diff --git a/CHANGES b/CHANGES
index cabd1eac71..27cf14612f 100644
--- a/CHANGES
+++ b/CHANGES
@@ -14,7 +14,11 @@
 
 5481.	[placeholder]
 
-5480.	[placeholder]
+5480.	[security]	When BIND 9 was compiled with native PKCS#11 support, it
+			was possible to trigger an assertion failure in code
+			determining the number of bits in the PKCS#11 RSA public
+			key with a specially crafted packet. (CVE-2020-8623)
+			[GL #2037]
 
 5479.	[security]	named could crash in certain query resolution scenarios
 			where QNAME minimization and forwarding were both
diff --git a/bin/tests/system/run.sh.in b/bin/tests/system/run.sh.in
index c59a3e2cfd..06c8d72011 100644
--- a/bin/tests/system/run.sh.in
+++ b/bin/tests/system/run.sh.in
@@ -70,7 +70,7 @@ if ! $do_run; then
     if [ "$baseport" -eq 0 ]; then
 	log_flags="$log_flags -p 5300"
     fi
-    env - PATH="$PATH" ${LD_LIBRARY_PATH:+"LD_LIBRARY_PATH=${LD_LIBRARY_PATH}"} TESTS="$*" TEST_SUITE_LOG=run.log LOG_DRIVER_FLAGS="--verbose yes --color-tests yes" LOG_FLAGS="$log_flags" make -e check
+    env - SLOT="$SLOT" SOFTHSM2_CONF="$SOFTHSM2_CONF" PATH="$PATH" ${LD_LIBRARY_PATH:+"LD_LIBRARY_PATH=${LD_LIBRARY_PATH}"} TESTS="$*" TEST_SUITE_LOG=run.log LOG_DRIVER_FLAGS="--verbose yes --color-tests yes" LOG_FLAGS="$log_flags" make -e check
     exit $?
 fi
 
diff --git a/doc/notes/notes-current.rst b/doc/notes/notes-current.rst
index f5fdc44bee..175a15b362 100644
--- a/doc/notes/notes-current.rst
+++ b/doc/notes/notes-current.rst
@@ -36,6 +36,14 @@ Security Fixes
   ISC would like to thank Dave Feldman, Jeff Warren, and Joel Cunningham
   of Oracle for bringing this vulnerability to our attention. [GL #2028]
 
+- When BIND 9 was compiled with native PKCS#11 support, it was possible
+  to trigger an assertion failure in code determining the number of bits
+  in the PKCS#11 RSA public key with a specially crafted packet. This
+  was disclosed in CVE-2020-8623.
+
+  ISC would like to thank Lyu Chiy for bringing this vulnerability to
+  our attention. [GL #2037]
+
 Known Issues
 ~~~~~~~~~~~~
 
diff --git a/lib/dns/pkcs11rsa_link.c b/lib/dns/pkcs11rsa_link.c
index fc5efe3bb8..c31ae33da9 100644
--- a/lib/dns/pkcs11rsa_link.c
+++ b/lib/dns/pkcs11rsa_link.c
@@ -292,6 +292,7 @@ pkcs11rsa_createctx_verify(dst_key_t *key, unsigned int maxbits,
 		key->key_alg == DST_ALG_NSEC3RSASHA1 ||
 		key->key_alg == DST_ALG_RSASHA256 ||
 		key->key_alg == DST_ALG_RSASHA512);
+	REQUIRE(maxbits <= RSA_MAX_PUBEXP_BITS);
 
 	/*
 	 * Reject incorrect RSA key lengths.
