by setting a `ClientSessionCache`. We verified that indeed Railgun uses this functionality and wrote this small test: ``` package main import ( "crypto/tls" ) func main() { conf := &tls.Config{ InsecureSkipVerify: true, ClientSessionCache: tls.NewLRUClientSessionCache(32), } conn, err := tls.Dial("tcp", "redacted:443", conf) if err != nil { panic("failed to connect: " + err.Error()) } conn.Close() conn, err = tls.Dial("tcp", "redacted:443", conf) if err != nil { panic("failed to resume: " + err.Error()) } conn.Close() } ``` And sure enough, `local error: unexpected message`. ## crypto/tls diving Once I had it reproduced in a local `crypto/tls` it became a home game. `crypto/tls` error messages tend to be short of details, but a quick tweak allows us to pinpoint where they are generated. Every time a fatal error occurs, `setErrorLocked` is called to record the error and make sure that all following operations fail. That function is usually called from the site of the error. A well placed `panic(err)` will drop a stack trace that should show us _what_ message is unexpected. ``` diff --git a/src/crypto/tls/conn.go b/src/crypto/tls/conn.go index 77fd6d3254..017350976a 100644 --- a/src/crypto/tls/conn.go +++ b/src/crypto/tls/conn.go @@ -150,8 +150,7 @@ type halfConn struct { } func (hc *halfConn) setErrorLocked(err error) error { - hc.err = err - return err + panic(err) } // prepareCipherSpec sets the encryption and MAC states ``` ``` panic: local error: tls: unexpected message goroutine 1 [running]: panic(0x185340, 0xc42006fae0) /Users/filippo/code/go/src/runtime/panic.go:500 +0x1a1 crypto/tls.(*halfConn).setErrorLocked(0xc42007da38, 0x25e6e0, 0xc42006fae0, 0x25eee0, 0xc4200c0af0) /Users/filippo/code/go/src/crypto/tls/conn.go:153 +0x4d crypto/tls.(*Conn).sendAlertLocked(0xc42007d880, 0x1c390a, 0xc42007da38, 0x2d) /Users/filippo/code/go/src/crypto/tls/conn.go:719 +0x147 crypto/tls.(*Conn).sendAlert(0xc42007d880, 0xc42007990a, 0x0, 0x0) /Users/filippo/code/go/src/crypto/tls/conn.go:727 +0x8c crypto/tls.(*Conn).readRecord(0xc42007d880, 0xc400000016, 0x0, 0x0) /Users/filippo/code/go/src/crypto/tls/conn.go:672 +0x719 crypto/tls.(*Conn).readHandshake(0xc42007d880, 0xe7a37, 0xc42006c3f0, 0x1030e, 0x0) /Users/filippo/code/go/src/crypto/tls/conn.go:928 +0x8f crypto/tls.(*clientHandshakeState).doFullHandshake(0xc4200b7c10, 0xc420070480, 0x55) /Users/filippo/code/go/src/crypto/tls/handshake_client.go:262 +0x8c crypto/tls.(*Conn).clientHandshake(0xc42007d880, 0x1c3928, 0xc42007d988) /Users/filippo/code/go/src/crypto/tls/handshake_client.go:228 +0xfd1 crypto/tls.(*Conn).Handshake(0xc42007d880, 0x0, 0x0) /Users/filippo/code/go/src/crypto/tls/conn.go:1259 +0x1b8 crypto/tls.DialWithDialer(0xc4200b7e40, 0x1ad310, 0x3, 0x1af02b, 0xf, 0xc420092580, 0x4ff80, 0xc420072000, 0xc42007d118) /Users/filippo/code/go/src/crypto/tls/tls.go:146 +0x1f8 crypto/tls.Dial(0x1ad310, 0x3, 0x1af02b, 0xf, 0xc420092580, 0xc42007ce00, 0x0, 0x0) /Users/filippo/code/go/src/crypto/tls/tls.go:170 +0x9d ``` Sweet, let's see where the unexpected message alert is sent, at `conn.go:672`. ``` 670 case recordTypeChangeCipherSpec: 671 if typ != want || len(data) != 1 || data[0] != 1 { 672 c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage)) 673 break 674 } 675 err := c.in.changeCipherSpec() 676 if err != nil { 677 c.in.setErrorLocked(c.sendAlert(err.(alert))) 678 } ``` So the message we didn't expect is the ChangeCipherSpec. Let's see if the higher stack frames give us an indication as to what we expected instead. Let's chase `handshake_client.go:262`. ``` 259 func (hs *clientHandshakeState) doFullHandshake() error { 260 c := hs.c 261 262 msg, err := c.readHandshake() 263 if err != nil { 264 return err 265 } ``` Ah, `doFullHandshake`. Wait. The server here is clearly doing a resumption (sending a Change Cipher Spec immediately after the Server Hello), while the client... tries to do a full handshake? It looks like the client offers a Session Ticket, the server _accepts it_, but the client _doesn't realize and carries on_. ## RFC diving At this point I had to fill a gap in my TLS 1.2 knowledge. How does a server signal acceptance of a Session Ticket? [RFC 5077](https://tools.ietf.org/html/rfc5077), which obsoletes RFC 4507, says: > When presenting a ticket, the client MAY generate and include a Session ID in the TLS ClientHello. If the server accepts the ticket and the Session ID is not empty, then it MUST respond with the same Session ID present in the ClientHello. So a client that doesn't want to guess whether a Session Ticket is accepted or not will send a Session ID and look for it to be echoed back by