Microsoft Authenticode Signature Verification

## About Microsoft Authenticode Authenticode is Microsoft’s system for using digital signatures to ensure that programs to be run/installed on Windows systems come from a verified source and has not been modified by anyone else. At a high level, it works by having software developers: 1. obtain a code-signing certificate from a certificate authority trusted by the Windows OS 2. compute digital signatures for executables and related software installation files using that certificate 3. include the signatures as part of the software execution/installation process so that Windows can use them in the verification process In addition, Authenticode signatures can be countersigned by a time-stamping service that allows signature verification to succeed even if the code-signing certificate expires or gets revoked. For more information, check-out the following resources: - Everything you need to know about Authenticode code-signing - Authenticode Digital Signatures - Time Stamping Authenticode Signatures - Caveats for Authenticode Code Signing ## Authenticode and ClamAV ClamAV supports parsing the Authenticode section and performing signature verification on a given executable to determine whether it should be whitelisted (based on rules loaded in from ClamAV .crb files). An overview of this process, including information on the .crb file format and on how to add new whitelist entries, is explained in the Authenticode Certificate Chain Verification ClamAV blog post.

There are a few things not covered in the blog post that are worth mentioning: - Leaf certificates (the ones actually issued to the entity signing the binary) cannot currently be used for whitelisting. Instead, only certificates that issued the leaf certificate (and certificates higher up in the chain) can be used. - The .crb format supports blacklist rule entries, but these cannot currently be used as a basis for malware detection. Instead, as currently implemented, these entries just override .crb rules which would otherwise whitelist a given sample. - sigtool offers the ‘–print-certs’ flag, which can be used to show information about embedded Authenticode signatures without having to first match on a signature (which is currently a requirement for clamscan) - External Authenticode signatures contained in .cat files can be loaded in to ClamAV by passing a ‘-d’ flag and indicating the path to the .cat file from which to load signatures. Note, however, that at least one certificate in the .cat file’s certificate chain must be trusted (in other words, it must have a backing .crb file whitelist rule.)

Helpful Info for Working with Authenticode Signatures

Below is some useful information collected when improving ClamAV support for Authenticode signatures. ## Format Specifications The Windows Authenticode 2008 specification document can be found at the link below. Note, however, that it is not 100% accurate. For instance, the documented steps for computing the Authenticode hash are not correct in the case where you have sections that overlap with the PE header or with one another. - Windows Authenticode PE Signature Format ## Verifying the Signature On Linux, osslsigncode can be used to verify a signature: ``` $ osslsigncode verify /path/to/signed/file Current PE checksum : 00092934 Calculated PE checksum: 00092934

Message digest algorithm : SHA256 Current message digest : 56924EB391B1B04572B1841ED5D5C10927CE7D6E9553A69F994B9BA855A73933 Calculated message digest : 56924EB391B1B04572B1841ED5D5C10927CE7D6E9553A69F994B9BA855A73933

Signature verification: ok

Number of signers: 1 Signer #0: Subject: /C=US/ST=California/L=Mountain View/O=Google Inc/CN=Google Inc Issuer : /C=US/O=Symantec Corporation/OU=Symantec Trust Network/CN=Symantec Class 3 SHA256 Code Signing CA

Number of certificates: 2 Cert #0: Subject: /C=US/ST=California/L=Mountain View/O=Google Inc/CN=Google Inc Issuer : /C=US/O=Symantec Corporation/OU=Symantec Trust Network/CN=Symantec Class 3 SHA256 Code Signing CA Cert #1: Subject: /C=US/O=Symantec Corporation/OU=Symantec Trust Network/CN=Symantec Class 3 SHA256 Code Signing CA Issuer : /C=US/O=VeriSign, Inc./OU=VeriSign Trust Network/OU=(c) 2006 VeriSign, Inc. - For authorized use only/CN=VeriSign Class 3 Public Primary Certification Authority - G5

Succeeded ``` On Windows, AnalyzePESig is a great tool for displaying signature information. In addition, signtool can be used.

