yara/libyara/modules/pe/authenticode-parser/authenticode.c

/* Copyright (c) 2021 Avast Software

Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights to
use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
of the Software, and to permit persons to whom the Software is furnished to do
so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

#include <openssl/asn1.h>
#include <openssl/evp.h>
#include <openssl/objects.h>
#include <openssl/ossl_typ.h>
#include <openssl/pkcs7.h>
#include <openssl/safestack.h>
#include <openssl/sha.h>
#include <openssl/x509.h>
#include <openssl/x509_vfy.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>

#include <authenticode-parser/authenticode.h>

#include "certificate.h"
#include "countersignature.h"
#include "helper.h"
#include "structs.h"

#define MAX_NESTED_COUNT 16

/* Moves signatures from src to dst, returns 0 on success,
 * else 1. If error occurs, arguments are unchanged */
static int authenticode_array_move(AuthenticodeArray* dst, AuthenticodeArray* src)
{
    size_t newCount = dst->count + src->count;

    Authenticode** tmp = (Authenticode**)realloc(dst->signatures, newCount * sizeof(Authenticode*));
    if (!tmp)
        return 1;

    dst->signatures = tmp;

    for (size_t i = 0; i < src->count; ++i)
        dst->signatures[i + dst->count] = src->signatures[i];

    dst->count = newCount;

    free(src->signatures);
    src->signatures = NULL;
    src->count = 0;

    return 0;
}

static SpcIndirectDataContent* get_content(PKCS7* content)
{
    if (!content)
        return NULL;

    if (OBJ_obj2nid(content->type) != OBJ_txt2nid(NID_spc_indirect_data))
        return NULL;

    SpcIndirectDataContent* spcContent = SpcIndirectDataContent_new();
    if (!spcContent)
        return NULL;

    int len = content->d.other->value.sequence->length;
    const uint8_t* data = content->d.other->value.sequence->data;

    d2i_SpcIndirectDataContent(&spcContent, &data, len);

    return spcContent;
}

static char* parse_program_name(ASN1_TYPE* spcAttr)
{
    const uint8_t* spcData = spcAttr->value.sequence->data;
    int spcLen = spcAttr->value.sequence->length;
    SpcSpOpusInfo* spcInfo = d2i_SpcSpOpusInfo(NULL, &spcData, spcLen);
    if (!spcInfo)
        return NULL;

    char* result = NULL;

    if (spcInfo->programName) {
        uint8_t* data = NULL;
        /* Should be Windows UTF16..., try to convert it to UTF8 */
        int nameLen = ASN1_STRING_to_UTF8(&data, spcInfo->programName->value.unicode);
        if (nameLen >= 0 && nameLen < spcLen) {
            result = (char*)malloc(nameLen + 1);
            if (result) {
                memcpy(result, data, nameLen);
                result[nameLen] = 0;
            }
            OPENSSL_free(data);
        }
    }

    SpcSpOpusInfo_free(spcInfo);
    return result;
}

/* Parses X509* certs into internal representation and inserts into CertificateArray
 * Array is assumed to have enough space to hold all certificates storted in the STACK */
static void parse_certificates(const STACK_OF(X509) * certs, CertificateArray* result)
{
    int certCount = sk_X509_num(certs);
    int i = 0;
    for (; i < certCount; ++i) {
        Certificate* cert = certificate_new(sk_X509_value(certs, i));
        if (!cert)
            break;

        /* Write to the result */
        result->certs[i] = cert;
    }
    result->count = i;
}

static void parse_nested_authenticode(PKCS7_SIGNER_INFO* si, AuthenticodeArray* result)
{
    STACK_OF(X509_ATTRIBUTE)* attrs = PKCS7_get_attributes(si);
    int idx = X509at_get_attr_by_NID(attrs, OBJ_txt2nid(NID_spc_nested_signature), -1);
    X509_ATTRIBUTE* attr = X509at_get_attr(attrs, idx);

    int attrCount = X509_ATTRIBUTE_count(attr);
    if (!attrCount)
        return;

