libcurl: Restrict redirect schemes

All protocols except for CURLPROTO_FILE/CURLPROTO_SMB and their TLS
counterpart were allowed for redirect. This vastly broadens the
exploitation surface in case of a vulnerability such as SSRF [1], where
libcurl-based clients are forced to make requests to arbitrary hosts.

For instance, CURLPROTO_GOPHER can be used to smuggle any TCP-based
protocol by URL-encoding a payload in the URI. Gopher will open a TCP
connection and send the payload.

Only HTTP/HTTPS and FTP are allowed. All other protocols have to be
explicitly enabled for redirects through CURLOPT_REDIR_PROTOCOLS.

[1]: https://www.acunetix.com/blog/articles/server-side-request-forgery-vulnerability/

Signed-off-by: Linos Giannopoulos <lgian@skroutz.gr>

Closes #4094

1 files changed, 1 insertions, 3 deletions

diff --git a/lib/url.c b/lib/url.c
index 22b930015..258f60c8f 100644
--- a/lib/url.c
+++ b/lib/url.c
@@ -488,9 +488,7 @@ CURLcode Curl_init_userdefined(struct Curl_easy *data)
      define since we internally only use the lower 16 bits for the passed
      in bitmask to not conflict with the private bits */
   set->allowed_protocols = CURLPROTO_ALL;
-  set->redir_protocols = CURLPROTO_ALL &  /* All except FILE, SCP and SMB */
-                          ~(CURLPROTO_FILE | CURLPROTO_SCP | CURLPROTO_SMB |
-                            CURLPROTO_SMBS);
+  set->redir_protocols = CURLPROTO_HTTP | CURLPROTO_HTTPS | CURLPROTO_FTP;
 
 #if defined(HAVE_GSSAPI) || defined(USE_WINDOWS_SSPI)
   /*