NOTE that the machine on which these commands is run should have Internet connectivity so that revocation lists can be consulted. Otherwise, Windows may default to assuming that none of the certificates are revoked.

There is also the verify-sigs python script that performs verification, but this script is no longer maintained. ## Extracting the Signature On Linux, the osslsigncode command can be used to extract the contents of the PE security section: osslsigncode extract-signature -in /path/to/exe -out /path/to/extracted Note: This will also extract the 8-byte WIN_CERTIFICATE structure data. To skip this data, use: dd if=/path/to/extracted of=/path/to/extracted.p7b bs=1 skip=8

Inspecting the Signature

On Linux, openssl has some useful functions for printing the certificate information and parsing the PKCS7 ASN1: ``` $ openssl pkcs7 -inform der -print_certs -in extracted.p7b -noout -text Certificate: Data: Version: 3 (0x2) Serial Number: 2a:9c:21:ac:aa:a6:3a:3c:58:a7:b9:32:2b:ee:94:8d Signature Algorithm: sha256WithRSAEncryption Issuer: C=US, O=Symantec Corporation, OU=Symantec Trust Network, CN=Symantec Class 3 SHA256 Code Signing CA Validity Not Before: Dec 16 00:00:00 2015 GMT Not After : Dec 16 23:59:59 2018 GMT Subject: C=US, ST=California, L=Mountain View, O=Google Inc, CN=Google Inc Subject Public Key Info: Public Key Algorithm: rsaEncryption Public-Key: (2048 bit) Modulus: 00:c4:0d:82:c4:41:29:28:e5:fd:0c:3f:a5:c7:0e: 66:bd:a5:c4:8b:b3:8a:ac:84:03:9f:84:2e:38:df: 06:b1:4e:fd:33:60:58:38:36:dd:22:cf:df:f1:50: 1f:47:f1:55:05:c1:81:01:e7:28:3e:ff:5f:89:12: 09:ea:df:aa:17:49:2c:71:ab:48:d1:9d:2e:f4:51: e0:03:e0:f7:16:6c:7b:0c:22:75:6d:7e:1f:49:c4: 43:28:88:41:dc:6c:ed:13:2a:03:99:eb:62:14:f9: 35:26:6e:12:2c:03:e2:f7:81:b9:1a:05:67:06:7c: a6:1a:5b:ed:20:15:e5:2d:83:de:8e:36:fa:1e:08: 41:1c:1a:48:9f:b6:f1:c3:2f:02:13:4b:a7:ca:ba: ef:1c:58:6f:8e:d3:0f:14:a4:0b:2b:5d:ba:f4:5a: a3:0d:64:34:a5:8a:d7:8f:4d:22:66:4d:a4:ae:e1: f9:cd:c6:58:e6:c6:11:77:32:df:ba:df:39:48:8a: d1:27:d7:33:77:a8:c9:e4:5e:ed:fa:12:cf:f3:fd: fa:ee:ab:80:86:13:34:eb:5a:7e:6f:6c:1b:ee:d8: 4b:b2:cc:77:98:87:ac:ca:f5:bb:64:6f:49:1e:5b: 91:63:50:1f:63:2d:83:27:73:07:9f:2b:16:f4:7b: 71:29 Exponent: 65537 (0x10001) X509v3 extensions: X509v3 Basic Constraints: CA:FALSE X509v3 Key Usage: critical Digital Signature X509v3 Extended Key Usage: Code Signing X509v3 Certificate Policies: Policy: 2.16.840.1.113733. CPS: User Notice: Explicit Text:

        X509v3 Authority Key Identifier: 

        X509v3 CRL Distribution Points: 

            Full Name:

        Authority Information Access: 
            OCSP - URI:
            CA Issuers - URI:

