From 66d3b2e0e596a6eac1ebcd15c83a8d9368fe7b34 Mon Sep 17 00:00:00 2001
From: Tobias Brunner <tobias@strongswan.org>
Date: Fri, 22 Jul 2022 15:37:43 +0200
Subject: [PATCH] credential-manager: Do online revocation checks only after
 basic trust chain validation

This avoids querying URLs of potentially untrusted certificates, e.g. if
an attacker sends a specially crafted end-entity and intermediate CA
certificate with a CDP that points to a server that completes the
TCP handshake but then does not send any further data, which will block
the fetcher thread (depending on the plugin) for as long as the default
timeout for TCP.  Doing that multiple times will block all worker threads,
leading to a DoS attack.

The logging during the certificate verification obviously changes.  The
following example shows the output of `pki --verify` for the current
strongswan.org certificate:

new:

  using certificate "CN=www.strongswan.org"
  using trusted intermediate ca certificate "C=US, O=Let's Encrypt, CN=R3"
  using trusted ca certificate "C=US, O=Internet Security Research Group, CN=ISRG Root X1"
  reached self-signed root ca with a path length of 1
checking certificate status of "CN=www.strongswan.org"
  requesting ocsp status from 'http://r3.o.lencr.org' ...
  ocsp response correctly signed by "C=US, O=Let's Encrypt, CN=R3"
  ocsp response is valid: until Jul 27 12:59:58 2022
certificate status is good
checking certificate status of "C=US, O=Let's Encrypt, CN=R3"
ocsp response verification failed, no signer certificate 'C=US, O=Let's Encrypt, CN=R3' found
  fetching crl from 'http://x1.c.lencr.org/' ...
  using trusted certificate "C=US, O=Internet Security Research Group, CN=ISRG Root X1"
  crl correctly signed by "C=US, O=Internet Security Research Group, CN=ISRG Root X1"
  crl is valid: until Apr 18 01:59:59 2023
certificate status is good
certificate trusted, lifetimes valid, certificate not revoked

old:

  using certificate "CN=www.strongswan.org"
  using trusted intermediate ca certificate "C=US, O=Let's Encrypt, CN=R3"
checking certificate status of "CN=www.strongswan.org"
  requesting ocsp status from 'http://r3.o.lencr.org' ...
  ocsp response correctly signed by "C=US, O=Let's Encrypt, CN=R3"
  ocsp response is valid: until Jul 27 12:59:58 2022
certificate status is good
  using trusted ca certificate "C=US, O=Internet Security Research Group, CN=ISRG Root X1"
checking certificate status of "C=US, O=Let's Encrypt, CN=R3"
ocsp response verification failed, no signer certificate 'C=US, O=Let's Encrypt, CN=R3' found
  fetching crl from 'http://x1.c.lencr.org/' ...
  using trusted certificate "C=US, O=Internet Security Research Group, CN=ISRG Root X1"
  crl correctly signed by "C=US, O=Internet Security Research Group, CN=ISRG Root X1"
  crl is valid: until Apr 18 01:59:59 2023
certificate status is good
  reached self-signed root ca with a path length of 1
certificate trusted, lifetimes valid, certificate not revoked

Note that this also fixes an issue with the previous dual-use of the
`trusted` flag.  It not only indicated whether the chain is trusted but
also whether the current issuer is the root anchor (the corresponding
flag in the `cert_validator_t` interface is called `anchor`).  This was
a problem when building multi-level trust chains for pre-trusted
end-entity certificates (i.e. where `trusted` is TRUE from the start).
This caused the main loop to get aborted after the first intermediate CA
certificate and the mentioned `anchor` flag wasn't correct in any calls
to `cert_validator_t` implementations.

Fixes: CVE-2022-40617
---
 .../credentials/credential_manager.c          | 54 +++++++++++++++----
 1 file changed, 45 insertions(+), 9 deletions(-)

diff --git a/src/libstrongswan/credentials/credential_manager.c b/src/libstrongswan/credentials/credential_manager.c
index e93b5943a3a7..798785544e41 100644
--- a/src/libstrongswan/credentials/credential_manager.c
+++ b/src/libstrongswan/credentials/credential_manager.c
@@ -556,7 +556,7 @@ static void cache_queue(private_credential_manager_t *this)
  */
 static bool check_lifetime(private_credential_manager_t *this,
 						   certificate_t *cert, char *label,
-						   int pathlen, bool trusted, auth_cfg_t *auth)
+						   int pathlen, bool anchor, auth_cfg_t *auth)
 {
 	time_t not_before, not_after;
 	cert_validator_t *validator;
@@ -571,7 +571,7 @@ static bool check_lifetime(private_credential_manager_t *this,
 			continue;
 		}
 		status = validator->check_lifetime(validator, cert,
-										   pathlen, trusted, auth);
+										   pathlen, anchor, auth);
 		if (status != NEED_MORE)
 		{
 			break;
@@ -604,13 +604,13 @@ static bool check_lifetime(private_credential_manager_t *this,
  */
 static bool check_certificate(private_credential_manager_t *this,
 				certificate_t *subject, certificate_t *issuer, bool online,
-				int pathlen, bool trusted, auth_cfg_t *auth)
+				int pathlen, bool anchor, auth_cfg_t *auth)
 {
 	cert_validator_t *validator;
 	enumerator_t *enumerator;
 
