OpenSSL-CVE-2015-1793漏洞分析

引言


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。

1.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。

2.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:
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。

3.漏洞验证


笔者修改了部分代码并做了个Poc 。修改代码:
int X509_verify_cert(X509_STORE_CTX *ctx)
{
X509 [i]x, [/i]xtmp, [i]xtmp2, [/i]chain_ss = NULL;
int bad_chain = 0;
X509_VERIFY_PARAM *param = ctx->param;
int depth, i, ok = 0;
int num, j, retry;
int ([i]cb) (int xok, X509_STORE_CTX [/i]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;
}

/[i] We use a temporary STACK so we can chop and hack at it [/i]/
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 (;;) {
/[i] If we have enough, we break [/i]/
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
[i] later. [/i]/

/[i] If we are self signed, we break [/i]/
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;
}
}

/[i] If we were passed a cert chain, use it first [/i]/
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;
}

/[i] Remember how many untrusted certs we have [/i]/
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)) {
/[i] we have a self signed certificate [/i]/
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);
}
}
/[i] We now lookup certs from the certificate store [/i]/
for (;;) {
/[i] If we have enough, we break [/i]/
if (depth < num)
break;
/[i] If we are self signed, we break [/i]/
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++;
}

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

/[i] If explicitly rejected error [/i]/
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;
/[i] Check if we found an alternate chain [/i]/
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");
/[i] We have the chain complete: now we need to check its purpose [/i]/
ok = check_chain_extensions(ctx);

if (!ok)
goto end;

printf("flag=2\n");
/[i] Check name constraints [/i]/

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");
/[i] We may as well copy down any DSA parameters that are required [/i]/
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");
/[i] At this point, we have a chain and need to verify it [/i]/
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
/[i] RFC 3779 path validation, now that CRL check has been done [/i]/
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");
/[i] If we get this far evaluate policies [/i]/
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) [i]load_certs_from_file(const char [/i]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

4.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

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