Fix a padding oracle in PKCS7_dataDecode and CMS_decrypt_set1_pkey

An attack is simple, if the first CMS_recipientInfo is valid but the
second CMS_recipientInfo is chosen ciphertext. If the second
recipientInfo decodes to PKCS #1 v1.5 form plaintext, the correct
encryption key will be replaced by garbage, and the message cannot be
decoded, but if the RSA decryption fails, the correct encryption key is
used and the recipient will not notice the attack.

As a work around for this potential attack the length of the decrypted
key must be equal to the cipher default key length, in case the
certifiate is not given and all recipientInfo are tried out.

The old behaviour can be re-enabled in the CMS code by setting the
CMS_DEBUG_DECRYPT flag.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9777)

(cherry picked from commit 5840ed0cd1e6487d247efbc1a04136a41d7b3a37)

5 files changed, 45 insertions, 5 deletions

diff --git a/CHANGES b/CHANGES
index b4400d20cc..b12cf3ae07 100644
--- a/CHANGES
+++ b/CHANGES
@@ -39,6 +39,20 @@
      (CVE-2019-1547)
      [Billy Bob Brumley]
 
+  *) Fixed a padding oracle in PKCS7_dataDecode and CMS_decrypt_set1_pkey.
+     An attack is simple, if the first CMS_recipientInfo is valid but the
+     second CMS_recipientInfo is chosen ciphertext. If the second
+     recipientInfo decodes to PKCS #1 v1.5 form plaintext, the correct
+     encryption key will be replaced by garbage, and the message cannot be
+     decoded, but if the RSA decryption fails, the correct encryption key is
+     used and the recipient will not notice the attack.
+     As a work around for this potential attack the length of the decrypted
+     key must be equal to the cipher default key length, in case the
+     certifiate is not given and all recipientInfo are tried out.
+     The old behaviour can be re-enabled in the CMS code by setting the
+     CMS_DEBUG_DECRYPT flag.
+     [Bernd Edlinger]
+
   *) Use Windows installation paths in the mingw builds
 
      Mingw isn't a POSIX environment per se, which means that Windows
diff --git a/crypto/cms/cms_env.c b/crypto/cms/cms_env.c
index fe5076ec02..269e97f3fa 100644
--- a/crypto/cms/cms_env.c
+++ b/crypto/cms/cms_env.c
@@ -363,6 +363,7 @@ static int cms_RecipientInfo_ktri_decrypt(CMS_ContentInfo *cms,
     unsigned char *ek = NULL;
     size_t eklen;
     int ret = 0;
+    size_t fixlen = 0;
     CMS_EncryptedContentInfo *ec;
     ec = cms->d.envelopedData->encryptedContentInfo;
 
@@ -371,6 +372,19 @@ static int cms_RecipientInfo_ktri_decrypt(CMS_ContentInfo *cms,
         return 0;
     }
 
+    if (cms->d.envelopedData->encryptedContentInfo->havenocert
+            && !cms->d.envelopedData->encryptedContentInfo->debug) {
+        X509_ALGOR *calg = ec->contentEncryptionAlgorithm;
+        const EVP_CIPHER *ciph = EVP_get_cipherbyobj(calg->algorithm);
+
+        if (ciph == NULL) {
+            CMSerr(CMS_F_CMS_RECIPIENTINFO_KTRI_DECRYPT, CMS_R_UNKNOWN_CIPHER);
+            return 0;
+        }
+
+        fixlen = EVP_CIPHER_key_length(ciph);
+    }
+
     ktri->pctx = EVP_PKEY_CTX_new(pkey, NULL);
     if (ktri->pctx == NULL)
         return 0;
@@ -401,7 +415,9 @@ static int cms_RecipientInfo_ktri_decrypt(CMS_ContentInfo *cms,
 