@@ -334,6 +335,7 @@ pkcs11rsa_createctx_verify(dst_key_t *key, unsigned int maxbits,
 
 	for (attr = pk11_attribute_first(rsa); attr != NULL;
 	     attr = pk11_attribute_next(rsa, attr))
+	{
 		switch (attr->type) {
 		case CKA_MODULUS:
 			INSIST(keyTemplate[5].type == attr->type);
@@ -350,13 +352,16 @@ pkcs11rsa_createctx_verify(dst_key_t *key, unsigned int maxbits,
 			memmove(keyTemplate[6].pValue, attr->pValue,
 				attr->ulValueLen);
 			keyTemplate[6].ulValueLen = attr->ulValueLen;
-			if (pk11_numbits(attr->pValue, attr->ulValueLen) >
-				    maxbits &&
-			    maxbits != 0) {
+			unsigned int bits;
+			ret = pk11_numbits(attr->pValue, attr->ulValueLen,
+					   &bits);
+			if (ret != ISC_R_SUCCESS ||
+			    (bits > maxbits && maxbits != 0)) {
 				DST_RET(DST_R_VERIFYFAILURE);
 			}
 			break;
 		}
+	}
 	pk11_ctx->object = CK_INVALID_HANDLE;
 	pk11_ctx->ontoken = false;
 	PK11_RET(pkcs_C_CreateObject,
@@ -957,14 +962,17 @@ pkcs11rsa_verify(dst_context_t *dctx, const isc_region_t *sig) {
 			keyTemplate[5].ulValueLen = attr->ulValueLen;
 			break;
 		case CKA_PUBLIC_EXPONENT:
+			unsigned int bits;
 			INSIST(keyTemplate[6].type == attr->type);
 			keyTemplate[6].pValue = isc_mem_get(dctx->mctx,
 							    attr->ulValueLen);
 			memmove(keyTemplate[6].pValue, attr->pValue,
 				attr->ulValueLen);
 			keyTemplate[6].ulValueLen = attr->ulValueLen;
-			if (pk11_numbits(attr->pValue, attr->ulValueLen) >
-			    RSA_MAX_PUBEXP_BITS) {
+			ret = pk11_numbits(attr->pValue, attr->ulValueLen,
+					   &bits);
+			if (ret != ISC_R_SUCCESS || bits > RSA_MAX_PUBEXP_BITS)
+			{
 				DST_RET(DST_R_VERIFYFAILURE);
 			}
 			break;
@@ -1335,6 +1343,8 @@ pkcs11rsa_fromdns(dst_key_t *key, isc_buffer_t *data) {
 	CK_BYTE *exponent = NULL, *modulus = NULL;
 	CK_ATTRIBUTE *attr;
 	unsigned int length;
+	unsigned int bits;
+	isc_result_t ret = ISC_R_SUCCESS;
 
 	isc_buffer_remainingregion(data, &r);
 	if (r.length == 0) {
@@ -1351,9 +1361,7 @@ pkcs11rsa_fromdns(dst_key_t *key, isc_buffer_t *data) {
 
 	if (e_bytes == 0) {
 		if (r.length < 2) {
-			isc_safe_memwipe(rsa, sizeof(*rsa));
-			isc_mem_put(key->mctx, rsa, sizeof(*rsa));
-			return (DST_R_INVALIDPUBLICKEY);
+			DST_RET(DST_R_INVALIDPUBLICKEY);
 		}
 		e_bytes = (*r.base) << 8;
 		isc_region_consume(&r, 1);
@@ -1362,16 +1370,18 @@ pkcs11rsa_fromdns(dst_key_t *key, isc_buffer_t *data) {
 	}
 
 	if (r.length < e_bytes) {
-		isc_safe_memwipe(rsa, sizeof(*rsa));
-		isc_mem_put(key->mctx, rsa, sizeof(*rsa));
-		return (DST_R_INVALIDPUBLICKEY);
+		DST_RET(DST_R_INVALIDPUBLICKEY);
 	}
 	exponent = r.base;
 	isc_region_consume(&r, e_bytes);
