diff --git a/src/bio.c b/src/bio.c
index 13fdef3a5d..b9dfe6b7dd 100644
--- a/src/bio.c
+++ b/src/bio.c
@@ -120,6 +120,16 @@ static int wolfSSL_BIO_MEMORY_read(WOLFSSL_BIO* bio, void* buf, int len)
     WOLFSSL_ENTER("wolfSSL_BIO_MEMORY_read");
     }
 
+    /* Reject a negative length up front. Callers are expected to validate, but
+     * guarding here too prevents a negative len from defeating the signed
+     * bounds checks below (sz/memSz comparisons) and reaching XMEMCPY with a
+     * length of (size_t)-1. Return the same error the public wolfSSL_BIO_read()
+     * does for a negative length rather than silently reporting 0 bytes read. A
+     * zero length is handled correctly by the logic below (copies nothing). */
+    if (len < 0) {
+        return WOLFSSL_BIO_ERROR;
+    }
+
     sz = wolfSSL_BIO_pending(bio);
     if (sz > 0) {
         int memSz;
diff --git a/tests/api/test_ossl_bio.c b/tests/api/test_ossl_bio.c
index 45c8e94086..b111fd4468 100644
--- a/tests/api/test_ossl_bio.c
+++ b/tests/api/test_ossl_bio.c
@@ -980,6 +980,45 @@ int test_wolfSSL_BIO_write(void)
     return EXPECT_RESULT();
 }
 
+/* A negative length must never defeat the memory-BIO read bounds check and
+ * reach XMEMCPY with (size_t)-1. This exercises the public BIO_read() boundary
+ * (which rejects a negative length before dispatch); the matching guard in the
+ * static wolfSSL_BIO_MEMORY_read() sink is defense-in-depth and not separately
+ * reachable through the public API. Verify a negative length is rejected with
+ * an error without copying, a zero length reads nothing, and the pending data
+ * is left intact and still readable. */
+int test_wolfSSL_BIO_read_negative_len(void)
+{
+    EXPECT_DECLS;
+#if defined(OPENSSL_EXTRA)
+    BIO*  bio = NULL;
+    char  msg[] = "negative length test";
+    int   msgLen = (int)XSTRLEN(msg);
+    char  out[64];
+
+    ExpectNotNull(bio = BIO_new(BIO_s_mem()));
+    ExpectIntEQ(BIO_write(bio, msg, msgLen), msgLen);
+
+    /* Negative length: must be rejected with an error, not a wild copy and not
+     * a silent 0-byte read. */
+    XMEMSET(out, 0, sizeof(out));
+    ExpectIntLT(BIO_read(bio, out, -1), 0);
+    /* Data must be untouched - still all pending. */
+    ExpectIntEQ(BIO_pending(bio), msgLen);
+
+    /* Zero length: nothing read, data still pending. */
+    ExpectIntEQ(BIO_read(bio, out, 0), 0);
+    ExpectIntEQ(BIO_pending(bio), msgLen);
+
+    /* A normal read still returns the intact message. */
+    ExpectIntEQ(BIO_read(bio, out, (int)sizeof(out)), msgLen);
+    ExpectIntEQ(XMEMCMP(out, msg, msgLen), 0);
+
+    BIO_free(bio);
+#endif
+    return EXPECT_RESULT();
+}
+
 
 int test_wolfSSL_BIO_printf(void)
 {
diff --git a/tests/api/test_ossl_bio.h b/tests/api/test_ossl_bio.h
index d401193b14..acf13fb776 100644
--- a/tests/api/test_ossl_bio.h
+++ b/tests/api/test_ossl_bio.h
@@ -35,6 +35,7 @@ int test_wolfSSL_BIO_datagram(void);
 int test_wolfSSL_BIO_s_null(void);
 int test_wolfSSL_BIO_accept(void);
 int test_wolfSSL_BIO_write(void);
+int test_wolfSSL_BIO_read_negative_len(void);
 int test_wolfSSL_BIO_printf(void);
 int test_wolfSSL_BIO_f_md(void);
 int test_wolfSSL_BIO_up_ref(void);
@@ -55,6 +56,7 @@ int test_wolfSSL_BIO_get_init(void);
     TEST_DECL_GROUP("ossl_bio", test_wolfSSL_BIO_should_retry),   \
     TEST_DECL_GROUP("ossl_bio", test_wolfSSL_BIO_s_null),         \
     TEST_DECL_GROUP("ossl_bio", test_wolfSSL_BIO_write),          \
+    TEST_DECL_GROUP("ossl_bio", test_wolfSSL_BIO_read_negative_len), \
     TEST_DECL_GROUP("ossl_bio", test_wolfSSL_BIO_printf),         \
     TEST_DECL_GROUP("ossl_bio", test_wolfSSL_BIO_f_md),           \
     TEST_DECL_GROUP("ossl_bio", test_wolfSSL_BIO_up_ref),         \