        Netscape Cert Type: 
            Object Signing 
Signature Algorithm: sha256WithRSAEncryption
     4e:5c:ac:bf ... ``` ``` $ openssl asn1parse -inform der -i -in extracted.p7b
0:d=0  hl=4 l=6984 cons: SEQUENCE          
4:d=1  hl=2 l=   9 prim:  OBJECT            :pkcs7-signedData    15:d=1  hl=4 l=6969 cons:  cont [ 0 ]            19:d=2  hl=4 l=6965 cons:   SEQUENCE              23:d=3  hl=2 l=   1 prim:    INTEGER           :01    26:d=3  hl=2 l=  15 cons:    SET                   28:d=4  hl=2 l=  13 cons:     SEQUENCE              30:d=5  hl=2 l=   9 prim:      OBJECT            :sha256    41:d=5  hl=2 l=   0 prim:      NULL                  43:d=3  hl=2 l=  92 cons:    SEQUENCE              45:d=4  hl=2 l=  10 prim:     OBJECT            :    57:d=4  hl=2 l=  78 cons:     cont [ 0 ]            59:d=5  hl=2 l=  76 cons:      SEQUENCE              61:d=6  hl=2 l=  23 cons:       SEQUENCE              63:d=7  hl=2 l=  10 prim:        OBJECT            :    75:d=7  hl=2 l=   9 cons:        SEQUENCE              77:d=8  hl=2 l=   1 prim:         BIT STRING            80:d=8  hl=2 l=   4 cons:         cont [ 0 ]            82:d=9  hl=2 l=   2 cons:          cont [ 2 ]            84:d=10 hl=2 l=   0 prim:           cont [ 0 ]            86:d=6  hl=2 l=  49 cons:       SEQUENCE              88:d=7  hl=2 l=  13 cons:        SEQUENCE              90:d=8  hl=2 l=   9 prim:         OBJECT            :sha256   101:d=8  hl=2 l=   0 prim:         NULL                 103:d=7  hl=2 l=  32 prim:        OCTET STRING      [HEX DUMP]:56924EB391B1B04572B1841ED5D5C10927CE7D6E9553A69F994B9BA855A73933 ... ```

On Windows, the certutil executable has a great ASN parser: C:\>certutil -asn extracted.p7b 0000: 30 82 1b 48 ; SEQUENCE (1b48 Bytes) 0004: | 06 09 ; OBJECT_ID (9 Bytes) 0006: | | 2a 86 48 86 f7 0d 01 07 02 | | ; 1.2.840.113549.1.7.2 PKCS 7 Signed 000f: | a0 82 1b 39 ; OPTIONAL[0] (1b39 Bytes) 0013: | 30 82 1b 35 ; SEQUENCE (1b35 Bytes) 0017: | 02 01 ; INTEGER (1 Bytes) 0019: | | 01 001a: | 31 0f ; SET (f Bytes) 001c: | | 30 0d ; SEQUENCE (d Bytes) 001e: | | 06 09 ; OBJECT_ID (9 Bytes) 0020: | | | 60 86 48 01 65 03 04 02 01 | | | ; 2.16.840. sha256 (sha256NoSign) 0029: | | 05 00 ; NULL (0 Bytes) 002b: | 30 5c ; SEQUENCE (5c Bytes) 002d: | | 06 0a ; OBJECT_ID (a Bytes) 002f: | | | 2b 06 01 04 01 82 37 02 01 04 | | | ; SPC_INDIRECT_DATA_OBJID 0039: | | a0 4e ; OPTIONAL[0] (4e Bytes) 003b: | | 30 4c ; SEQUENCE (4c Bytes) 003d: | | 30 17 ; SEQUENCE (17 Bytes) 003f: | | | 06 0a ; OBJECT_ID (a Bytes) 0041: | | | | 2b 06 01 04 01 82 37 02 01 0f | | | | ; SPC_PE_IMAGE_DATA_OBJID 004b: | | | 30 09 ; SEQUENCE (9 Bytes) ... There is also a website that offers ASN1 parser and allows you to interactively hide/view parts of the structure: - ASN1 JavaScript Parser