    /* Limit the maximum amount of nested attributes to be safe from malformed samples */
    attrCount = attrCount > MAX_NESTED_COUNT ? MAX_NESTED_COUNT : attrCount;

    for (int i = 0; i < attrCount; ++i) {
        ASN1_TYPE* nested = X509_ATTRIBUTE_get0_type(attr, i);
        if (nested == NULL)
            break;
        int len = nested->value.sequence->length;
        const uint8_t* data = nested->value.sequence->data;
        AuthenticodeArray* auth = authenticode_new(data, len);
        if (!auth)
            continue;

        authenticode_array_move(result, auth);
        authenticode_array_free(auth);
    }
}

static void parse_pkcs9_countersig(PKCS7* p7, Authenticode* auth)
{
    PKCS7_SIGNER_INFO* si = sk_PKCS7_SIGNER_INFO_value(PKCS7_get_signer_info(p7), 0);

    STACK_OF(X509_ATTRIBUTE)* attrs = PKCS7_get_attributes(si);

    int idx = X509at_get_attr_by_NID(attrs, NID_pkcs9_countersignature, -1);
    X509_ATTRIBUTE* attr = X509at_get_attr(attrs, idx);

    int attrCount = X509_ATTRIBUTE_count(attr);
    if (!attrCount)
        return;

    /* Limit the maximum amount of nested attributes to be safe from malformed samples */
    attrCount = attrCount > MAX_NESTED_COUNT ? MAX_NESTED_COUNT : attrCount;

    for (int i = 0; i < attrCount; ++i) {
        ASN1_TYPE* nested = X509_ATTRIBUTE_get0_type(attr, i);
        if (nested == NULL)
            break;
        int len = nested->value.sequence->length;
        const uint8_t* data = nested->value.sequence->data;

        Countersignature* sig = pkcs9_countersig_new(data, len, p7->d.sign->cert, si->enc_digest);
        if (!sig)
            continue;

        countersignature_array_insert(auth->countersigs, sig);
    }
}

/* Extracts X509 certificates from MS countersignature and stores them into result */
static void extract_ms_counter_certs(const uint8_t* data, int len, CertificateArray* result)
{
    PKCS7* p7 = d2i_PKCS7(NULL, &data, len);
    if (!p7)
        return;

    STACK_OF(X509)* certs = p7->d.sign->cert;
    CertificateArray* certArr = certificate_array_new(sk_X509_num(certs));
    if (!certArr) {
        PKCS7_free(p7);
        return;
    }
    parse_certificates(certs, certArr);
    certificate_array_move(result, certArr);
    certificate_array_free(certArr);

    PKCS7_free(p7);
}

static void parse_ms_countersig(PKCS7* p7, Authenticode* auth)
{
    PKCS7_SIGNER_INFO* si = sk_PKCS7_SIGNER_INFO_value(PKCS7_get_signer_info(p7), 0);

    STACK_OF(X509_ATTRIBUTE)* attrs = PKCS7_get_attributes(si);

    int idx = X509at_get_attr_by_NID(attrs, OBJ_txt2nid(NID_spc_ms_countersignature), -1);
    X509_ATTRIBUTE* attr = X509at_get_attr(attrs, idx);

    int attrCount = X509_ATTRIBUTE_count(attr);
    if (!attrCount)
        return;

    /* Limit the maximum amount of nested attributes to be safe from malformed samples */
    attrCount = attrCount > MAX_NESTED_COUNT ? MAX_NESTED_COUNT : attrCount;

    for (int i = 0; i < attrCount; ++i) {
        ASN1_TYPE* nested = X509_ATTRIBUTE_get0_type(attr, i);
        if (nested == NULL)
            break;
        int len = nested->value.sequence->length;
        const uint8_t* data = nested->value.sequence->data;

        Countersignature* sig = ms_countersig_new(data, len, si->enc_digest);
        if (!sig)
            return;

