0x00 前言

OpenSSL官方在7月9日发布了编号为 CVE-2015-1793 的交叉证书验证绕过漏洞，其中主要影响了OpenSSL的1.0.1和1.0.2分支。1.0.0和0.9.8分支不受影响。

360安全研究员au2o3t对该漏洞进行了原理上的分析，确认是一个绕过交叉链类型证书验证的高危漏洞，可以让攻击者构造证书来绕过交叉验证，用来形成诸如“中间人”等形式的攻击。

0x01 漏洞基本原理

直接看最简单的利用方法（利用方法包括但不限于此）：

攻击者从一公共可信的 CA （C）处签得一证书 X，并以此证书签发另一证书 V（含对X的交叉引用），那么攻击者发出的证书链 V, R （R为任意证书）对信任 C 的用户将是可信的。

显然用户对 V, R 链的验证会返回失败。

对不支持交叉链认证的老版本来说，验证过程将以失败结束。

对支持交叉认证的版本，则将会尝试构建交叉链 V, X, C，并继续进行验证。

虽然 V, X, C 链能通过可信认证，但会因 X 的用法不包括 CA 而导致验证失败。

但在 openssl-1.0.2c 版本，因在对交叉链的处理中，对最后一个不可信证书位置计数的错误，导致本应对 V, X 记为不可信并验证，错记为了仅对 V 做验证，而没有验证攻击者的证书 X，返回验证成功。

0x02 具体漏洞分析

漏洞代码位于文件：openssl-1.0.2c/crypto/x509/x509_vfy.c

函数：X509_verify_cert() 中

第 392 行：ctx->last_untrusted–;

对问题函数 X509_verify_cert 的简单分析：

（ 为方便阅读，仅保留与证书验证强相关的代码，去掉了诸如变量定义、错误处理、资源释放等非主要代码）

问题在于由 <1> 处加入颁发者时及 <2> 处验证（颁发者）后，证书链计数增加，但 最后一个不可信证书位置计数 并未增加， 而在 <4> 处去除过程中 最后一个不可信证书位置计数 额外减少了，导致后面验证过程中少验。

（上述 V, X, C 链中应验 V, X 但少验了 X）

代码分析如下

#!c++
int X509_verify_cert(X509_STORE_CTX *ctx)
{
    // 将 ctx->cert 做为不信任证书压入需验证链  ctx->chain
    // STACK_OF(X509) *chain 将被构造为证书链，并最终送到 internal_verify() 中去验证
    sk_X509_push(ctx->chain,ctx->cert); 
    // 当前链长度（==1）
    num = sk_X509_num(ctx->chain);
     // 取出第 num 个证书
    x = sk_X509_value(ctx->chain, num - 1);
     // 存在不信任链则复制之
    if (ctx->untrusted != NULL
        && (sktmp = sk_X509_dup(ctx->untrusted)) == NULL) {
        X509err(X509_F_X509_VERIFY_CERT, ERR_R_MALLOC_FAILURE);
         goto end;
    }
     // 预设定的最大链深度（100）
    depth = param->depth;
    // 构造需验证证书链
    for (;;) {
        // 超长退出
        if (depth < num)
            break;
        // 遇自签退出（链顶）
        if (cert_self_signed(x))
            break;
         if (ctx->untrusted != NULL) {
            xtmp = find_issuer(ctx, sktmp, x);
            // 当前证书为不信任颁发者（应需CA标志）颁发
            if (xtmp != NULL) {
                // 则加入需验证链
                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);
                // 最后一个不可信证书位置计数 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);
        // 若最顶证书是自签的
        if (cert_self_signed(x)) {
            // 若需验证链长度 == 1
            if (sk_X509_num(ctx->chain) == 1) {
                // 在可信链中查找其颁发者（找自己）
                ok = ctx->get_issuer(&xtmp, ctx, x);

               // 没找到或不是相同证书
                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;
                    // 入到可信链
                    (void)sk_X509_set(ctx->chain, i - 1, x);
                    // 最后一个不可信证书位置计数 置0
                    ctx->last_untrusted = 0;
                }
            // 最顶为自签证书 且 证书链长度>1
            } else {
                // 弹出
                chain_ss = sk_X509_pop(ctx->chain);
                // 最后一个不可信证书位置计数 自减
                ctx->last_untrusted--;
                num--;
                j--;
                // 保持指向当前最顶证书
                x = sk_X509_value(ctx->chain, num - 1);
            }
        }
        // <1>
        // 继续构造证书链（加入颁发者）
        for (;;) {
            // 自签退出
            if (cert_self_signed(x))
                break;
            // 在可信链中查找其颁发者
            ok = ctx->get_issuer(&xtmp, ctx, x);
            // 出错
            if (ok < 0)
                return ok;
            // 没找到
            if (ok == 0)
                 break;
            x = xtmp;
            // 将不可信证书的颁发者（证书）加入需验证证书链
            if (!sk_X509_push(ctx->chain, x)) {
                X509_free(xtmp);
                X509err(X509_F_X509_VERIFY_CERT, ERR_R_MALLOC_FAILURE);
                return 0;
            }
            num++;
        }
        // <2>
        // 验证 for(;;) 中加入的颁发者链
        i = check_trust(ctx);
        if (i == X509_TRUST_REJECTED)
            goto end;
        retry = 0;
         // <3>
        // 检查交叉链
        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);
                 // 其实得到一个“看似合理”的证书就返回，这里实际上仅仅根据 CN域 查找颁发者
                ok = ctx->get_issuer(&xtmp, ctx, xtmp2);
                if (ok < 0)
                    goto end;
                // 存在交叉链
                if (ok > 0) {
                    X509_free(xtmp);

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

官方的解决方法是在 <5> 处重新计算 最后一个不可信证书位置计数 的值为链长：

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

并去掉 <4> 处的 最后一个不可信证书位置计数 自减运算（其实去不去掉都无所谓）。 另一个解决办法可以是在 <1> <2> 后，在 <3> 处重置 最后一个不可信证书位置计数，加一行：

ctx->last_untrusted = num;

这样 <4> 处不用删除，而逻辑也是合理并前后一致的。

0x03 漏洞验证

笔者修改了部分代码并做了个Poc 。 修改代码：

#!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:
?
//
//里头的证书文件自己去找一个，这个不提供了
//
#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;
}

将代码中 X509_verify_cert() 函数加入输出信息如下： 编译，以伪造证书测试，程序输出信息为：

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

认证成功 将 <1> 处注释代码去掉，编译，再以伪造证书测试，程序输出信息为：

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

认证失败

0x04 安全建议

建议使用受影响版本（OpenSSL 1.0.2b/1.0.2c 和 OpenSSL 1.0.1n/1.0.1o）的 产品或代码升级OpenSSL到最新版本