diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 04c8eba..dfadcc5 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -134,7 +134,7 @@
 #error "MBEDTLS_ECDSA_DETERMINISTIC defined, but not all prerequisites"
 #endif
 
-#if defined(MBEDTLS_ECP_C) && ( !defined(MBEDTLS_BIGNUM_C) || (   \
+#if defined(MBEDTLS_ECP_C) && ( !defined(MBEDTLS_BIGNUM_C) || (    \
     !defined(MBEDTLS_ECP_DP_SECP192R1_ENABLED) &&                  \
     !defined(MBEDTLS_ECP_DP_SECP224R1_ENABLED) &&                  \
     !defined(MBEDTLS_ECP_DP_SECP256R1_ENABLED) &&                  \
@@ -145,7 +145,9 @@
     !defined(MBEDTLS_ECP_DP_BP512R1_ENABLED)   &&                  \
     !defined(MBEDTLS_ECP_DP_SECP192K1_ENABLED) &&                  \
     !defined(MBEDTLS_ECP_DP_SECP224K1_ENABLED) &&                  \
-    !defined(MBEDTLS_ECP_DP_SECP256K1_ENABLED) ) )
+    !defined(MBEDTLS_ECP_DP_SECP256K1_ENABLED) &&                  \
+    !defined(MBEDTLS_ECP_DP_CURVE25519_ENABLED) &&                 \
+    !defined(MBEDTLS_ECP_DP_CURVE448_ENABLED) ) )
 #error "MBEDTLS_ECP_C defined, but not all prerequisites"
 #endif
 
diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index 24951a6..f89faec 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -458,6 +458,10 @@
  *
  * \note            For RSA, md_alg may be MBEDTLS_MD_NONE if hash_len != 0.
  *                  For ECDSA, md_alg may never be MBEDTLS_MD_NONE.
+ *
+ * \note            In order to ensure enough space for the signature, the
+ *                  \p sig buffer size must be of at least
+ *                  `max(MBEDTLS_ECDSA_MAX_LEN, MBEDTLS_MPI_MAX_SIZE)` bytes.
  */
 int mbedtls_pk_sign( mbedtls_pk_context *ctx, mbedtls_md_type_t md_alg,
              const unsigned char *hash, size_t hash_len,
@@ -472,6 +476,10 @@
  *                  \c mbedtls_ecp_set_max_ops() to reduce blocking for ECC
  *                  operations. For RSA, same as \c mbedtls_pk_sign().
  *
+ * \note            In order to ensure enough space for the signature, the
+ *                  \p sig buffer size must be of at least
+ *                  `max(MBEDTLS_ECDSA_MAX_LEN, MBEDTLS_MPI_MAX_SIZE)` bytes.
+ *
  * \param ctx       The PK context to use. It must have been set up
  *                  with a private key.
  * \param md_alg    Hash algorithm used (see notes)
diff --git a/include/mbedtls/rsa.h b/include/mbedtls/rsa.h
index 489f2ed..610e804 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -907,7 +907,8 @@
  *                 the size of the hash corresponding to \p md_alg.
  * \param sig      The buffer to hold the signature. This must be a writable
  *                 buffer of length \c ctx->len Bytes. For example, \c 256 Bytes
- *                 for an 2048-bit RSA modulus.
+ *                 for an 2048-bit RSA modulus. A buffer length of
+ *                 #MBEDTLS_MPI_MAX_SIZE is always safe.
  *
  * \return         \c 0 if the signing operation was successful.
  * \return         An \c MBEDTLS_ERR_RSA_XXX error code on failure.
@@ -954,7 +955,8 @@
  *                 the size of the hash corresponding to \p md_alg.
  * \param sig      The buffer to hold the signature. This must be a writable
  *                 buffer of length \c ctx->len Bytes. For example, \c 256 Bytes
- *                 for an 2048-bit RSA modulus.
+ *                 for an 2048-bit RSA modulus. A buffer length of
+ *                 #MBEDTLS_MPI_MAX_SIZE is always safe.
  *
  * \return         \c 0 if the signing operation was successful.
  * \return         An \c MBEDTLS_ERR_RSA_XXX error code on failure.
@@ -1015,7 +1017,8 @@
  *                 the size of the hash corresponding to \p md_alg.
  * \param sig      The buffer to hold the signature. This must be a writable
  *                 buffer of length \c ctx->len Bytes. For example, \c 256 Bytes
- *                 for an 2048-bit RSA modulus.
+ *                 for an 2048-bit RSA modulus. A buffer length of
+ *                 #MBEDTLS_MPI_MAX_SIZE is always safe.
  *
  * \return         \c 0 if the signing operation was successful.
  * \return         An \c MBEDTLS_ERR_RSA_XXX error code on failure.
diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 3c23214..03fb3fd 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -45,6 +45,16 @@
 #include "mbedtls/pem.h"
 #endif /* MBEDTLS_PEM_WRITE_C */
 
