[BRLY-2024-023] Use of RSA test keys for BMC RoT implementation

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-2024-023
• CVSS v3.1: 9.1 Critical AV:N/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H

Affected Supermicro firmware with confirmed impact by Binarly team

We also noticed that the same key is present in the firmware used by these series of servers, indicating that they are also likely to be affected:

• B13DEE
• B13DET
• B13SEE-CPU-25G
• B13SEG
• B4SA1-CPU
• B4SC1-CPU
• G1SMH
• G1SMH-G
• H13SRH
• X12DGQ-R
• X12DPG-QR
• X13DEG-QT
• X13DEM
• X13DSF-A
• X13OEI-CPU
• X13QEH+
• X13SEDW-F
• X13SEED-F
• X13SEED-SF
• X13SEFR-A
• X13SEM-F
• X13SEM-TF
• X13SET-G
• X13SET-GC
• X13SET-PT

Potential impact

If the threat actor obtains the private part of the RSA signing key (which is quite possible as this is a test untrusted key), they can generate and sign BMC firmware with arbitrary content and it will be successfully validated during the boot process. For example, such firmware may contain an implanted backdoor, allowing an attacker to remotely control the BMC system and the target host operating system.

Vulnerability description

The following figure shows the RoT chain implementation used by the Supermicro BMC firmware:

This process can be summarised in 3 main steps:

1. U-Boot SPL bootloader verification

In general, the code in the ROM is responsible for verifying the U-Boot SPL bootloader with the public keys stored in the OTP (one time programmable) memory. Since we don't have the actual device, we can't be sure which keys are stored in the OTP. However, we have noticed that the logic in the custom U-Boot regular bootloader also performs the U-Boot SPL verification, trying 3 RSA modules located at 0xBDCO, 0xBFCO, 0xC1CO in sequence and exponent 65537, using the signature located at 0xC400. Subsequent execution is only allowed if the signature is successfully validated with at least one of the keys. It is therefore highly likely that the same keys are stored in the OTP. Crucially, the RSA module located at 0xBDCO matches the module of the "BMC Test Key".

2. U-Boot regular bootloader verification

The logic in the customised U-Boot SPL performs the verification of the regular U-Boot bootloader by trying 2 RSA modules located at 0xB9C0, 0xBBC0 in sequence and exponent 65537, using the signature located at 0xFFC00. Subsequent execution is only allowed if the signature is successfully validated with at least one of the keys. Crucially, the RSA module located at 0xB9C0 matches the module of the "BMC Test Key".

3. Rest of the firmware (Kernel, DTB, initramfs, Squashfs) verification

The logic in the custom U-Boot regular bootloader performs the verification of the rest of the firmware by trying 2 RSA modules located at 0xFEA00, 0xFEC00 in sequence and exponent 65537, using the signature located at 0x34F0400. Subsequent execution is only allowed if the signature is successfully validated with at least one of the keys. Crucially, the RSA module located at 0xFEA00 matches the module of the "BMC Test Key".

Note: The certificate of the above mentioned "BMC Test Key" is located in the firmware at offset 0xFE004, the public exponent of the RSA key is 65537.

Note: BINARLY REsearch team has not identified the presence of the private part of the key in the wild at this moment.

Note: The expiration date of the "BMC Test Key" is Feb 1 03:14:28 2070 GMT.

Note: We noticed that U-Boot SPL and U-Boot regular bootloaders are signed with the same private key. It is better to use different keys to sign different stages, in which case a compromise of a single key will only affect one stage. Note: We have noticed that the U-Boot SPL of some Supermicro BMC images is signed with "BMC Test Key", which may indicate that this key is stored in the OTP memory for these devices:

Disclosure Timeline

This bug is subject to a 90 day disclosure deadline. After 90 days elapsed or a patch has been made broadly available (whichever is earlier), the bug report will become visible to the public

Acknowledgements

BINARLY team

Appendix

Test key certificate:

Data:
        Version: 3 (0x2)
        Serial Number:
            1a:da:e6:cf:23:66:6a:36:d9:dd:69:4c:2f:ba:30:14:90:f7:3d:5e
        Signature Algorithm: sha512WithRSAEncryption
        Issuer: C = US, ST = CA, L = SanJose, O = Super Micro Computer Inc., CN = RD1 BMC Test Key - DO NOT TRUST
        Validity
            Not Before: Feb 14 03:14:28 2020 GMT
            Not After : Feb  1 03:14:28 2070 GMT
        Subject: C = US, ST = CA, L = SanJose, O = Super Micro Computer Inc., CN = RD1 BMC Test Key - DO NOT TRUST
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (4096 bit)
                Modulus:
                    00:c6:b3:42:c9:36:c3:a1:24:0c:ec:e5:1a:31:96:
                    5b:1d:a6:c7:85:66:50:bf:59:78:9c:2d:8d:07:5e:
                    6f:9b:f0:a0:70:7a:42:f0:0a:68:bd:e1:aa:80:ef:
                    2c:70:bd:7a:36:59:6a:ca:2a:1d:21:f1:1c:a1:31:
                    f3:d6:3d:2c:ea:32:0f:d6:62:99:01:57:99:e1:13:
                    fd:82:1a:78:c7:29:2b:4c:2d:70:43:b0:c5:28:94:
                    78:33:d5:c2:0b:d3:84:6c:86:08:18:e5:2e:6a:40:
                    d6:f4:fe:41:02:7f:ad:7a:96:16:af:86:9d:01:d7:
                    71:74:bb:0b:7b:24:3d:26:31:23:2b:91:05:2f:f9:
                    c5:6a:bb:b8:c1:85:bc:be:e7:6b:2f:bf:f9:5b:fa:
                    e9:c8:ce:f0:b7:af:4c:c7:eb:5c:f0:32:9f:f0:a3:
                    7a:a9:b3:eb:27:cd:a5:f3:3a:24:81:5d:01:e4:ac:
                    44:9a:59:fc:ee:04:86:03:9c:89:ce:65:ec:4d:34:
                    b5:9a:d8:86:71:97:8e:a7:b6:ec:91:61:89:b3:6c:
                    b1:7c:d8:8d:6b:fa:0d:51:0e:2a:cb:89:d6:06:f1:
                    c4:6b:27:25:66:92:d2:37:0f:5b:dc:fb:22:8f:18:
                    32:a4:42:52:68:55:5f:bd:8f:f5:20:c1:1e:1a:9a:
                    e4:32:ca:a4:e0:93:1d:f4:7f:41:8f:b7:78:9a:f4:
                    b0:b4:89:3f:93:d7:96:d7:cf:61:77:96:65:6f:03:
                    cf:82:e4:3d:c1:20:2e:1d:60:10:41:7d:2d:7f:5a:
                    0f:c3:52:0b:96:0b:a1:56:3b:47:6e:67:db:54:b1:
                    76:10:61:e7:34:1e:5f:63:b3:6c:27:9d:76:6f:d3:
                    39:11:e9:34:07:66:55:1d:fc:32:53:b6:91:54:d5:
                    5c:44:73:01:98:75:cc:71:04:8d:9d:a5:ca:f8:68:
                    26:a6:52:ab:bc:53:ac:65:08:56:ba:ad:f8:e9:51:
                    59:d5:24:d4:34:1b:c1:b4:f3:43:c7:69:ee:36:2d:
                    75:26:6c:7e:20:12:83:6b:1f:6f:fc:05:f7:7c:ae:
                    c8:3f:ca:49:7c:e6:a1:91:44:b5:db:55:5a:5c:ea:
                    f1:b1:46:71:1b:2a:fe:4e:9f:db:c1:e7:10:91:fb:
                    80:c2:8f:04:74:c6:31:aa:ed:71:0f:a4:b3:03:2d:
                    57:70:0c:ff:d0:aa:ed:f5:d7:1f:02:d5:75:8a:11:
                    d9:ca:07:9c:e7:02:39:2d:8b:1a:78:62:98:e4:d9:
                    1d:f2:2a:30:a4:29:ce:03:c9:56:08:d8:79:12:2e:
                    53:0d:b0:3f:0e:be:b4:51:06:a2:77:c2:dc:04:0b:
                    4a:a8:c5
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Subject Key Identifier: 
                2A:F0:A8:8C:87:67:4E:0A:D2:AD:20:B6:26:40:CA:A2:C4:01:1D:7F
            X509v3 Authority Key Identifier: 
                2A:F0:A8:8C:87:67:4E:0A:D2:AD:20:B6:26:40:CA:A2:C4:01:1D:7F
            X509v3 Basic Constraints: critical
                CA:TRUE
    Signature Algorithm: sha512WithRSAEncryption
    Signature Value:
        bc:05:cd:3b:e8:62:06:e6:ff:b0:80:5b:a8:02:2a:0c:20:9d:
        0b:eb:c6:fd:19:74:62:5c:34:aa:37:8d:46:84:2c:39:97:0d:
        08:ea:1a:8f:ae:cb:ff:dd:2e:1d:1d:85:3c:29:a5:fc:ed:99:
        d6:1b:17:2e:c6:d2:e0:28:b1:aa:62:58:06:11:fc:17:45:1b:
        a4:fd:4f:14:79:fb:42:5d:4c:35:72:73:3f:09:32:c6:c7:dc:
        b0:d2:63:4e:41:54:39:47:0e:a8:46:50:c3:43:7a:15:d9:a0:
        1e:ba:95:20:34:db:97:9f:7c:6e:c6:07:7a:76:2a:a2:b2:f2:
        06:c7:00:f3:a8:ef:5e:63:86:5e:e8:aa:de:07:f5:83:92:93:
        92:1a:86:04:85:bd:0b:c6:9a:04:3f:1e:10:8e:8f:8a:04:61:
        e7:b3:b2:af:5a:40:b2:ac:28:56:2e:b5:13:98:22:b3:71:df:
        5f:f0:8d:aa:c8:3e:20:66:bc:44:8a:cc:f5:e2:0e:aa:aa:9e:
        2b:43:46:cf:97:dc:b0:7b:b4:24:40:11:e0:bb:87:de:00:76:
        b1:2e:2d:c8:9a:e0:4b:b6:7c:33:ee:8c:0d:f8:72:15:ad:d4:
        35:9d:8f:79:ad:7f:f2:a1:da:a3:51:b0:55:5e:13:eb:72:bf:
        d8:91:7e:9f:65:83:ec:8a:70:23:4e:f2:e5:14:ef:b7:c5:67:
        67:50:76:71:1c:b1:e0:73:1c:ff:07:a7:2e:92:7d:e4:d3:4c:
        f9:2e:39:3a:e1:34:e0:35:55:19:91:62:54:5b:ac:04:13:3b:
        29:4a:cc:d2:18:59:65:60:0f:c2:c5:77:f3:a3:17:f2:68:1c:
        94:61:ef:70:48:90:c9:25:17:ad:e5:51:6e:90:12:97:45:67:
        ce:cc:16:e4:f7:67:e8:89:e4:39:40:c5:7f:84:5a:70:db:0c:
        1d:99:ea:32:58:43:39:2c:f4:6a:5a:5e:a9:a3:f1:f8:20:9e:
        42:f3:6f:35:60:6f:80:1d:ac:98:96:b3:45:51:fb:e7:6f:7e:
        3d:91:d7:96:1d:cf:5b:cd:89:56:f8:94:d0:b9:d0:92:b7:d0:
        51:01:11:54:57:7c:2a:21:91:91:42:37:35:63:52:d2:67:d3:
        47:6f:9e:27:3f:26:07:f0:d5:65:d4:a7:cf:3e:1c:88:ac:35:
        69:47:f4:1f:fd:2b:a2:c7:26:fd:b0:96:53:0e:5c:98:18:e3:
        16:c1:ae:d3:c8:61:34:92:17:3a:10:86:ea:46:1e:ad:16:98:
        0d:8c:fa:16:3a:3f:af:49:60:ee:76:a8:3a:61:a9:fe:03:c7:
        28:25:a3:be:3f:03:92:35
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