diff --git a/crypto/CMakeLists.txt b/crypto/CMakeLists.txt
index 5f6e327..2684750 100644
--- a/crypto/CMakeLists.txt
+++ b/crypto/CMakeLists.txt
@@ -260,6 +260,7 @@
   hmac_extra/hmac_test.cc
   lhash/lhash_test.cc
   obj/obj_test.cc
+  pem/pem_test.cc
   pkcs7/pkcs7_test.cc
   pkcs8/pkcs8_test.cc
   pkcs8/pkcs12_test.cc
diff --git a/crypto/pem/pem_lib.c b/crypto/pem/pem_lib.c
index 8f89096..5180e55 100644
--- a/crypto/pem/pem_lib.c
+++ b/crypto/pem/pem_lib.c
@@ -188,6 +188,26 @@
     return 0;
 }
 
+static const EVP_CIPHER *cipher_by_name(const char *name)
+{
+    /* This is similar to the (deprecated) function |EVP_get_cipherbyname|. Note
+     * the PEM code assumes that ciphers have at least 8 bytes of IV, at most 20
+     * bytes of overhead and generally behave like CBC mode. */
+    if (0 == strcmp(name, SN_des_cbc)) {
+        return EVP_des_cbc();
+    } else if (0 == strcmp(name, SN_des_ede3_cbc)) {
+        return EVP_des_ede3_cbc();
+    } else if (0 == strcmp(name, SN_aes_128_cbc)) {
+        return EVP_aes_128_cbc();
+    } else if (0 == strcmp(name, SN_aes_192_cbc)) {
+        return EVP_aes_192_cbc();
+    } else if (0 == strcmp(name, SN_aes_256_cbc)) {
+        return EVP_aes_256_cbc();
+    } else {
+        return NULL;
+    }
+}
+
 int PEM_bytes_read_bio(unsigned char **pdata, long *plen, char **pnm,
                        const char *name, BIO *bp, pem_password_cb *cb,
                        void *u)
@@ -265,7 +285,9 @@
 
     if (enc != NULL) {
         objstr = OBJ_nid2sn(EVP_CIPHER_nid(enc));
-        if (objstr == NULL || EVP_CIPHER_iv_length(enc) == 0) {
+        if (objstr == NULL ||
+            cipher_by_name(objstr) == NULL ||
+            EVP_CIPHER_iv_length(enc) < 8) {
             OPENSSL_PUT_ERROR(PEM, PEM_R_UNSUPPORTED_CIPHER);
             goto err;
         }
@@ -393,26 +415,6 @@
     return (1);
 }
 
-static const EVP_CIPHER *cipher_by_name(const char *name)
-{
-    /* This is similar to the (deprecated) function |EVP_get_cipherbyname|. */
-    if (0 == strcmp(name, SN_rc4)) {
-        return EVP_rc4();
-    } else if (0 == strcmp(name, SN_des_cbc)) {
-        return EVP_des_cbc();
-    } else if (0 == strcmp(name, SN_des_ede3_cbc)) {
-        return EVP_des_ede3_cbc();
-    } else if (0 == strcmp(name, SN_aes_128_cbc)) {
-        return EVP_aes_128_cbc();
-    } else if (0 == strcmp(name, SN_aes_192_cbc)) {
-        return EVP_aes_192_cbc();
-    } else if (0 == strcmp(name, SN_aes_256_cbc)) {
-        return EVP_aes_256_cbc();
-    } else {
-        return NULL;
-    }
-}
-
 int PEM_get_EVP_CIPHER_INFO(char *header, EVP_CIPHER_INFO *cipher)
 {
     const EVP_CIPHER *enc = NULL;
@@ -420,6 +422,7 @@
     char **header_pp = &header;
 
     cipher->cipher = NULL;
+    OPENSSL_memset(cipher->iv, 0, sizeof(cipher->iv));
     if ((header == NULL) || (*header == '\0') || (*header == '\n'))
         return (1);
     if (strncmp(header, "Proc-Type: ", 11) != 0) {
@@ -466,6 +469,13 @@
         OPENSSL_PUT_ERROR(PEM, PEM_R_UNSUPPORTED_ENCRYPTION);
         return (0);
     }
+    // The IV parameter must be at least 8 bytes long to be used as the salt in
+    // the KDF. (This should not happen given |cipher_by_name|.)
+    if (EVP_CIPHER_iv_length(enc) < 8) {
+        assert(0);
+        OPENSSL_PUT_ERROR(PEM, PEM_R_UNSUPPORTED_ENCRYPTION);
+        return 0;
+    }
     if (!load_iv(header_pp, &(cipher->iv[0]), EVP_CIPHER_iv_length(enc)))
         return (0);
 
diff --git a/crypto/pem/pem_test.cc b/crypto/pem/pem_test.cc
new file mode 100644
index 0000000..aed523c
--- /dev/null
+++ b/crypto/pem/pem_test.cc
@@ -0,0 +1,45 @@
+/* Copyright (c) 2018, Google Inc.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
+
+#include <openssl/pem.h>
+
+#include <gtest/gtest.h>
+
+#include <openssl/bio.h>
+#include <openssl/err.h>
+#include <openssl/rsa.h>
+
+
+// Test that implausible ciphers, notably an IV-less RC4, aren't allowed in PEM.
+// This is a regression test for https://github.com/openssl/openssl/issues/6347,
+// though our fix differs from upstream.
+TEST(PEMTest, NoRC4) {
+  static const char kPEM[] =
+      "-----BEGIN RSA PUBLIC KEY-----\n"
+      "Proc-Type: 4,ENCRYPTED\n"
+      "DEK-Info: RC4 -\n"
+      "extra-info\n"
+      "router-signature\n"
+      "\n"
+      "Z1w=\n"
+      "-----END RSA PUBLIC KEY-----\n";
+  bssl::UniquePtr<BIO> bio(BIO_new_mem_buf(kPEM, sizeof(kPEM) - 1));
+  ASSERT_TRUE(bio);
+  bssl::UniquePtr<RSA> rsa(PEM_read_bio_RSAPublicKey(
+      bio.get(), nullptr, nullptr, const_cast<char *>("password")));
+  EXPECT_FALSE(rsa);
+  uint32_t err = ERR_get_error();
+  EXPECT_EQ(ERR_LIB_PEM, ERR_GET_LIB(err));
+  EXPECT_EQ(PEM_R_UNSUPPORTED_ENCRYPTION, ERR_GET_REASON(err));
+}