the server. The code in `crypto/tls`, clear as always, does exactly that. ``` func (hs *clientHandshakeState) serverResumedSession() bool { // If the server responded with the same sessionId then it means the // sessionTicket is being used to resume a TLS session. return hs.session != nil && hs.hello.sessionId != nil && bytes.Equal(hs.serverHello.sessionId, hs.hello.sessionId) } ``` ## Session IDs diving Something must be going wrong there. Let's practice some healthy print-based debugging. ``` diff --git a/src/crypto/tls/handshake_client.go b/src/crypto/tls/handshake_client.go index f789e6f888..2868802d82 100644 --- a/src/crypto/tls/handshake_client.go +++ b/src/crypto/tls/handshake_client.go @@ -552,6 +552,8 @@ func (hs *clientHandshakeState) establishKeys() error { func (hs *clientHandshakeState) serverResumedSession() bool { // If the server responded with the same sessionId then it means the // sessionTicket is being used to resume a TLS session. + println(hex.Dump(hs.hello.sessionId)) + println(hex.Dump(hs.serverHello.sessionId)) return hs.session != nil && hs.hello.sessionId != nil && bytes.Equal(hs.serverHello.sessionId, hs.hello.sessionId) } ``` ``` 00000000 a8 73 2f c4 c9 80 e2 ef b8 e0 b7 da cf 0d 71 e5 |.s/...........q.| 00000000 a8 73 2f c4 c9 80 e2 ef b8 e0 b7 da cf 0d 71 e5 |.s/...........q.| 00000010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| ``` Ah. The F5 server is padding the Session ID to its maximum length of 32 bytes, instead of returning it as the client sent it. crypto/tls in Go uses 16 byte Session IDs. From there the failure mode is clear: the server thinks it told the client to use the ticket, the client thinks the server started a new session, and things get unexpected. In the TLS space we have seen quite some incompatibilities like this. Notoriously, ClientHellos have to be either shorter than 256 bytes or longer than 512 [not to clash with some server implementations](https://bugs.chromium.org/p/chromium/issues/detail?id=315828). I was about to write this up as just another real world TLS quirk when... ``` 00000000 79 bd e5 a8 77 55 8b 92 41 e9 89 45 e1 50 31 25 |y...wU..A..E.P1%| 00000000 79 bd e5 a8 77 55 8b 92 41 e9 89 45 e1 50 31 25 |y...wU..A..E.P1%| 00000010 04 27 a8 4f 63 22 de 8b ef f9 a3 13 dd 66 5c ee |.'.Oc".......f\.| ``` Uh oh. Wait. Those are not zeroes. That's not padding. That's... memory? At this point the impression of dealing with a Heartbleed-like vulnerability got pretty clear. The server is allocating a buffer as big as the client's Session ID, and then sending back always 32 bytes, bringing along whatever unallocated memory was in the extra bytes. ## Browser diving I had one last source of skepticism: how could this not have been noticed before? The answer is banal: all browsers use 32-byte Session IDs to negotiate Session Tickets. Together with Nick Sullivan I checked NSS, OpenSSL and BoringSSL to confirm. [Here's BoringSSL for example](https://github.com/google/boringssl/blob/33fe4a0d1406f423e7424ea7367e1d1a51c2edc1/ssl/handshake_client.c#L1901-L1908). ``` /* Generate a session ID for this session based on the session ticket. We use * the session ID mechanism for detecting ticket resumption. This also fits in * with assumptions elsewhere in OpenSSL.*/ if (!EVP_Digest(CBS_data(&ticket), CBS_len(&ticket), session->session_id, &session->session_id_length, EVP_sha256(), NULL)) { goto err; } ``` BoringSSL uses a SHA256 hash of the Session Ticket, which is exactly 32 bytes. (Interestingly, from speaking to people in the TLS field, there was an idle intention to switch to 1-byte Session IDs but no one had tested it widely yet.) As for Go, it’s probably the case that client-side Session Tickets are not enabled that often. ## Disclosure diving After realizing the security implications of this issue we compartmentalized it inside the company, made sure our Support team would advise our customer to simply disable Session Tickets, and sought to contact F5. After a couple misdirected emails that were met by requests for Serial Numbers, we got in contact with the F5 SIRT, exchanged PGP keys, and provided a report and a PoC. The report was escalated to the development team, and confirmed to be an uninitialized memory disclosure limited to the Session Ticket functionality. It's unclear what data might be exfiltrated via this vulnerability, but Heartbleed and the [Cloudflare Heartbleed Challenge](https://blog.cloudflare.com/the-results-of-the-cloudflare-challenge/) taught us not to make assumptions of safety with uninitialized memory. In planning a timeline, the F5 team was faced with a rigid release