diff --git a/ssl/s3_clnt.c b/ssl/s3_clnt.c
index 516ca4a..eb0bbd0 100644
--- a/ssl/s3_clnt.c
+++ b/ssl/s3_clnt.c
@@ -1188,7 +1188,7 @@
       goto err;
     }
 
-    if (DH_size(dh) < 512 / 8) {
+    if (DH_num_bits(dh) < 1024) {
       OPENSSL_PUT_ERROR(SSL, ssl3_get_server_key_exchange,
                         SSL_R_BAD_DH_P_LENGTH);
       goto err;
diff --git a/ssl/test/runner/common.go b/ssl/test/runner/common.go
index 2e6ddf3..e1479e9 100644
--- a/ssl/test/runner/common.go
+++ b/ssl/test/runner/common.go
@@ -707,6 +707,10 @@
 
 	// BadFinished, if true, causes the Finished hash to be broken.
 	BadFinished bool
+
+	// DHGroupPrime, if not nil, is used to define the (finite field)
+	// Diffie-Hellman group. The generator used is always two.
+	DHGroupPrime *big.Int
 }
 
 func (c *Config) serverInit() {
diff --git a/ssl/test/runner/key_agreement.go b/ssl/test/runner/key_agreement.go
index 5e44b54..2ee0087 100644
--- a/ssl/test/runner/key_agreement.go
+++ b/ssl/test/runner/key_agreement.go
@@ -561,11 +561,18 @@
 }
 
 func (ka *dheKeyAgreement) generateServerKeyExchange(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
-	// 2048-bit MODP Group with 256-bit Prime Order Subgroup (RFC
-	// 5114, Section 2.3)
-	ka.p, _ = new(big.Int).SetString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
-	ka.g, _ = new(big.Int).SetString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
-	q, _ := new(big.Int).SetString("8CF83642A709A097B447997640129DA299B1A47D1EB3750BA308B0FE64F5FBD3", 16)
+	var q *big.Int
+	if p := config.Bugs.DHGroupPrime; p != nil {
+		ka.p = p
+		ka.g = big.NewInt(2)
+		q = p
+	} else {
+		// 2048-bit MODP Group with 256-bit Prime Order Subgroup (RFC
+		// 5114, Section 2.3)
+		ka.p, _ = new(big.Int).SetString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
+		ka.g, _ = new(big.Int).SetString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
+		q, _ = new(big.Int).SetString("8CF83642A709A097B447997640129DA299B1A47D1EB3750BA308B0FE64F5FBD3", 16)
+	}
 
 	var err error
 	ka.xOurs, err = rand.Int(config.rand(), q)
diff --git a/ssl/test/runner/runner.go b/ssl/test/runner/runner.go
index 6c16020..c892c37 100644
--- a/ssl/test/runner/runner.go
+++ b/ssl/test/runner/runner.go
@@ -11,6 +11,7 @@
 	"fmt"
 	"io"
 	"io/ioutil"
+	"math/big"
 	"net"
 	"os"
 	"os/exec"
@@ -1569,6 +1570,14 @@
 	return !hasComponent(suiteName, "RC4")
 }
 
+func bigFromHex(hex string) *big.Int {
+	ret, ok := new(big.Int).SetString(hex, 16)
+	if !ok {
+		panic("failed to parse hex number 0x" + hex)
+	}
+	return ret
+}
+
 func addCipherSuiteTests() {
 	for _, suite := range testCipherSuites {
 		const psk = "12345"
@@ -1667,6 +1676,21 @@
 			}
 		}
 	}
+
+	testCases = append(testCases, testCase{
+		name: "WeakDH",
+		config: Config{
+			CipherSuites: []uint16{TLS_DHE_RSA_WITH_AES_128_GCM_SHA256},
+			Bugs: ProtocolBugs{
+				// This is a 1023-bit prime number, generated
+				// with:
+				// openssl gendh 1023 | openssl asn1parse -i
+				DHGroupPrime: bigFromHex("518E9B7930CE61C6E445C8360584E5FC78D9137C0FFDC880B495D5338ADF7689951A6821C17A76B3ACB8E0156AEA607B7EC406EBEDBB84D8376EB8FE8F8BA1433488BEE0C3EDDFD3A32DBB9481980A7AF6C96BFCF490A094CFFB2B8192C1BB5510B77B658436E27C2D4D023FE3718222AB0CA1273995B51F6D625A4944D0DD4B"),
+			},
+		},
+		shouldFail:    true,
+		expectedError: "BAD_DH_P_LENGTH",
+	})
 }
 
 func addBadECDSASignatureTests() {