        /* Because MS TimeStamp countersignature has it's own SET of certificates
         * extract it back into parent signature for consistency with PKCS9 */
        countersignature_array_insert(auth->countersigs, sig);
        extract_ms_counter_certs(data, len, auth->certs);
    }
}

static bool authenticode_verify(PKCS7* p7, PKCS7_SIGNER_INFO* si, X509* signCert)
{
    const uint8_t* contentData = p7->d.sign->contents->d.other->value.sequence->data;
    long contentLen = p7->d.sign->contents->d.other->value.sequence->length;

    uint64_t version = 0;
    ASN1_INTEGER_get_uint64(&version, p7->d.sign->version);
    if (version == 1) {
        /* Move the pointer to the actual contents - skip OID and length */
        int pclass = 0, ptag = 0;
        ASN1_get_object(&contentData, &contentLen, &ptag, &pclass, contentLen);
    }

    BIO* contentBio = BIO_new_mem_buf(contentData, contentLen);
    /* Create `digest` type BIO to calculate content digest for verification */
    BIO* p7bio = PKCS7_dataInit(p7, contentBio);

    char buf[4096];
    /* We now have to 'read' from p7bio to calculate content digest */
    while (BIO_read(p7bio, buf, sizeof(buf)) > 0)
        continue;

    /* Pass it to the PKCS7_signatureVerify, to do the hard work for us */
    bool isValid = PKCS7_signatureVerify(p7bio, p7, si, signCert) == 1;

    BIO_free_all(p7bio);

    return isValid;
}

/* Creates all the Authenticode objects so we can parse them with OpenSSL, is not thread-safe, needs
 * to be called once before any multi-threading environmentt - https://github.com/openssl/openssl/issues/13524 */
void initialize_authenticode_parser()
{
    OBJ_create("1.3.6.1.4.1.311.2.1.12", "spcSpOpusInfo", "SPC_SP_OPUS_INFO_OBJID");
    OBJ_create("1.3.6.1.4.1.311.3.3.1", "spcMsCountersignature", "SPC_MICROSOFT_COUNTERSIGNATURE");
    OBJ_create("1.3.6.1.4.1.311.2.4.1", "spcNestedSignature", "SPC_NESTED_SIGNATUREs");
    OBJ_create("1.3.6.1.4.1.311.2.1.4", "spcIndirectData", "SPC_INDIRECT_DATA");
}

/* Return array of Authenticode signatures stored in the data, there can be multiple
 * of signatures as Authenticode signatures are often nested through unauth attributes */
AuthenticodeArray* authenticode_new(const uint8_t* data, long len)
{
    if (!data || len == 0)
        return NULL;

    AuthenticodeArray* result = (AuthenticodeArray*)calloc(1, sizeof(*result));
    if (!result)
        return NULL;

    result->signatures = (Authenticode**)malloc(sizeof(Authenticode*));
    if (!result->signatures) {
        free(result);
        return NULL;
    }

    Authenticode* auth = (Authenticode*)calloc(1, sizeof(*auth));
    if (!auth) {
        free(result->signatures);
        free(result);
        return NULL;
    }

    result->count = 1;
    result->signatures[0] = auth;

    /* Let openssl parse the PKCS7 structure */
    PKCS7* p7 = d2i_PKCS7(NULL, &data, len);
    if (!p7) {
        auth->verify_flags = AUTHENTICODE_VFY_CANT_PARSE;
        goto end;
    }

    /* We expect SignedData type of PKCS7 */
    if (!PKCS7_type_is_signed(p7)) {
        auth->verify_flags = AUTHENTICODE_VFY_WRONG_PKCS7_TYPE;
        goto end;
    }

    PKCS7_SIGNED* p7data = p7->d.sign;

    uint64_t version = 0;
    if (ASN1_INTEGER_get_uint64(&version, p7data->version))
        auth->version = version;