Creating Signed Executables

For Linux, Didier Stevens has a great post about how to create signed binaries using self-signed certificates: - Signing Windows Executables on Kali

On Windows, a program called signtool ships with the Windows SDK and can be used. See the following for tutorials/examples: - Authenticode Code Signing with Microsoft SignTool - Signtool Examples ## Samples with Interesting Authenticode Signatures Below are some PE files with interesting Authenticode signatures. These are probably only interesting to other researchers who are looking at Authenticode in-depth. All samples are available via VirusTotal.

  • SHA256-based code-signing signature without a countersignature
    • 8886d96e9ed475e4686ffba3d242e97836de8a56b75cc915e21bb324cc89de03
  • SHA256-based code-signing sig and SHA1-based timestamping countersig
    • 20367d0e3a5ad12154095d424b8d9818c33e7d6087525e6a3304ef6c22a53665
  • SHA384-based cert used in the code-signing chain
    • 2249611fef630d666f667ac9dc7b665d3b9382956e41f10704e40bd900decbb8
  • Uses SHA512 to compute the Authenticode hash
    • eeb5469a214d5aac1dcd7e8415f93eca14edc38f47d1e360d3d97d432695207a
  • Signed by an MS root cert that doesn’t have a KU or EKU specified
    • 69b61b2c00323cea3686315617d0f452e205dae10c47e02cbe1ea96fea38f582
  • Has a v1 x509 cert and uses MD2-based hashing
    • 1cb16f94cebdcad7dd05c8537375a6ff6379fcdb08528fc83889f26efaa84e2a
  • Countersignature with version 0 instead of version 1
    • 145fbbf59b1071493686bf41f4eb364627d8be3c9dc8fb927bbe853e593864ec
  • Countersignature with contentType == timestampToken instead of pkcs7-data
    • 8a364e0881fd7201cd6f0a0ff747451c9b93182d5699afb28ad8466f7f726660
  • Countersignature with AlgoIdentifier sha1WithRSAEncryption instead of SHA1
    • 2aa6b18d509090c60c3e4ecdd8aeb16e5f149807e3404c86892112710eab576d
  • Countersignature uses v1 x509 certs
    • 934860a4ac2446240e4c7053ddc27ff4c2463d4ad433cc28c9fcc2ea4690fb86
  • Certificate chain has old certificates without a version
    • 374a31b20fbafdcd31d52ae11a0dcad58baba556c8942a3cdfae0bb96ae117a1
  • Has extra data after the PKCS7, a violation of MS13-098
    • 0ee196bb23f0eafe4f61d30bf6c676fd7365cb12ae66a6bde278851e91901ac1
  • Authenticode signature covers data not in a section
    • 0123c163ac981e639565caff72ee3af2df7174613ee12003ac89124be461c6e6
  • Authenticode signature with a section that overlaps the PE header (UPX-packed)
    • 0059fb3f225c5784789622eeccb97197d591972851b63d59f5bd107ddfdb7a21
  • Authenticode signature with overlapping sections
    • 014b66cf2cef39620e9a985d237971b8cf272043e9ac372d5dcef44db754a1d2
  • Uses certs with no NULL after AlgorithmIdentifier OID
    • 66b797b3b4f99488f53c2b676610dfe9868984c779536891a8d8f73ee214bc4b
  • Uses an ASN1 indefinite length object
    • 8ca912e397a9e1b0cc54c216144ff550da9d43610392208193c0781b1aa5d695
  • Unexpected contentType for embedded mode signature (copied from a .cat?)
    • 6ed9b5f6d32f94b3d06456b176c8536be123e1047763cc0a31c6e8fd6a0242b1