diff --git a/ChangeLog b/ChangeLog
index f59fed05b..bb6222d6e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -32,6 +32,12 @@ Bugfix
      #878, #1019.
    * Fix variable used before assignment compilation warnings with IAR
      toolchain. Found by gkerrien38.
+   * Fix unchecked return codes from AES, DES and 3DES functions in
+     pem_aes_decrypt(), pem_des_decrypt() and pem_des3_decrypt() respectively.
+     If a call to one of the functions of the cryptographic primitive modules
+     failed, the error may not be noticed by the function
+     mbedtls_pem_read_buffer() causing it to return invalid values. Found by
+     Guido Vranken. #756
 
 = mbed TLS 2.6.0 branch released 2017-08-10
 
diff --git a/library/pem.c b/library/pem.c
index 8dd86a4ac..87401ba55 100644
--- a/library/pem.c
+++ b/library/pem.c
@@ -134,45 +134,55 @@ static void pem_pbkdf1( unsigned char *key, size_t keylen,
 /*
  * Decrypt with DES-CBC, using PBKDF1 for key derivation
  */
-static void pem_des_decrypt( unsigned char des_iv[8],
-                               unsigned char *buf, size_t buflen,
-                               const unsigned char *pwd, size_t pwdlen )
+static int pem_des_decrypt( unsigned char des_iv[8],
+                            unsigned char *buf, size_t buflen,
+                            const unsigned char *pwd, size_t pwdlen )
 {
     mbedtls_des_context des_ctx;
     unsigned char des_key[8];
+    int ret;
 
     mbedtls_des_init( &des_ctx );
 
     pem_pbkdf1( des_key, 8, des_iv, pwd, pwdlen );
 
-    mbedtls_des_setkey_dec( &des_ctx, des_key );
-    mbedtls_des_crypt_cbc( &des_ctx, MBEDTLS_DES_DECRYPT, buflen,
+    if( ( ret = mbedtls_des_setkey_dec( &des_ctx, des_key ) ) != 0 )
+        goto exit;
+    ret = mbedtls_des_crypt_cbc( &des_ctx, MBEDTLS_DES_DECRYPT, buflen,
                      des_iv, buf, buf );
 
+exit:
     mbedtls_des_free( &des_ctx );
     mbedtls_zeroize( des_key, 8 );
+
+    return( ret );
 }
 
 /*
  * Decrypt with 3DES-CBC, using PBKDF1 for key derivation
  */
-static void pem_des3_decrypt( unsigned char des3_iv[8],
-                               unsigned char *buf, size_t buflen,
-                               const unsigned char *pwd, size_t pwdlen )
+static int pem_des3_decrypt( unsigned char des3_iv[8],
+                             unsigned char *buf, size_t buflen,
+                             const unsigned char *pwd, size_t pwdlen )
 {
     mbedtls_des3_context des3_ctx;
     unsigned char des3_key[24];
+    int ret;
 
     mbedtls_des3_init( &des3_ctx );
 
     pem_pbkdf1( des3_key, 24, des3_iv, pwd, pwdlen );
 
-    mbedtls_des3_set3key_dec( &des3_ctx, des3_key );
-    mbedtls_des3_crypt_cbc( &des3_ctx, MBEDTLS_DES_DECRYPT, buflen,
+    if( ( ret = mbedtls_des3_set3key_dec( &des3_ctx, des3_key ) ) != 0 )
+        goto exit;
+    ret = mbedtls_des3_crypt_cbc( &des3_ctx, MBEDTLS_DES_DECRYPT, buflen,
                      des3_iv, buf, buf );
 
+exit:
     mbedtls_des3_free( &des3_ctx );
     mbedtls_zeroize( des3_key, 24 );
+
+    return( ret );
 }
 #endif /* MBEDTLS_DES_C */
 
@@ -180,23 +190,28 @@ static void pem_des3_decrypt( unsigned char des3_iv[8],
 /*
  * Decrypt with AES-XXX-CBC, using PBKDF1 for key derivation
  */
-static void pem_aes_decrypt( unsigned char aes_iv[16], unsigned int keylen,
-                               unsigned char *buf, size_t buflen,
-                               const unsigned char *pwd, size_t pwdlen )
+static int pem_aes_decrypt( unsigned char aes_iv[16], unsigned int keylen,
+                            unsigned char *buf, size_t buflen,
+                            const unsigned char *pwd, size_t pwdlen )
 {
     mbedtls_aes_context aes_ctx;
     unsigned char aes_key[32];
+    int ret;
 
     mbedtls_aes_init( &aes_ctx );
 
     pem_pbkdf1( aes_key, keylen, aes_iv, pwd, pwdlen );
 
