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Advisory: Monkey HTTPd improper input validation vulnerability

Note: The original advisory can be found here.

Affected products: Monkey web server versions ≤ 0.9.2
Class: Improper Input Validation (CWE-20), Incorrect Calculation (CWE-682)
Remote: Yes
Discovered by: Patroklos Argyroudis

We have discovered a remotely exploitable “improper input validation” vulnerability in the Monkey web server that allows an attacker to perform denial of service attacks by repeatedly crashing worker threads that process HTTP requests.


Monkey is a fast, efficient, small and easy to configure HTTP/1.1 compliant web server. It has been designed to be scalable with low memory and CPU consumption. More information about its features can be found here.

Monkey (up to and including version 0.9.2) employs an insufficient input validation method for handling HTTP requests with invalid connection headers. Specifically, the vulnerability is in the calculation for the end of the request body buffer related to newline characters in function Request_Find_Variable() in the file src/request.c:

364: char *Request_Find_Variable(char *request_body,  char *string)
365: {
366:   int pos_init_var=0, pos_end_var=0;
367:   char *var_value = 0;
369:   /* Existe *string en request_body ??? */        
370:   if (strstr2(request_body, string) == NULL)
371:       return NULL;
373:   pos_init_var = str_search(request_body, string, strlen(string));
374:   pos_end_var = str_search(request_body+pos_init_var, "\n", 1) - 1;
376:   if(pos_init_var<=0 || pos_end_var<=0){
377:       return  NULL;   
378:   }
380:   pos_init_var += strlen(string) + 1;
381:   pos_end_var = (unsigned int) (pos_init_var  + pos_end_var) - (strlen(string) +1);
383:   var_value = m_copy_string(request_body, pos_init_var, pos_end_var);
385:   return (char *) var_value;
386: }

With a specially crafted request body the pos_init_var integer can take the value 0x1c (28 in decimal) and the pos_end_var integer can take the value 0x1a (26 in decimal). Then in the m_copy_string() function, the calculation for the unsigned integer size in line 428 (file src/utils.c) leads to a signedness bug and m_copy_string() returns NULL (line 438, file src/utils.c):

423: char *m_copy_string(const char *string, int pos_init, int pos_end)
424: {
425:   unsigned int size, bytes;
426:   char *buffer=0;
428:   size = (unsigned int) (pos_end - pos_init ) + 1;
429:   if(size<=2) size=4;
431:   buffer = M_malloc(size);
433:   if(!buffer){
434:       return NULL;
435:   }
437:   if(pos_end>strlen(string) || (pos_init > pos_end)){
438:       return NULL;
439:   }

This causes Request_Find_Variable() to return NULL (line 344, file src/request.c) and this to be used in the strstr2() call at line 345 of file src/request.c:

344:   sr->connection = Request_Find_Variable(request_body, RH_CONNECTION);
345:   if((strstr2(sr->connection,"Keep-Alive"))!=NULL){
346:       sr->keep_alive=VAR_ON;
347:   }

This vulnerability can allow an attacker to perform denial of service attacks by repeatedly crashing Monkey worker threads that process HTTP requests. We have developed a proof-of-concept exploit to demonstrate the vulnerability.

The maintainer of Monkey has been contacted and a new version of the web server (0.9.3) has been released that addresses this issue. All affected parties are advised to upgrade to the latest version available.