diff --git a/mongoose.c b/mongoose.c
index b9af66e0..2663634c 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -17600,7 +17600,6 @@ static int mg_tls_verify_cert_signature(const struct mg_tls_cert *cert,
       return 0;
     }
   } else {
-    uint8_t r;
     const uint8_t *n;
     size_t nlen;
     uint8_t sig2[512];  // 4096 bits
@@ -17620,15 +17619,7 @@ static int mg_tls_verify_cert_signature(const struct mg_tls_cert *cert,
       return 0;
     }
 
-    r = 0;
-    {
-      const uint8_t *p, *q;
-      size_t i;
-      p = sig2 + nlen - cert->tbshashsz;
-      q = cert->tbshash;
-      for (i = 0; i < cert->tbshashsz; i++) r |= (uint8_t)(*p++ ^ *q++);
-    }
-    return r == 0;
+    return mg_rsa_pkcs_verify(sig2, nlen, cert->tbshash, cert->tbshashsz);
   }
 }
 
@@ -21721,6 +21712,43 @@ bool mg_rsa_verify(const uint8_t *em, size_t nlen, const uint8_t *mhash) {
   return pss_verify_sha256(em, nlen, mhash);
 }
 
+// Full RSASSA-PKCS1-v1_5-VERIFY for rsa_pkcs1_sha256/sha384 (RFC 8017 9.2)
+// em      - output of RSA public-key primitive (nlen bytes)
+// nlen    - modulus byte length
+// hash    - SHA-256 or SHA-384 of the certificate TBS data
+// hashlen - hash length, 32 for SHA-256 or 48 for SHA-384
+static bool pkcs1_v15_verify(const uint8_t *em, size_t nlen,
+                             const uint8_t *hash, size_t hashlen) {
+  static const uint8_t sha256_di[] = {
+      0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01,
+      0x65, 0x03, 0x04, 0x02, 0x01, 0x05, 0x00, 0x04, 0x20};
+  static const uint8_t sha384_di[] = {
+      0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01,
+      0x65, 0x03, 0x04, 0x02, 0x02, 0x05, 0x00, 0x04, 0x30};
+  const uint8_t *di = hashlen == 32 ? sha256_di : sha384_di;
+  size_t disz = hashlen == 32 ? sizeof(sha256_di) : sizeof(sha384_di);
+  size_t pslen, i;
+  uint8_t bad = 0;
+
+  if (hashlen != 32 && hashlen != 48) return false;
+  if (nlen < 3 + 8 + disz + hashlen) return false;
+  pslen = nlen - 3 - disz - hashlen;
+
+  bad |= (uint8_t) (em[0] | (em[1] ^ 0x01));
+  for (i = 0; i < pslen; i++) bad |= (uint8_t) (em[2 + i] ^ 0xff);
+  bad |= em[2 + pslen];
+  for (i = 0; i < disz; i++) bad |= (uint8_t) (em[3 + pslen + i] ^ di[i]);
+  for (i = 0; i < hashlen; i++)
+    bad |= (uint8_t) (em[3 + pslen + disz + i] ^ hash[i]);
+
+  return bad == 0;
+}
+
+bool mg_rsa_pkcs_verify(const uint8_t *em, size_t nlen, const uint8_t *hash,
+                        size_t hashlen) {
+  return pkcs1_v15_verify(em, nlen, hash, hashlen);
+}
+
 #endif /* MG_TLS == MG_TLS_BUILTIN */
 