     if (EVP_PKEY_decrypt(ktri->pctx, ek, &eklen,
                          ktri->encryptedKey->data,
-                         ktri->encryptedKey->length) <= 0) {
+                         ktri->encryptedKey->length) <= 0
+            || eklen == 0
+            || (fixlen != 0 && eklen != fixlen)) {
         CMSerr(CMS_F_CMS_RECIPIENTINFO_KTRI_DECRYPT, CMS_R_CMS_LIB);
         goto err;
     }
diff --git a/crypto/cms/cms_lcl.h b/crypto/cms/cms_lcl.h
index d0c0e81363..671fd801b4 100644
--- a/crypto/cms/cms_lcl.h
+++ b/crypto/cms/cms_lcl.h
@@ -129,6 +129,8 @@ struct CMS_EncryptedContentInfo_st {
     size_t keylen;
     /* Set to 1 if we are debugging decrypt and don't fake keys for MMA */
     int debug;
+    /* Set to 1 if we have no cert and need extra safety measures for MMA */
+    int havenocert;
 };
 
 struct CMS_RecipientInfo_st {
diff --git a/crypto/cms/cms_smime.c b/crypto/cms/cms_smime.c
index 5dcf803f4b..3a26108b8c 100644
--- a/crypto/cms/cms_smime.c
+++ b/crypto/cms/cms_smime.c
@@ -743,6 +743,10 @@ int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pk, X509 *cert,
         cms->d.envelopedData->encryptedContentInfo->debug = 1;
     else
         cms->d.envelopedData->encryptedContentInfo->debug = 0;
+    if (!cert)
+        cms->d.envelopedData->encryptedContentInfo->havenocert = 1;
+    else
+        cms->d.envelopedData->encryptedContentInfo->havenocert = 0;
     if (!pk && !cert && !dcont && !out)
         return 1;
     if (pk && !CMS_decrypt_set1_pkey(cms, pk, cert))
diff --git a/crypto/pkcs7/pk7_doit.c b/crypto/pkcs7/pk7_doit.c
index e6e80f08d3..ee23a52db8 100644
--- a/crypto/pkcs7/pk7_doit.c
+++ b/crypto/pkcs7/pk7_doit.c
@@ -137,7 +137,8 @@ static int pkcs7_encode_rinfo(PKCS7_RECIP_INFO *ri,
 }
 
 static int pkcs7_decrypt_rinfo(unsigned char **pek, int *peklen,
-                               PKCS7_RECIP_INFO *ri, EVP_PKEY *pkey)
+                               PKCS7_RECIP_INFO *ri, EVP_PKEY *pkey,
+                               size_t fixlen)
 {
     EVP_PKEY_CTX *pctx = NULL;
     unsigned char *ek = NULL;
@@ -170,7 +171,9 @@ static int pkcs7_decrypt_rinfo(unsigned char **pek, int *peklen,
     }
 
     if (EVP_PKEY_decrypt(pctx, ek, &eklen,
-                         ri->enc_key->data, ri->enc_key->length) <= 0) {
+                         ri->enc_key->data, ri->enc_key->length) <= 0
+            || eklen == 0
+            || (fixlen != 0 && eklen != fixlen)) {
         ret = 0;
         PKCS7err(PKCS7_F_PKCS7_DECRYPT_RINFO, ERR_R_EVP_LIB);
         goto err;
@@ -499,13 +502,14 @@ BIO *PKCS7_dataDecode(PKCS7 *p7, EVP_PKEY *pkey, BIO *in_bio, X509 *pcert)
             for (i = 0; i < sk_PKCS7_RECIP_INFO_num(rsk); i++) {
                 ri = sk_PKCS7_RECIP_INFO_value(rsk, i);
 
-                if (pkcs7_decrypt_rinfo(&ek, &eklen, ri, pkey) < 0)
+                if (pkcs7_decrypt_rinfo(&ek, &eklen, ri, pkey,
+                        EVP_CIPHER_key_length(evp_cipher)) < 0)
                     goto err;
                 ERR_clear_error();
             }
         } else {
             /* Only exit on fatal errors, not decrypt failure */
-            if (pkcs7_decrypt_rinfo(&ek, &eklen, ri, pkey) < 0)
+            if (pkcs7_decrypt_rinfo(&ek, &eklen, ri, pkey, 0) < 0)
                 goto err;
             ERR_clear_error();
         }