Apache2: Incorrect handling of large requests in mod_proxy_uwsgi

mod_proxy_uwsgi as included in current versions of Apache httpd incorrectly handles large
HTTP requests. The UWSGI line protocol uses uint16_t length values for both header name/values 
and the overall packet size, but mod_proxy_uwsgi does not verify that these size fields do not overflow:

// modules/proxy/mod_proxy_uwsgi.c
static int uwsgi_send_headers(request_rec *r, proxy_conn_rec * conn)
{
  char *buf, *ptr;

  const apr_array_header_t *env_table;
  const apr_table_entry_t *env;

  apr_size_t headerlen = 4;
  apr_uint16_t pktsize, keylen, vallen;

  [...]

  env_table = apr_table_elts(r->subprocess_env);
  env = (apr_table_entry_t *) env_table->elts;

  for (j = 0; j < env_table->nelts; ++j) {
    headerlen += 2 + strlen(env[j].key) + 2 + strlen(env[j].val); 
  }

  ptr = buf = apr_palloc(r->pool, headerlen);

  ptr += 4;

  for (j = 0; j < env_table->nelts; ++j) {
    keylen = strlen(env[j].key); ** A ** 
    *ptr++ = (apr_byte_t) (keylen & 0xff);
    *ptr++ = (apr_byte_t) ((keylen >> 8) & 0xff);
    memcpy(ptr, env[j].key, keylen);
    ptr += keylen;

    vallen = strlen(env[j].val); ** B **
    *ptr++ = (apr_byte_t) (vallen & 0xff);
    *ptr++ = (apr_byte_t) ((vallen >> 8) & 0xff);
    memcpy(ptr, env[j].val, vallen);
    ptr += vallen;
  }

  pktsize = headerlen - 4; ** C **

  buf[0] = 0;
  buf[1] = (apr_byte_t) (pktsize & 0xff);
  buf[2] = (apr_byte_t) ((pktsize >> 8) & 0xff);
  buf[3] = 0;

  return uwsgi_send(conn, buf, headerlen, r);
}

A malicious request can easily overflow pktsize (C) by sending
a small amount of headers with a length that is close to the LimitRequestFieldSize
default value of 8190. This can be used to trick UWSGI into parsing parts of 
the serialized subprocess environment as part of the POST body. 
In most configurations the security impact of this seems to be limited. However,
an attacker might be able to leak sensitive environment variables as part of the POST body and/or 
strip security sensitive headers from the request. 
If UWSGI is explicitly configured in persistent mode (puwsgi), this can
also be used to smuggle a second UWSGI request leading to remote code execution.
(In its standard configuration UWSGI only supports a single request per connection, 
making request smuggling impossible)

RCE against a standard UWSGI config is possible if an attacker can put a controlled 
name or value into subprocess_env that is longer than 0xFFFF bytes: 
This would overflow the size calculation in (A) or (B) and
makes it possible to inject malicious key/value pairs into the UWSGI request. This can be turned
into code execution by setting a malicious UWSGI_FILE var 
(see https://github.com/wofeiwo/webcgi-exploits/blob/master/python/uwsgi-rce-zh.md)

Using an oversized HTTP header for this attack requires a LimitRequestFieldSize 
bypass and should not be possible in normal configurations. 
However, mod_http2 incorrectly enforced LimitRequestFieldSize before 
R1863276 (https://svn.apache.org/viewvc?view=revision&revision=1863276) so systems 
without this commit can be exploited easily. Other config dependent attack vectors might exist. 


Credits: 
Felix Wilhelm of Google Project Zero

This bug is subject to a 90 day disclosure deadline. After 90 days elapse, the bug report 
will become visible to the public. The scheduled disclosure date is 2020-07-23. 
Disclosure at an earlier date is also possible if agreed upon by all parties.

Related CVE Numbers: CVE-2020-11984.



Found by: fwilhelm@google.com