 #ifdef MG_ENABLE_LINES
diff --git a/mongoose.h b/mongoose.h
index 4c879fa3..077cdab9 100644
--- a/mongoose.h
+++ b/mongoose.h
@@ -3479,6 +3479,8 @@ int mg_rsa_crt_sign(const uint8_t *em, size_t em_len,
                     const uint8_t *q, size_t q_len,
                     const uint8_t *qInv, size_t qInv_len,
                     uint8_t *signature, size_t sig_len);
+bool mg_rsa_pkcs_verify(const uint8_t *em, size_t nlen, const uint8_t *hash,
+                        size_t hashlen);
 bool mg_rsa_verify(const uint8_t *em, size_t nlen, const uint8_t *mhash);
 #endif // TLS_RSA_H
 
diff --git a/src/tls_builtin.c b/src/tls_builtin.c
index 8083981f..c1c40989 100644
--- a/src/tls_builtin.c
+++ b/src/tls_builtin.c
@@ -1704,7 +1704,6 @@ static int mg_tls_verify_cert_signature(const struct mg_tls_cert *cert,
       return 0;
     }
   } else {
-    uint8_t r;
     const uint8_t *n;
     size_t nlen;
     uint8_t sig2[512];  // 4096 bits
@@ -1724,15 +1723,7 @@ static int mg_tls_verify_cert_signature(const struct mg_tls_cert *cert,
       return 0;
     }
 
-    r = 0;
-    {
-      const uint8_t *p, *q;
-      size_t i;
-      p = sig2 + nlen - cert->tbshashsz;
-      q = cert->tbshash;
-      for (i = 0; i < cert->tbshashsz; i++) r |= (uint8_t)(*p++ ^ *q++);
-    }
-    return r == 0;
+    return mg_rsa_pkcs_verify(sig2, nlen, cert->tbshash, cert->tbshashsz);
   }
 }
 
diff --git a/src/tls_rsa.c b/src/tls_rsa.c
index ff214c49..b5edc47b 100644
--- a/src/tls_rsa.c
+++ b/src/tls_rsa.c
@@ -950,4 +950,41 @@ bool mg_rsa_verify(const uint8_t *em, size_t nlen, const uint8_t *mhash) {
   return pss_verify_sha256(em, nlen, mhash);
 }
 
