Limit the number of allowed X-Forwarded-For hops (#1589)

Squid will ignore all X-Forwarded-For elements listed after the first 64
addresses allowed by the follow_x_forwarded_for directive. A different
limit can be specified by defining a C++ SQUID_X_FORWARDED_FOR_HOP_MAX
macro, but that macro is not a supported Squid configuration interface
and may change or disappear at any time.

Squid will log a cache.log ERROR if the hop limit has been reached.

This change works around problematic ACLChecklist and/or slow ACLs
implementation that results in immediate nonBlockingCheck() callbacks.
Such callbacks have caused many bugs and development complications. In
clientFollowXForwardedForCheck() context, they lead to indirect
recursion that was bound only by the number of allowed XFF entries,
which could reach thousands and exhaust Squid process call stack.

This recursion bug was discovered and detailed by Joshua Rogers at
https://megamansec.github.io/Squid-Security-Audit/xff-stackoverflow.html
where it was filed as "X-Forwarded-For Stack Overflow".

diff --git a/src/ClientRequestContext.h b/src/ClientRequestContext.h
index f5316f9cf6917f92d42c4d06f6ac08e47c775e9e..8651d101ae0f868c0cccb3c030eb875c12df10d6 100644 (file)
--- a/src/ClientRequestContext.h
+++ b/src/ClientRequestContext.h
@@ -80,6 +80,10 @@ public:
 #endif
     ErrorState *error; ///< saved error page for centralized/delayed processing
     bool readNextRequest; ///< whether Squid should read after error handling
+
+#if FOLLOW_X_FORWARDED_FOR
+    size_t currentXffHopNumber = 0; ///< number of X-Forwarded-For header values processed so far
+#endif
 };
 
 #endif /* SQUID_CLIENTREQUESTCONTEXT_H */

diff --git a/src/client_side_request.cc b/src/client_side_request.cc
index 2f49ca1495d73eafee2a69eca14ddf322550d321..890357835aecd95e8b33658a171b749fa84c18ba 100644 (file)
--- a/src/client_side_request.cc
+++ b/src/client_side_request.cc
@@ -81,6 +81,11 @@
 static const char *const crlf = "\r\n";
 
 #if FOLLOW_X_FORWARDED_FOR
+
+#if !defined(SQUID_X_FORWARDED_FOR_HOP_MAX)
+#define SQUID_X_FORWARDED_FOR_HOP_MAX 64
+#endif
+
 static void clientFollowXForwardedForCheck(Acl::Answer answer, void *data);
 #endif /* FOLLOW_X_FORWARDED_FOR */
 
@@ -486,8 +491,16 @@ clientFollowXForwardedForCheck(Acl::Answer answer, void *data)
                 /* override the default src_addr tested if we have to go deeper than one level into XFF */
                 Filled(calloutContext->acl_checklist)->src_addr = request->indirect_client_addr;
             }
-            calloutContext->acl_checklist->nonBlockingCheck(clientFollowXForwardedForCheck, data);
-            return;
+            if (++calloutContext->currentXffHopNumber < SQUID_X_FORWARDED_FOR_HOP_MAX) {
+                calloutContext->acl_checklist->nonBlockingCheck(clientFollowXForwardedForCheck, data);
+                return;
+            }
+            const auto headerName = Http::HeaderLookupTable.lookup(Http::HdrType::X_FORWARDED_FOR).name;
+            debugs(28, DBG_CRITICAL, "ERROR: Ignoring trailing " << headerName << " addresses" <<
+                   Debug::Extra << "addresses allowed by follow_x_forwarded_for: " << calloutContext->currentXffHopNumber <<
+                   Debug::Extra << "last/accepted address: " << request->indirect_client_addr <<
+                   Debug::Extra << "ignored trailing addresses: " << request->x_forwarded_for_iterator);
+            // fall through to resume clientAccessCheck() processing
         }
     }