-    mbedtls_aes_setkey_dec( &aes_ctx, aes_key, keylen * 8 );
-    mbedtls_aes_crypt_cbc( &aes_ctx, MBEDTLS_AES_DECRYPT, buflen,
+    if( ( ret = mbedtls_aes_setkey_dec( &aes_ctx, aes_key, keylen * 8 ) ) != 0 )
+        goto exit;
+    ret = mbedtls_aes_crypt_cbc( &aes_ctx, MBEDTLS_AES_DECRYPT, buflen,
                      aes_iv, buf, buf );
 
+exit:
     mbedtls_aes_free( &aes_ctx );
     mbedtls_zeroize( aes_key, keylen );
+
+    return( ret );
 }
 #endif /* MBEDTLS_AES_C */
 
@@ -345,22 +360,30 @@ int mbedtls_pem_read_buffer( mbedtls_pem_context *ctx, const char *header, const
             return( MBEDTLS_ERR_PEM_PASSWORD_REQUIRED );
         }
 
+        ret = 0;
+
 #if defined(MBEDTLS_DES_C)
         if( enc_alg == MBEDTLS_CIPHER_DES_EDE3_CBC )
-            pem_des3_decrypt( pem_iv, buf, len, pwd, pwdlen );
+            ret = pem_des3_decrypt( pem_iv, buf, len, pwd, pwdlen );
         else if( enc_alg == MBEDTLS_CIPHER_DES_CBC )
-            pem_des_decrypt( pem_iv, buf, len, pwd, pwdlen );
+            ret = pem_des_decrypt( pem_iv, buf, len, pwd, pwdlen );
 #endif /* MBEDTLS_DES_C */
 
 #if defined(MBEDTLS_AES_C)
         if( enc_alg == MBEDTLS_CIPHER_AES_128_CBC )
-            pem_aes_decrypt( pem_iv, 16, buf, len, pwd, pwdlen );
+            ret = pem_aes_decrypt( pem_iv, 16, buf, len, pwd, pwdlen );
         else if( enc_alg == MBEDTLS_CIPHER_AES_192_CBC )
-            pem_aes_decrypt( pem_iv, 24, buf, len, pwd, pwdlen );
+            ret = pem_aes_decrypt( pem_iv, 24, buf, len, pwd, pwdlen );
         else if( enc_alg == MBEDTLS_CIPHER_AES_256_CBC )
-            pem_aes_decrypt( pem_iv, 32, buf, len, pwd, pwdlen );
+            ret = pem_aes_decrypt( pem_iv, 32, buf, len, pwd, pwdlen );
 #endif /* MBEDTLS_AES_C */
 
+        if( ret != 0 )
+        {
+            mbedtls_free( buf );
+            return( ret );
+        }
+
         /*
          * The result will be ASN.1 starting with a SEQUENCE tag, with 1 to 3
          * length bytes (allow 4 to be sure) in all known use cases.
diff --git a/tests/suites/test_suite_pem.data b/tests/suites/test_suite_pem.data
index 065e4a2b5..77546c586 100644
--- a/tests/suites/test_suite_pem.data
+++ b/tests/suites/test_suite_pem.data
@@ -17,11 +17,22 @@ PEM write (exactly two lines + 1)
 mbedtls_pem_write_buffer:"-----START TEST-----\n":"-----END TEST-----\n":"000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F00":"-----START TEST-----\nAAECAwQFBgcICQoLDA0ODwABAgMEBQYHCAkKCwwNDg8AAQIDBAUGBwgJCgsMDQ4P\nAAECAwQFBgcICQoLDA0ODwABAgMEBQYHCAkKCwwNDg8AAQIDBAUGBwgJCgsMDQ4P\nAA==\n-----END TEST-----\n"
 
 PEM read (DES-EDE3-CBC + invalid iv)
-mbedtls_pem_read_buffer:"^":"$":"^\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: DES-EDE3-CBC,00$":MBEDTLS_ERR_PEM_INVALID_ENC_IV
+mbedtls_pem_read_buffer:"^":"$":"^\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: DES-EDE3-CBC,00$":"pwd":MBEDTLS_ERR_PEM_INVALID_ENC_IV
 