+// Full RSASSA-PKCS1-v1_5-VERIFY for rsa_pkcs1_sha256/sha384 (RFC 8017 9.2)
+// em      - output of RSA public-key primitive (nlen bytes)
+// nlen    - modulus byte length
+// hash    - SHA-256 or SHA-384 of the certificate TBS data
+// hashlen - hash length, 32 for SHA-256 or 48 for SHA-384
+static bool pkcs1_v15_verify(const uint8_t *em, size_t nlen,
+                             const uint8_t *hash, size_t hashlen) {
+  static const uint8_t sha256_di[] = {
+      0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01,
+      0x65, 0x03, 0x04, 0x02, 0x01, 0x05, 0x00, 0x04, 0x20};
+  static const uint8_t sha384_di[] = {
+      0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01,
+      0x65, 0x03, 0x04, 0x02, 0x02, 0x05, 0x00, 0x04, 0x30};
+  const uint8_t *di = hashlen == 32 ? sha256_di : sha384_di;
+  size_t disz = hashlen == 32 ? sizeof(sha256_di) : sizeof(sha384_di);
+  size_t pslen, i;
+  uint8_t bad = 0;
+
+  if (hashlen != 32 && hashlen != 48) return false;
+  if (nlen < 3 + 8 + disz + hashlen) return false;
+  pslen = nlen - 3 - disz - hashlen;
+
+  bad |= (uint8_t) (em[0] | (em[1] ^ 0x01));
+  for (i = 0; i < pslen; i++) bad |= (uint8_t) (em[2 + i] ^ 0xff);
+  bad |= em[2 + pslen];
+  for (i = 0; i < disz; i++) bad |= (uint8_t) (em[3 + pslen + i] ^ di[i]);
+  for (i = 0; i < hashlen; i++)
+    bad |= (uint8_t) (em[3 + pslen + disz + i] ^ hash[i]);
+
+  return bad == 0;
+}
+
+bool mg_rsa_pkcs_verify(const uint8_t *em, size_t nlen, const uint8_t *hash,
+                        size_t hashlen) {
+  return pkcs1_v15_verify(em, nlen, hash, hashlen);
+}
+
 #endif /* MG_TLS == MG_TLS_BUILTIN */
diff --git a/src/tls_rsa.h b/src/tls_rsa.h
index 5beb0b4c..6b9ada27 100644
--- a/src/tls_rsa.h
+++ b/src/tls_rsa.h
@@ -10,5 +10,7 @@ int mg_rsa_crt_sign(const uint8_t *em, size_t em_len,
                     const uint8_t *q, size_t q_len,
                     const uint8_t *qInv, size_t qInv_len,
                     uint8_t *signature, size_t sig_len);
+bool mg_rsa_pkcs_verify(const uint8_t *em, size_t nlen, const uint8_t *hash,
+                        size_t hashlen);
 bool mg_rsa_verify(const uint8_t *em, size_t nlen, const uint8_t *mhash);
 #endif // TLS_RSA_H
diff --git a/test/unit_test.c b/test/unit_test.c
index c5f3d9b5..ef697646 100644
--- a/test/unit_test.c
+++ b/test/unit_test.c
@@ -4311,6 +4311,51 @@ static void test_rsa(void) {
     ASSERT(!mg_rsa_verify(em, 256, tv_mhash));
     em[50] = tv_em[50];
   }
+  {
+    static const uint8_t sha256_di[] = {
+        0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01,
+        0x65, 0x03, 0x04, 0x02, 0x01, 0x05, 0x00, 0x04, 0x20};
+    static const uint8_t sha384_di[] = {
+        0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01,
+        0x65, 0x03, 0x04, 0x02, 0x02, 0x05, 0x00, 0x04, 0x30};
+    uint8_t hash[48], em[256];
+    size_t i, pslen;
+
+    for (i = 0; i < sizeof(hash); i++) hash[i] = (uint8_t) i;
+
+    memset(em, 0xff, sizeof(em));
+    em[0] = 0, em[1] = 1;
+    pslen = sizeof(em) - 3 - sizeof(sha256_di) - 32;
+    em[2 + pslen] = 0;
+    memcpy(em + 3 + pslen, sha256_di, sizeof(sha256_di));
+    memcpy(em + 3 + pslen + sizeof(sha256_di), hash, 32);
+    ASSERT(mg_rsa_pkcs_verify(em, sizeof(em), hash, 32));
+
+    em[1] = 2;  // bad block type
+    ASSERT(!mg_rsa_pkcs_verify(em, sizeof(em), hash, 32));
+    em[1] = 1;
+    em[10] = 0;  // bad padding
+    ASSERT(!mg_rsa_pkcs_verify(em, sizeof(em), hash, 32));
+    em[10] = 0xff;
+    em[3 + pslen + 15] ^= 1;  // bad DigestInfo
+    ASSERT(!mg_rsa_pkcs_verify(em, sizeof(em), hash, 32));
+    em[3 + pslen + 15] ^= 1;
+    em[sizeof(em) - 1] ^= 1;  // bad hash
+    ASSERT(!mg_rsa_pkcs_verify(em, sizeof(em), hash, 32));
+
+    memset(em, 0, sizeof(em));  // former suffix-only bypass
+    memcpy(em + sizeof(em) - 32, hash, 32);
+    ASSERT(!mg_rsa_pkcs_verify(em, sizeof(em), hash, 32));
+
+    memset(em, 0xff, sizeof(em));
+    em[0] = 0, em[1] = 1;
+    pslen = sizeof(em) - 3 - sizeof(sha384_di) - 48;
+    em[2 + pslen] = 0;
+    memcpy(em + 3 + pslen, sha384_di, sizeof(sha384_di));
+    memcpy(em + 3 + pslen + sizeof(sha384_di), hash, 48);
+    ASSERT(mg_rsa_pkcs_verify(em, sizeof(em), hash, 48));
+    ASSERT(!mg_rsa_pkcs_verify(em, sizeof(em), hash, 20));
+  }
 #endif
 }