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 16c33bba4bc84d74e5fe59d0f5be7c351bf07349..0997f5d22b6d7d34980f462aa4913e61473f68f3 100644 (file)
--- a/src/ClientRequestContext.h
+++ b/src/ClientRequestContext.h
@@ -80,8 +80,13 @@ public:
 #if USE_OPENSSL
     bool sslBumpCheckDone = false;
 #endif
-    ErrorState *error = nullptr; ///< saved error page for centralized/delayed processing
+
     bool readNextRequest = false; ///< whether Squid should read after error handling
+    ErrorState *error = nullptr; ///< saved error page for centralized/delayed processing
+
+#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 5b5b5af8086d29122246b2686b8cd0f74255f2eb..7f802d4219e3f95f383b42723dff69aa371d61ae 100644 (file)
--- a/src/client_side_request.cc
+++ b/src/client_side_request.cc
@@ -75,6 +75,11 @@
 #endif
 
 #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 */
 
@@ -437,8 +442,16 @@ 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
         }
     }