    STACK_OF(X509)* certs = p7data->cert;

    auth->certs = certificate_array_new(sk_X509_num(certs));
    if (!auth->certs) {
        auth->verify_flags = AUTHENTICODE_VFY_INTERNAL_ERROR;
        goto end;
    }
    parse_certificates(certs, auth->certs);

    /* Get Signature content that contains the message digest and it's algorithm */
    SpcIndirectDataContent* dataContent = get_content(p7data->contents);
    if (!dataContent) {
        auth->verify_flags = AUTHENTICODE_VFY_BAD_CONTENT;
        goto end;
    }

    DigestInfo* messageDigest = dataContent->messageDigest;

    int digestnid = OBJ_obj2nid(messageDigest->digestAlgorithm->algorithm);
    auth->digest_alg = strdup(OBJ_nid2ln(digestnid));

    int digestLen = messageDigest->digest->length;
    const uint8_t* digestData = messageDigest->digest->data;
    byte_array_init(&auth->digest, digestData, digestLen);

    SpcIndirectDataContent_free(dataContent);

    Signer* signer = (Signer*)calloc(1, sizeof(Signer));
    if (!signer) {
        auth->verify_flags = AUTHENTICODE_VFY_INTERNAL_ERROR;
        goto end;
    }
    auth->signer = signer;

    /* Authenticode is supposed to have only one SignerInfo value
     * that contains all information for actual signing purposes
     * and nested signatures or countersignatures */
    PKCS7_SIGNER_INFO* si = sk_PKCS7_SIGNER_INFO_value(PKCS7_get_signer_info(p7), 0);
    if (!si) {
        auth->verify_flags = AUTHENTICODE_VFY_NO_SIGNER_INFO;
        goto end;
    }

    auth->countersigs = (CountersignatureArray*)calloc(1, sizeof(CountersignatureArray));
    if (!auth->countersigs) {
        auth->verify_flags = AUTHENTICODE_VFY_INTERNAL_ERROR;
        goto end;
    }
    /* Authenticode can contain SET of nested Authenticode signatures
     * and countersignatures in unauthenticated attributes */
    parse_nested_authenticode(si, result);
    parse_pkcs9_countersig(p7, auth);
    parse_ms_countersig(p7, auth);

    /* Get the signing certificate for the first SignerInfo */
    STACK_OF(X509)* signCertStack = PKCS7_get0_signers(p7, certs, 0);

    X509* signCert = sk_X509_value(signCertStack, 0);
    if (!signCert) {
        auth->verify_flags = AUTHENTICODE_VFY_NO_SIGNER_CERT;
        sk_X509_free(signCertStack);
        goto end;
    }

    sk_X509_free(signCertStack);

    signer->chain = parse_signer_chain(signCert, certs);

    /* Get the Signers digest of Authenticode content */
    ASN1_TYPE* digest = PKCS7_get_signed_attribute(si, NID_pkcs9_messageDigest);
    if (!digest) {
        auth->verify_flags = AUTHENTICODE_VFY_DIGEST_MISSING;
        goto end;
    }

    digestnid = OBJ_obj2nid(si->digest_alg->algorithm);
    signer->digest_alg = strdup(OBJ_nid2ln(digestnid));

    digestLen = digest->value.asn1_string->length;
    digestData = digest->value.asn1_string->data;
    byte_array_init(&signer->digest, digestData, digestLen);

    /* Authenticode stores optional programName in non-optional SpcSpOpusInfo attribute */
    ASN1_TYPE* spcInfo = PKCS7_get_signed_attribute(si, OBJ_txt2nid(NID_spc_info));
    if (spcInfo)
        signer->program_name = parse_program_name(spcInfo);

