In subversion 1.7.7 in tools/server-side/mod_dontdothat/mod_dontdothat.c, there is the following code:

      ctx->xmlp = XML_ParserCreate(NULL);
      apr_pool_cleanup_register(r->pool, ctx->xmlp,
                                clean_up_parser,
                                apr_pool_cleanup_null);
      XML_SetUserData(ctx->xmlp, ctx);
      XML_SetElementHandler(ctx->xmlp, start_element, end_element);
      XML_SetCharacterDataHandler(ctx->xmlp, cdata);

This doesn't disable entity expansion for the internal DTD subset, so there is a denial-of-service vector ("billion laughs attack").  I'm marking this as a security bug because it probably allows to crash Apache or trigger the kernel OOM handler.  This should probably be fixed in coordination with Subversion upstream.

Adding the following handler using

  XML_SetEntityDeclHandler(ctx->xmlp, EntityDeclHandler);

should be sufficient to address this issue.

// Stop the parser when an entity declaration is encountered.
static void
EntityDeclHandler(void *userData,
		  const XML_Char *entityName, int is_parameter_entity,
		  const XML_Char *value, int value_length,
		  const XML_Char *base, const XML_Char *systemId,
		  const XML_Char *publicId, const XML_Char *notationName)
{
  XML_StopParser((XML_Parser)userData, XML_FALSE);
}