diff --git a/http-backend.c b/http-backend.c
index 8e08f057dd..f729488fc5 100644
--- a/http-backend.c
+++ b/http-backend.c
@@ -108,6 +108,7 @@ static const char *get_parameter(const char *name)
 	return i ? i->util : NULL;
 }
 
+__attribute__((format (printf, 2, 3)))
 static void format_write(int fd, const char *fmt, ...)
 {
 	static char buffer[1024];
@@ -165,6 +166,7 @@ static void end_headers(void)
 	safe_write(1, "\r\n", 2);
 }
 
+__attribute__((format (printf, 1, 2)))
 static NORETURN void not_found(const char *err, ...)
 {
 	va_list params;
@@ -180,6 +182,7 @@ static NORETURN void not_found(const char *err, ...)
 	exit(0);
 }
 
+__attribute__((format (printf, 1, 2)))
 static NORETURN void forbidden(const char *err, ...)
 {
 	va_list params;