diff --git a/src/utils/base64.c b/src/utils/base64.c
index 0eadb8142..13fc511dc 100644
--- a/src/utils/base64.c
+++ b/src/utils/base64.c
@@ -43,6 +43,8 @@ unsigned char * base64_encode(const unsigned char *src, size_t len,
 	olen = len * 4 / 3 + 4; /* 3-byte blocks to 4-byte */
 	olen += olen / 72; /* line feeds */
 	olen++; /* nul termination */
+	if (olen < len)
+		return NULL; /* integer overflow */
 	out = os_malloc(olen);
 	if (out == NULL)
 		return NULL;