[bitcoin-dev] Alert key disclosure

[Bryan Bishop <kanzure@gmail.com>]

The bitcoin alert keys are disclosed in this email, followed by a
disclosure of various known vulnerabilities in what was once the alert
system. The bitcoin alert system has been completely retired. The
network is not at risk and this warning may be safely ignored if you
do not have an ancient node (running v0.12.x or older) using the
deprecated bitcoin alert system or its public keys.

mainnet public key:
04fc9702847840aaf195de8442ebecedf5b095cdbb9bc716bda9110971b28a49e0ead8564ff0db22209e0374782c093bb899692d524e9d6a6956e7c5ecbcd68284

mainnet private key:
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

testnet public key:
04302390343f91cc401d56d68b123028bf52e5fca1939df127f63c6467cdf9c8e2c14b61104cf817d0b780da337893ecc4aaff1309e536162dabbdb45200ca2b0a

testnet private key:
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

These are openssl-serialized private keys.

In 2016, a plan was proposed[1] for the completion of the retirement
of the bitcoin alert system which included the idea of revealing the
alert system private keys. The proposal still contains good
information regarding the purpose and intention of alert system
retirement and motivation for the disclosure of the private keys.
Additionally, an overview of the alert system retirement and its
timeline is available on the web at [2]. This disclosure was recently
discussed in an IRC meeting logs at [3]. A media site also recently
discussed this topic[4].

One of the reasons for disclosure of the keys is to mitigate the
effects of unknown dissemination and proliferation of the keys. By
broadcasting the values to make them available to everyone, the value
of the keys is intended to be to be eliminated, since now everyone
could feasibly sign messages, the value of the signed messages becomes
zero.

[1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-September/013104.html
[2] https://bitcoin.org/en/alert/2016-11-01-alert-retirement
[3] http://www.erisian.com.au/meetbot/bitcoin-core-dev/2018/bitcoin-core-dev.2018-06-21-19.00.log.html#l-30
[4] https://www.coindesk.com/long-secret-bitcoin-key-finally-revealed/


# Vulnerabilities in the bitcoin alert system

The following text[5] discloses a number of known vulnerabilities in
the alert system. Writeup contributed by achow101.

[5] https://gist.github.com/achow101/18a2dfc371c421419d494a3ae0447f66

The Alert System previously utilized by Bitcoin has several issues
(some of which may be classified as vulnerabilities). These issues no
longer exist in Bitcoin as of network protocol version 700013 which
was released with Bitcoin Core 0.13.0. Many altcoins and Bitcoin
client implementations were notified of the Alert System's removal and
have since removed the alert system themselves or transitioned to
using an Alert system that does not share an Alert Key with Bitcoin.

All of the issues described below allow an attacker in possession of
the Alert Key to perform a Denial of Service attack on nodes that
still support the Alert system. These issues involve the exhaustion of
memory which causes node software to crash or be killed due to
excessive memory usage.

Many of these issues were not known until the Alert System was removed
as developers inspected the code for vulnerabilities prior to
releasing the Alert Key. Due to these issues, the publication of the
Alert Key was delayed and affected altcoins and software were
notified.

As of this writing, less than 4% of Bitcoin nodes are vulnerable.
Furthermore, the Bitcoin Core developers have created a "final alert"
which is a maximum ID number alert which overrides all previous alerts
and displays a fixed "URGENT: Alert key compromised, upgrade required"
message on all vulnerable software. The Bitcoin Core developers
believe that so few vulnerable nodes are present on the network, and
risks to those nodes so minor, that it is safe to publish the Alert
Key.

An Alert contains these fields:

    int32_t nVersion;
    int64_t nRelayUntil;      // when newer nodes stop relaying to newer nodes
    int64_t nExpiration;
    int32_t nID;
    int32_t nCancel;
    std::set<int32_t> setCancel;
    int32_t nMinVer;            // lowest version inclusive
    int32_t nMaxVer;            // highest version inclusive
    std::set<std::string> setSubVer;  // empty matches all
    int32_t nPriority;

Alerts are also identified by their SHA256 hash. The above fields can
be freely modified to generate alerts with differing hashes.

# Infinitely sized map (CVE-2016-10724)

The Alert System was designed to support multiple Alerts
simultaneously. As such, Alerts were stored in memory in a map.
However, there is no limit on how large this map can be, thus an
attacker with the Alert Key can send a large number of Alerts to a
node. Eventually, the map containing all of the Alerts will be so
large that the node runs out of memory and crashes, thus causing a
Denial of Service attack.

The infinitely sized map is the basis for which the Alert system can
be used to cause Denial of Service attacks.

# Infinitely sized alerts

Although the infinitely sized map is what causes the crash itself, an
attacker can also send very large Alerts. Alerts themselves are not
limited in size explicitly, they are only limited by the maximum
network message size. This maximum network message size has varied
between versions. At times in the past, it has been 32 MB. For Bitcoin
Core 0.12.0 (the most recent version of Bitcoin Core with the alert
system enabled by default), the maximum message size is 2 MB.