+/*
+ * For the currently used signature algorithms the buffer to store any signature
+ * must be at least of size MAX(MBEDTLS_ECDSA_MAX_LEN, MBEDTLS_MPI_MAX_SIZE)
+ */
+#if MBEDTLS_ECDSA_MAX_LEN > MBEDTLS_MPI_MAX_SIZE
+#define SIGNATURE_MAX_SIZE MBEDTLS_ECDSA_MAX_LEN
+#else
+#define SIGNATURE_MAX_SIZE MBEDTLS_MPI_MAX_SIZE
+#endif
+
 void mbedtls_x509write_crt_init( mbedtls_x509write_cert *ctx )
 {
     memset( ctx, 0, sizeof( mbedtls_x509write_cert ) );
@@ -337,7 +347,7 @@
     size_t sig_oid_len = 0;
     unsigned char *c, *c2;
     unsigned char hash[64];
-    unsigned char sig[MBEDTLS_MPI_MAX_SIZE];
+    unsigned char sig[SIGNATURE_MAX_SIZE];
     size_t sub_len = 0, pub_len = 0, sig_and_oid_len = 0, sig_len;
     size_t len = 0;
     mbedtls_pk_type_t pk_alg;
diff --git a/library/x509write_csr.c b/library/x509write_csr.c
index 8dc39e7..0d62d1d 100644
--- a/library/x509write_csr.c
+++ b/library/x509write_csr.c
@@ -49,6 +49,16 @@
 #include "mbedtls/pem.h"
 #endif
 
+/*
+ * For the currently used signature algorithms the buffer to store any signature
+ * must be at least of size MAX(MBEDTLS_ECDSA_MAX_LEN, MBEDTLS_MPI_MAX_SIZE)
+ */
+#if MBEDTLS_ECDSA_MAX_LEN > MBEDTLS_MPI_MAX_SIZE
+#define SIGNATURE_MAX_SIZE MBEDTLS_ECDSA_MAX_LEN
+#else
+#define SIGNATURE_MAX_SIZE MBEDTLS_MPI_MAX_SIZE
+#endif
+
 void mbedtls_x509write_csr_init( mbedtls_x509write_csr *ctx )
 {
     memset( ctx, 0, sizeof( mbedtls_x509write_csr ) );
@@ -138,7 +148,7 @@
     size_t sig_oid_len = 0;
     unsigned char *c, *c2;
     unsigned char hash[64];
-    unsigned char sig[MBEDTLS_MPI_MAX_SIZE];
+    unsigned char sig[SIGNATURE_MAX_SIZE];
     unsigned char tmp_buf[2048];
     size_t pub_len = 0, sig_and_oid_len = 0, sig_len;
     size_t len = 0;
diff --git a/programs/pkey/pk_sign.c b/programs/pkey/pk_sign.c
index 47a098a..bdedca4 100644
--- a/programs/pkey/pk_sign.c
+++ b/programs/pkey/pk_sign.c
@@ -61,6 +61,16 @@
 #include <string.h>
 
 
+/*
+ * For the currently used signature algorithms the buffer to store any signature
+ * must be at least of size MAX(MBEDTLS_ECDSA_MAX_LEN, MBEDTLS_MPI_MAX_SIZE)
+ */
+#if MBEDTLS_ECDSA_MAX_LEN > MBEDTLS_MPI_MAX_SIZE
+#define SIGNATURE_MAX_SIZE MBEDTLS_ECDSA_MAX_LEN
+#else
+#define SIGNATURE_MAX_SIZE MBEDTLS_MPI_MAX_SIZE
+#endif
+
 int main( int argc, char *argv[] )
 {
     FILE *f;
@@ -70,7 +80,7 @@
     mbedtls_entropy_context entropy;
     mbedtls_ctr_drbg_context ctr_drbg;
     unsigned char hash[32];
-    unsigned char buf[MBEDTLS_MPI_MAX_SIZE];
+    unsigned char buf[SIGNATURE_MAX_SIZE];
     char filename[512];
     const char *pers = "mbedtls_pk_sign";
     size_t olen = 0;
diff --git a/tests/data_files/Makefile b/tests/data_files/Makefile
index 99d64eb..e75bf81 100644
--- a/tests/data_files/Makefile
+++ b/tests/data_files/Makefile
@@ -869,6 +869,14 @@
 	$(OPENSSL) pkey -in $< -inform DER -out $@
 all_final += ec_prv.pk8param.pem
 
+###
+### A generic SECP521R1 private key
+###
+
+secp521r1_prv.der:
+	$(OPENSSL) ecparam -genkey -name secp521r1 -noout -out secp521r1_prv.der
+all_final += secp521r1_prv.der
+
 ################################################################
 ### Generate CSRs for X.509 write test suite
 ################################################################
diff --git a/tests/data_files/secp521r1_prv.der b/tests/data_files/secp521r1_prv.der
new file mode 100644
index 0000000..4d342bd
--- /dev/null
+++ b/tests/data_files/secp521r1_prv.der
Binary files differ
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 22c8129..1b799f7 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -617,6 +617,23 @@
 #### Build and test many configurations and targets
 ################################################################
 
+component_test_large_ecdsa_key_signature () {
+
+    SMALL_MPI_MAX_SIZE=136 # Small enough to interfere with the EC signatures
+
+    msg "build: cmake + MBEDTLS_MPI_MAX_SIZE=${SMALL_MPI_MAX_SIZE}, gcc, ASan" # ~ 1 min 50s
+    scripts/config.pl set MBEDTLS_MPI_MAX_SIZE $SMALL_MPI_MAX_SIZE
+    CC=gcc cmake -D CMAKE_BUILD_TYPE:String=Asan .
+    make
+
+    INEVITABLY_PRESENT_FILE=Makefile
+    SIGNATURE_FILE="${INEVITABLY_PRESENT_FILE}.sig" # Warning, this is rm -f'ed below
+
+    msg "test: pk_sign secp521r1_prv.der for MBEDTLS_MPI_MAX_SIZE=${SMALL_MPI_MAX_SIZE} (ASan build)" # ~ 5s
+    if_build_succeeded programs/pkey/pk_sign tests/data_files/secp521r1_prv.der $INEVITABLY_PRESENT_FILE
+    rm -f $SIGNATURE_FILE
+}
+
 component_test_default_out_of_box () {
     msg "build: make, default config (out-of-box)" # ~1min
     make