 	modulus = r.base;
 	mod_bytes = r.length;
 
-	key->key_size = pk11_numbits(modulus, mod_bytes);
+	ret = pk11_numbits(modulus, mod_bytes, &bits);
+	if (ret != ISC_R_SUCCESS) {
+		goto err;
+	}
+	key->key_size = bits;
 
 	isc_buffer_forward(data, length);
 
@@ -1391,6 +1401,10 @@ pkcs11rsa_fromdns(dst_key_t *key, isc_buffer_t *data) {
 	key->keydata.pkey = rsa;
 
 	return (ISC_R_SUCCESS);
+err:
+	isc_safe_memwipe(rsa, sizeof(*rsa));
+	isc_mem_put(key->mctx, rsa, sizeof(*rsa));
+	return (ret);
 }
 
 static isc_result_t
@@ -1564,6 +1578,7 @@ pkcs11rsa_fetch(dst_key_t *key, const char *engine, const char *label,
 	pk11_object_t *pubrsa;
 	pk11_context_t *pk11_ctx = NULL;
 	isc_result_t ret;
+	unsigned int bits;
 
 	if (label == NULL) {
 		return (DST_R_NOENGINE);
@@ -1642,7 +1657,11 @@ pkcs11rsa_fetch(dst_key_t *key, const char *engine, const char *label,
 
 	attr = pk11_attribute_bytype(rsa, CKA_MODULUS);
 	INSIST(attr != NULL);
-	key->key_size = pk11_numbits(attr->pValue, attr->ulValueLen);
+	ret = pk11_numbits(attr->pValue, attr->ulValueLen, &bits);
+	if (ret != ISC_R_SUCCESS) {
+		goto err;
+	}
+	key->key_size = bits;
 
 	return (ISC_R_SUCCESS);
 
@@ -1734,6 +1753,7 @@ pkcs11rsa_parse(dst_key_t *key, isc_lex_t *lexer, dst_key_t *pub) {
 	CK_ATTRIBUTE *attr;
 	isc_mem_t *mctx = key->mctx;
 	const char *engine = NULL, *label = NULL;
+	unsigned int bits;
 
 	/* read private key file */
 	ret = dst__privstruct_parse(key, DST_ALG_RSA, lexer, mctx, &priv);
@@ -1871,12 +1891,20 @@ pkcs11rsa_parse(dst_key_t *key, isc_lex_t *lexer, dst_key_t *pub) {
 
 	attr = pk11_attribute_bytype(rsa, CKA_MODULUS);
 	INSIST(attr != NULL);
-	key->key_size = pk11_numbits(attr->pValue, attr->ulValueLen);
+	ret = pk11_numbits(attr->pValue, attr->ulValueLen, &bits);
+	if (ret != ISC_R_SUCCESS) {
+		goto err;
+	}
+	key->key_size = bits;
 
 	attr = pk11_attribute_bytype(rsa, CKA_PUBLIC_EXPONENT);
 	INSIST(attr != NULL);
-	if (pk11_numbits(attr->pValue, attr->ulValueLen) > RSA_MAX_PUBEXP_BITS)
-	{
+
+	ret = pk11_numbits(attr->pValue, attr->ulValueLen, &bits);
+	if (ret != ISC_R_SUCCESS) {
+		goto err;
+	}
+	if (bits > RSA_MAX_PUBEXP_BITS) {
 		DST_RET(ISC_R_RANGE);
 	}
 
@@ -1911,6 +1939,7 @@ pkcs11rsa_fromlabel(dst_key_t *key, const char *engine, const char *label,
 	pk11_context_t *pk11_ctx = NULL;
 	isc_result_t ret;
 	unsigned int i;
+	unsigned int bits;
 
 	UNUSED(pin);
 
@@ -1996,14 +2025,22 @@ pkcs11rsa_fromlabel(dst_key_t *key, const char *engine, const char *label,
 
 	attr = pk11_attribute_bytype(rsa, CKA_PUBLIC_EXPONENT);
 	INSIST(attr != NULL);
-	if (pk11_numbits(attr->pValue, attr->ulValueLen) > RSA_MAX_PUBEXP_BITS)