    /* If we got to this point, we got all we need to start verifying */
    bool isValid = authenticode_verify(p7, si, signCert);
    if (!isValid)
        auth->verify_flags = AUTHENTICODE_VFY_INVALID;

end:
    PKCS7_free(p7);
    return result;
}

static int authenticode_digest(
    const EVP_MD* md,
    const uint8_t* pe_data,
    uint32_t pe_hdr_offset,
    bool is_64bit,
    uint32_t cert_table_addr,
    uint8_t* digest)
{
    uint32_t buffer_size = 0xFFFF;
    uint8_t* buffer = (uint8_t*)malloc(buffer_size);

    /* BIO with the file data */
    BIO* bio = BIO_new_mem_buf(pe_data, cert_table_addr);

    EVP_MD_CTX* mdctx = EVP_MD_CTX_new();
    if (!buffer || !bio || !mdctx)
        goto error;

    if (!EVP_DigestInit(mdctx, md))
        goto error;

    /* Calculate size of the space between file start and PE header */
    /* Checksum starts at 0x58th byte of the header */
    uint32_t pe_checksum_offset = pe_hdr_offset + 0x58;
    /* Space between DOS and PE header could have arbitrary amount of data, read in chunks */
    uint32_t fpos = 0;
    while (fpos < pe_checksum_offset) {
        uint32_t len_to_read = pe_checksum_offset - fpos;
        if (len_to_read > buffer_size)
            len_to_read = buffer_size;

        int rlen = BIO_read(bio, buffer, len_to_read);
        if (rlen <= 0)
            goto error;

        if (!EVP_DigestUpdate(mdctx, buffer, rlen))
            goto error;

        fpos += rlen;
    }

    /* Skip the checksum */
    if (BIO_read(bio, buffer, 4) <= 0)
        goto error;

    /* 64bit PE file is larger than 32bit */
    uint32_t pe64_extra = is_64bit ? 16 : 0;

    /* Read up to certificate table*/
    uint32_t cert_table_offset = 0x3c + pe64_extra;

    if (BIO_read(bio, buffer, cert_table_offset) <= 0)
        goto error;

    if (!EVP_DigestUpdate(mdctx, buffer, cert_table_offset))
        goto error;

    /* Skip certificate table */
    if (BIO_read(bio, buffer, 8) <= 0)
        goto error;

    /* PE header with check sum + checksum + cert table offset + cert table len */
    fpos = pe_checksum_offset + 4 + cert_table_offset + 8;

    /* Hash everything up to the signature (assuming signature is stored in the
     * end of the file) */
    /* Read chunks of the file in case the file is large */
    while (fpos < cert_table_addr) {
        uint32_t len_to_read = cert_table_addr - fpos;
        if (len_to_read > buffer_size)
            len_to_read = buffer_size;

        int rlen = BIO_read(bio, buffer, len_to_read);
        if (rlen <= 0)
            goto error;

        if (!EVP_DigestUpdate(mdctx, buffer, rlen))
            goto error;
        fpos += rlen;
    }

    /* Calculate the digest, write it into digest */
    if (!EVP_DigestFinal(mdctx, digest, NULL))
        goto error;

    EVP_MD_CTX_free(mdctx);
    BIO_free_all(bio);
    free(buffer);
    return 0;

error:
    EVP_MD_CTX_free(mdctx);
    BIO_free_all(bio);
    free(buffer);
    return 1;
}

AuthenticodeArray* parse_authenticode(const uint8_t* pe_data, uint64_t pe_len)
{
    const uint64_t dos_hdr_size = 0x40;
    if (pe_len < dos_hdr_size)
        return NULL;

    /* Check if it has DOS signature, so we don't parse random gibberish */
    uint8_t dos_prefix[] = {0x4d, 0x5a};
    if (memcmp(pe_data, dos_prefix, sizeof(dos_prefix)) != 0)
        return NULL;

    /* offset to pointer in DOS header, that points to PE header */
    const int pe_hdr_ptr_offset = 0x3c;
    /* Read the PE offset */
    uint32_t pe_offset = letoh32(*(uint32_t*)(pe_data + pe_hdr_ptr_offset));
    /* Offset to Magic, to know the PE class (32/64bit) */
    uint32_t magic_addr = pe_offset + 0x18;

    if (pe_len < magic_addr + sizeof(uint16_t))
        return NULL;