schedule. Considering multiple factors, including the availability of an effective mitigation (disabling Session Tickets) and the apparent triviality of the vulnerability, I decided to adhere to the [industry-standard disclosure policy adopted by Google's Project Zero](https://googleprojectzero.blogspot.co.uk/2015/02/feedback-and-data-driven-updates-to.html): 90 days with 15 days of grace period if a fix is due to be released. By coincidence today coincides with both the expiration of those terms and the scheduled release of the first hotfix for one of the affected versions. I'd like to thank the F5 SIRT for their professionalism, transparency and collaboration, which were in pleasant contrast with the stories of adversarial behavior we hear too often in the industry. The issue was assigned CVE-2016-9244. ## Internet diving When we reported the issue to F5 I had tested the vulnerability against a single host, which quickly became unavailable after disabling Session Tickets. That meant having both low confidence in the extent of the vulnerability, and no way to reproduce it. This was the perfect occasion to perform an Internet scan. I picked the toolkit that powers Censys.io by the University of Michigan: zmap and zgrab. zmap is an IPv4-space scanning tool that detects open ports, while zgrab is a Go tool that follows up by connecting to those ports and collecting a number of protocol details. I added support for Session Ticket resumption to zgrab, and then wrote a simple Ticketbleed detector by having zgrab send a 31-byte Session ID, and comparing it with the one returned by the server. ``` diff --git a/ztools/ztls/handshake_client.go b/ztools/ztls/handshake_client.go index e6c506b..af098d3 100644 --- a/ztools/ztls/handshake_client.go +++ b/ztools/ztls/handshake_client.go @@ -161,7 +161,7 @@ func (c *Conn) clientHandshake() error { session, sessionCache = nil, nil hello.ticketSupported = true hello.sessionTicket = []byte(c.config.FixedSessionTicket) - hello.sessionId = make([]byte, 32) + hello.sessionId = make([]byte, 32-1) if _, err := io.ReadFull(c.config.rand(), hello.sessionId); err != nil { c.sendAlert(alertInternalError) return errors.New("tls: short read from Rand: " + err.Error()) @@ -658,8 +658,11 @@ func (hs *clientHandshakeState) processServerHello() (bool, error) { if c.config.FixedSessionTicket != nil { c.resumption = &Resumption{ - Accepted: hs.hello.sessionId != nil && bytes.Equal(hs.serverHello.sessionId, hs.hello.sessionId), - SessionID: hs.serverHello.sessionId, + Accepted: hs.hello.sessionId != nil && bytes.Equal(hs.serverHello.sessionId, hs.hello.sessionId), + TicketBleed: len(hs.serverHello.sessionId) > len(hs.hello.sessionId) && + bytes.Equal(hs.serverHello.sessionId[:len(hs.hello.sessionId)], hs.hello.sessionId), + ServerSessionID: hs.serverHello.sessionId, + ClientSessionID: hs.hello.sessionId, } return false, FixedSessionTicketError } ``` By picking 31 bytes I ensured the sensitive information leakage would be negligible. I then downloaded the latest zgrab results from the Censys website, which thankfully included information on what hosts supported Session Tickets, and completed the pipeline with abundant doses of `pv` and `jq`. After getting two hits in the first 1,000 hosts from the Alexa top 1m list in November, I interrupted the scan to avoid leaking the vulnerability and postponed to a date closer to the disclosure. While producing this writeup I completed the scan, and found between 0.1% and 0.2% of all hosts to be vulnerable, or 0.4% of the websites supporting Session Tickets. ## Read more For more details visit the [F5 K05121675 article](https://support.f5.com/csp/article/K05121675) or [ticketbleed.com](https://ticketbleed.com), where you'll find a technical summary, affected versions, mitigation instructions, a complete timeline, scan results, IPs of the scanning machines, and an online test. Otherwise, you might want to [follow me on Twitter](https://twitter.com/FiloSottile). |
| id | SSV:92673 |
| last seen | 2017-11-19 |
| modified | 2017-02-10 |
| published | 2017-02-10 |
| reporter | Root |
| source | https://www.seebug.org/vuldb/ssvid-92673 |
| title | F5 TLS vulnerability (CVE-2016-9244) (Ticketbleed) |
References
- https://support.f5.com/csp/article/K05121675
- http://www.securitytracker.com/id/1037800
- https://www.exploit-db.com/exploits/41298/
- https://filippo.io/Ticketbleed/
- https://blog.filippo.io/finding-ticketbleed/
- http://packetstormsecurity.com/files/141017/Ticketbleed-F5-TLS-Information-Disclosure.html
- https://github.com/0x00string/oldays/blob/master/CVE-2016-9244.py
- http://www.securityfocus.com/bid/96143