Although large Alerts do not directly cause a Denial of Service by
themselves, combined with the infinitely sized map, large Alerts can
more quickly cause a node to run out of memory.

* The setCancel field has no length limit (besides the maximum message
size) and is a std::set of 32-bit integers. Given that it has no size
constraints, an attacker can use this field to create a very large
Alert by filling the set with many integers.

* The setSubVer field, like setCancel, has no length limit and is a
std::set. However instead of integers it has std::strings. These
strings do not have a length limit themselves and can thus be
arbitrarily long to produce an Alert that is arbitrarily large.

* Bitcoin Core versions prior to 0.10.0 did not have a limit on the
length of the strComment, strStatusBar, and strReserved fields. These
strings can have an arbitrary length.

# The final alert

To protect against attackers abusing the Alert key following its
publication, the Bitcoin Core developers constructed a "final alert".
This final alert is a maximum ID alert which overrides all previous
alerts. All Bitcoin Core versions since and including Bitcoin Core
0.14.0 contain the final alert and will send it to any node which is
vulnerable to issues including the following disclosures. However this
protection is not enough to protect those nodes as a few issues were
found with the final alert implementation itself.

Final alerts are those which meet the following conditions:

    nExpiration == maxInt &&
    nCancel == (maxInt-1) &&
    nMinVer == 0 &&
    nMaxVer == maxInt &&
    setSubVer.empty() &&
    nPriority == maxInt &&
    strStatusBar == "URGENT: Alert key compromised, upgrade required"

maxInt is the maximum signed integer as defined by
std::numeric_limits<int>::max().

# Multiple final alerts

The definition for a final alert does not include a few fields.
Because alerts are identified by their hashes, changing the omitted
fields allows an Alert to be classified as a final alert but still be
an alert that is added to the infinitely sized map. The nCancel field
omits the maxInt ID number used by the final alert so all of the final
alerts share the same ID.

* Since setCancel is not required to be empty for an alert to be a
final alert, the setCancel field can contain different integers to
produce alerts that have different hashes and are thus different
alerts. Combined with the infinitely sized map and the infinitely
sized setCancel issues, many final alerts can be created which are
large, fill the map, and cause a node to run out of memory.

* The strComment field, while having a maximum length of 65536 bytes
(and no maximum length prior to Bitcoin Core version 0.10.0), is not
required to be a particular string in order for an alert to be a final
alert. Thus multiple final alerts can be crafted which have different
hashes by using different values for strComment

* The strReserved field, while having a maximum length of 256 bytes,
is not required to be a particular string in order for an alert to be
a final alert. Thus multiple final alerts can be crafted which have
different hashes by using different values for strReserved.

* The nVersion field is also not required to be a particular value.
Thus this can be used to construct final alerts with different hashes
by having different values for nVersion.

* nRelayUntil field is also not required to be a particular value.
Thus this can be used to construct final alerts with different hashes
by having different values for nRelayUntil.

# Final Alert Cancellation (CVE-2016-10725)

Although the final alert is supposed to be uncancellable, it
unfortunately is cancellable due to the order of actions when
processing an alert. Alerts are first processed by checking whether
they cancel any existing alert. Then they are checked whether any of
the remaining alerts cancels it. Because of this order, it is possible
to create an alert which cancels a final alert before the node checks
whether that alert is canceled by the final alert. Thus an attacker
can cancel a final alert with another alert allowing a node to be
vulnerable to all of the aforementioned attacks.

# Protecting against DoS attacks from the alert system

Fixing these issues is relatively easy. The first and most obvious
solution is to simply remove the Alert system entirely. As nodes
upgrade to versions without the Alert system, fewer nodes will be
vulnerable to attack should the Alert keys become public. This is the
option that Bitcoin has taken. However, because Bitcoin has retired
the Alert system entirely, the Alert key will also be published to
reduce the risk that the Alert Key is mistakenly depended upon in the
future.
Should altcoins wish to continue using the Alert system but with a
different Alert Key, a few very simple fixes will safeguard nodes from
the aforementioned issues. Limiting the number of alerts, the size of
setCancel and setSubVer, and only allowing one final alert altogether
fix the above issues. This patch[6], on top of Bitcoin Core 0.11 (a
vulnerable version), fixes the aforementioned issues. Altcoins that
still use the Alert system are recommended to port this patch to their
software. Outdated node software is still vulnerable.

[6] https://gist.github.com/achow101/02d03238090691558a68010a9ccbbf9d

This disclosure was authored primarily by Bryan Bishop (kanzure) and
Andrew Chow (achow101). Special thanks to reviewers. Also, an
interesting proposal was floated to not disclose the private keys in
WIF format-- one is that this is not how the original values were
received, and second (more importantly) to prevent users from
importing the key into their wallet and reusing it in their wallet key
circulation.

- Bryan
http://heybryan.org/
1 512 203 0507