    /* Read the magic and check if we have 64bit PE */
    uint16_t magic = letoh16(*(uint16_t*)(pe_data + magic_addr));
    bool is_64bit = magic == 0x20b;
    /* If PE is 64bit, header is 16 bytes larger */
    uint8_t pe64_extra = is_64bit ? 16 : 0;

    /* Calculate offset to certificate table directory */
    uint32_t pe_cert_table_addr = pe_offset + pe64_extra + 0x98;

    if (pe_len < pe_cert_table_addr + 2 * sizeof(uint32_t))
        return NULL;

    /* Use 64bit type due to the potential overflow in crafted binaries */
    uint64_t cert_addr = letoh32(*(uint32_t*)(pe_data + pe_cert_table_addr));
    uint64_t cert_len = letoh32(*(uint32_t*)(pe_data + pe_cert_table_addr + 4));

    /* we need atleast 8 bytes to read dwLength, revision and certType */
    if (cert_len < 8 || pe_len < cert_addr + 8)
        return NULL;

    uint32_t dwLength = letoh32(*(uint32_t*)(pe_data + cert_addr));
    if (pe_len < cert_addr + dwLength)
        return NULL;
    /* dwLength = offsetof(WIN_CERTIFICATE, bCertificate) + (size of the variable-length binary array contained within bCertificate) */
    AuthenticodeArray* auth_array = authenticode_new(pe_data + cert_addr + 0x8, dwLength - 0x8);
    if (!auth_array)
        return NULL;

    /* Compare valid signatures file digests to actual file digest, to complete verification */
    for (size_t i = 0; i < auth_array->count; ++i) {
        Authenticode* sig = auth_array->signatures[i];

        const EVP_MD* md = EVP_get_digestbyname(sig->digest_alg);
        if (!md || !sig->digest.len || !sig->digest.data) {
            /* If there is an verification error, keep the first error */
            if (sig->verify_flags == AUTHENTICODE_VFY_VALID)
                sig->verify_flags = AUTHENTICODE_VFY_UNKNOWN_ALGORITHM;

            continue;
        }

#if OPENSSL_VERSION_NUMBER >= 0x3000000fL
        int mdlen = EVP_MD_get_size(md);
#else
        int mdlen = EVP_MD_size(md);
#endif
        sig->file_digest.len = mdlen;
        sig->file_digest.data = (uint8_t*)malloc(mdlen);
        if (!sig->file_digest.data)
            continue;

        if (authenticode_digest(
                md, pe_data, pe_offset, is_64bit, cert_addr, sig->file_digest.data)) {

            /* If there is an verification error, keep the first error */
            if (sig->verify_flags == AUTHENTICODE_VFY_VALID)
                sig->verify_flags = AUTHENTICODE_VFY_INTERNAL_ERROR;
            break;
        }

        /* Complete the verification */
        if (memcmp(sig->file_digest.data, sig->digest.data, mdlen) != 0)
            sig->verify_flags = AUTHENTICODE_VFY_WRONG_FILE_DIGEST;
    }

    return auth_array;
}

static void signer_free(Signer* si)
{
    if (si) {
        free(si->digest.data);
        free(si->digest_alg);
        free(si->program_name);
        certificate_array_free(si->chain);
        free(si);
    }
}

static void authenticode_free(Authenticode* auth)
{
    if (auth) {
        free(auth->digest.data);
        free(auth->file_digest.data);
        free(auth->digest_alg);
        signer_free(auth->signer);
        certificate_array_free(auth->certs);
        countersignature_array_free(auth->countersigs);
        free(auth);
    }
}

void authenticode_array_free(AuthenticodeArray* arr)
{
    if (arr) {
        for (size_t i = 0; i < arr->count; ++i) {
            authenticode_free(arr->signatures[i]);
        }
        free(arr->signatures);
        free(arr);
    }
}