 	if (!check_lifetime(this, subject, "subject", pathlen, FALSE, auth) ||
-		!check_lifetime(this, issuer, "issuer", pathlen + 1, trusted, auth))
+		!check_lifetime(this, issuer, "issuer", pathlen + 1, anchor, auth))
 	{
 		return FALSE;
 	}
@@ -623,7 +623,7 @@ static bool check_certificate(private_credential_manager_t *this,
 			continue;
 		}
 		if (!validator->validate(validator, subject, issuer,
-								 online, pathlen, trusted, auth))
+								 online, pathlen, anchor, auth))
 		{
 			enumerator->destroy(enumerator);
 			return FALSE;
@@ -726,6 +726,7 @@ static bool verify_trust_chain(private_credential_manager_t *this,
 	auth_cfg_t *auth;
 	signature_params_t *scheme;
 	int pathlen;
+	bool is_anchor = FALSE;
 
 	auth = auth_cfg_create();
 	get_key_strength(subject, auth);
@@ -743,7 +744,7 @@ static bool verify_trust_chain(private_credential_manager_t *this,
 				auth->add(auth, AUTH_RULE_CA_CERT, issuer->get_ref(issuer));
 				DBG1(DBG_CFG, "  using trusted ca certificate \"%Y\"",
 							  issuer->get_subject(issuer));
-				trusted = TRUE;
+				trusted = is_anchor = TRUE;
 			}
 			else
 			{
@@ -778,11 +779,18 @@ static bool verify_trust_chain(private_credential_manager_t *this,
 				DBG1(DBG_CFG, "  issuer is \"%Y\"",
 					 current->get_issuer(current));
 				call_hook(this, CRED_HOOK_NO_ISSUER, current);
+				if (trusted)
+				{
+					DBG1(DBG_CFG, "  reached end of incomplete trust chain for "
+						 "trusted certificate \"%Y\"",
+						 subject->get_subject(subject));
+				}
 				break;
 			}
 		}
-		if (!check_certificate(this, current, issuer, online,
-							   pathlen, trusted, auth))
+		/* don't do online verification here */
+		if (!check_certificate(this, current, issuer, FALSE,
+							   pathlen, is_anchor, auth))
 		{
 			trusted = FALSE;
 			issuer->destroy(issuer);
@@ -794,7 +802,7 @@ static bool verify_trust_chain(private_credential_manager_t *this,
 		}
 		current->destroy(current);
 		current = issuer;
-		if (trusted)
+		if (is_anchor)
 		{
 			DBG1(DBG_CFG, "  reached self-signed root ca with a "
 				 "path length of %d", pathlen);
@@ -807,6 +815,34 @@ static bool verify_trust_chain(private_credential_manager_t *this,
 		DBG1(DBG_CFG, "maximum path length of %d exceeded", MAX_TRUST_PATH_LEN);
 		call_hook(this, CRED_HOOK_EXCEEDED_PATH_LEN, subject);
 	}
+	else if (trusted && online)
+	{
+		enumerator_t *enumerator;
+		auth_rule_t rule;
+
+		/* do online revocation checks after basic validation of the chain */
+		pathlen = 0;
+		current = subject;
+		enumerator = auth->create_enumerator(auth);
+		while (enumerator->enumerate(enumerator, &rule, &issuer))
+		{
+			if (rule == AUTH_RULE_CA_CERT || rule == AUTH_RULE_IM_CERT)
+			{
+				if (!check_certificate(this, current, issuer, TRUE, pathlen++,
+									   rule == AUTH_RULE_CA_CERT, auth))
+				{
+					trusted = FALSE;
+					break;
+				}
+				else if (rule == AUTH_RULE_CA_CERT)
+				{
+					break;
+				}
+				current = issuer;
+			}
+		}
+		enumerator->destroy(enumerator);
+	}
 	if (trusted)
 	{
 		result->merge(result, auth, FALSE);
-- 
2.25.1