-	{
+
+	ret = pk11_numbits(attr->pValue, attr->ulValueLen, &bits);
+	if (ret != ISC_R_SUCCESS) {
+		goto err;
+	}
+	if (bits > RSA_MAX_PUBEXP_BITS) {
 		DST_RET(ISC_R_RANGE);
 	}
 
 	attr = pk11_attribute_bytype(rsa, CKA_MODULUS);
 	INSIST(attr != NULL);
-	key->key_size = pk11_numbits(attr->pValue, attr->ulValueLen);
+	ret = pk11_numbits(attr->pValue, attr->ulValueLen, &bits);
+	if (ret != ISC_R_SUCCESS) {
+		goto err;
+	}
+	key->key_size = bits;
 
 	pk11_return_session(pk11_ctx);
 	isc_safe_memwipe(pk11_ctx, sizeof(*pk11_ctx));
diff --git a/lib/isc/include/pk11/internal.h b/lib/isc/include/pk11/internal.h
index dfdf526eeb..05be8997d3 100644
--- a/lib/isc/include/pk11/internal.h
+++ b/lib/isc/include/pk11/internal.h
@@ -30,8 +30,8 @@ pk11_mem_put(void *ptr, size_t size);
 CK_SLOT_ID
 pk11_get_best_token(pk11_optype_t optype);
 
-unsigned int
-pk11_numbits(CK_BYTE_PTR data, unsigned int bytecnt);
+isc_result_t
+pk11_numbits(CK_BYTE_PTR data, unsigned int bytecnt, unsigned int *bits);
 
 CK_ATTRIBUTE *
 pk11_attribute_first(const pk11_object_t *obj);
diff --git a/lib/isc/pk11.c b/lib/isc/pk11.c
index bc910d7fdf..315f1170e9 100644
--- a/lib/isc/pk11.c
+++ b/lib/isc/pk11.c
@@ -652,13 +652,14 @@ pk11_get_best_token(pk11_optype_t optype) {
 	return (token->slotid);
 }
 
-unsigned int
-pk11_numbits(CK_BYTE_PTR data, unsigned int bytecnt) {
+isc_result_t
+pk11_numbits(CK_BYTE_PTR data, unsigned int bytecnt, unsigned int *bits) {
 	unsigned int bitcnt, i;
 	CK_BYTE top;
 
 	if (bytecnt == 0) {
-		return (0);
+		*bits = 0;
+		return (ISC_R_SUCCESS);
 	}
 	bitcnt = bytecnt * 8;
 	for (i = 0; i < bytecnt; i++) {
@@ -668,33 +669,40 @@ pk11_numbits(CK_BYTE_PTR data, unsigned int bytecnt) {
 			continue;
 		}
 		if (top & 0x80) {
-			return (bitcnt);
+			*bits = bitcnt;
+			return (ISC_R_SUCCESS);
 		}
 		if (top & 0x40) {
-			return (bitcnt - 1);
+			*bits = bitcnt - 1;
+			return (ISC_R_SUCCESS);
 		}
 		if (top & 0x20) {
-			return (bitcnt - 2);
+			*bits = bitcnt - 2;
+			return (ISC_R_SUCCESS);
 		}
 		if (top & 0x10) {
-			return (bitcnt - 3);
+			*bits = bitcnt - 3;
+			return (ISC_R_SUCCESS);
 		}
 		if (top & 0x08) {
-			return (bitcnt - 4);
+			*bits = bitcnt - 4;
+			return (ISC_R_SUCCESS);
 		}
 		if (top & 0x04) {
-			return (bitcnt - 5);
+			*bits = bitcnt - 5;
+			return (ISC_R_SUCCESS);
 		}
 		if (top & 0x02) {
-			return (bitcnt - 6);
+			*bits = bitcnt - 6;
+			return (ISC_R_SUCCESS);
 		}
 		if (top & 0x01) {
-			return (bitcnt - 7);
+			*bits = bitcnt - 7;
+			return (ISC_R_SUCCESS);
 		}
 		break;
 	}
-	INSIST(0);
-	ISC_UNREACHABLE();
+	return (ISC_R_RANGE);
 }
 
 CK_ATTRIBUTE *