 PEM read (DES-CBC + invalid iv)
-mbedtls_pem_read_buffer:"^":"$":"^\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: DES-CBC,00$":MBEDTLS_ERR_PEM_INVALID_ENC_IV
+mbedtls_pem_read_buffer:"^":"$":"^\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: DES-CBC,00$":"pwd":MBEDTLS_ERR_PEM_INVALID_ENC_IV
 
 PEM read (unknown encryption algorithm)
-mbedtls_pem_read_buffer:"^":"$":"^\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: AES-,00$":MBEDTLS_ERR_PEM_UNKNOWN_ENC_ALG
+mbedtls_pem_read_buffer:"^":"$":"^\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: AES-,00$":"pwd":MBEDTLS_ERR_PEM_UNKNOWN_ENC_ALG
 
+PEM read (malformed PEM DES-CBC)
+depends_on:MBEDTLS_DES_C:MBEDTLS_CIPHER_MODE_CBC
+mbedtls_pem_read_buffer:"-----BEGIN EC PRIVATE KEY-----":"-----END EC PRIVATE KEY-----":"-----BEGIN EC PRIVATE KEY-----\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: DES-CBC,AA94892A169FA426\n\nMAAA\n-----END EC PRIVATE KEY-----":"pwd":MBEDTLS_ERR_DES_INVALID_INPUT_LENGTH
+
+PEM read (malformed PEM DES-EDE3-CBC)
+depends_on:MBEDTLS_DES_C:MBEDTLS_CIPHER_MODE_CBC
+mbedtls_pem_read_buffer:"-----BEGIN EC PRIVATE KEY-----":"-----END EC PRIVATE KEY-----":"-----BEGIN EC PRIVATE KEY-----\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: DES-EDE3-CBC,AA94892A169FA426\n\nMAAA\n-----END EC PRIVATE KEY-----":"pwd":MBEDTLS_ERR_DES_INVALID_INPUT_LENGTH
+
+PEM read (malformed PEM AES-128-CBC)
+depends_on:MBEDTLS_AES_C:MBEDTLS_CIPHER_MODE_CBC
+mbedtls_pem_read_buffer:"-----BEGIN EC PRIVATE KEY-----":"-----END EC PRIVATE KEY-----":"-----BEGIN EC PRIVATE KEY-----\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: AES-128-CBC,AA94892A169FA426AA94892A169FA426\n\nMAAA\n-----END EC PRIVATE KEY-----":"pwd":MBEDTLS_ERR_AES_INVALID_INPUT_LENGTH
diff --git a/tests/suites/test_suite_pem.function b/tests/suites/test_suite_pem.function
index 5e022109c..c24595d47 100644
--- a/tests/suites/test_suite_pem.function
+++ b/tests/suites/test_suite_pem.function
@@ -1,6 +1,8 @@
 /* BEGIN_HEADER */
 #include "mbedtls/base64.h"
 #include "mbedtls/pem.h"
+#include "mbedtls/des.h"
+#include "mbedtls/aes.h"
 /* END_HEADER */
 
 /* BEGIN_CASE depends_on:MBEDTLS_PEM_WRITE_C */
@@ -35,16 +37,19 @@ exit:
 /* END_CASE */
 
 /* BEGIN_CASE depends_on:MBEDTLS_PEM_PARSE_C:MBEDTLS_AES_C:MBEDTLS_DES_C:MBEDTLS_MD5_C:MBEDTLS_CIPHER_MODE_CBC */
-void mbedtls_pem_read_buffer( char *header, char *footer, char *data, int ret )
+void mbedtls_pem_read_buffer( char *header, char *footer, char *data,
+                              char *pwd, int res )
 {
     mbedtls_pem_context ctx;
+    int ret;
     size_t use_len = 0;
+    size_t pwd_len = strlen( pwd );
 
     mbedtls_pem_init( &ctx );
 
-    TEST_ASSERT( mbedtls_pem_read_buffer( &ctx, header, footer,
-                                          (const unsigned char *)data, NULL, 0,
-                                          &use_len ) == ret );
+    ret = mbedtls_pem_read_buffer( &ctx, header, footer, (unsigned char *)data,
+                (unsigned char *)pwd, pwd_len, &use_len );
+    TEST_ASSERT( ret == res );
 
 exit:
     mbedtls_pem_free( &ctx );