Merge branch 'disallow-ntlm-auth-by-default'

This topic branch addresses the following vulnerability:

- **CVE-2025-66413**:
  When a user clones a repository from an attacker-controlled server,
  Git may attempt NTLM authentication and disclose the user's NTLMv2 hash
  to the remote server. Since NTLM hashing is weak, the captured hash can
  potentially be brute-forced to recover the user's credentials. This is
  addressed by disabling NTLM authentication by default.
  (GHSA-hv9c-4jm9-jh3x)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

Documentation/config/http.txt

@@ -216,6 +216,11 @@ http.sslBackend::
 	This option is ignored if cURL lacks support for choosing the SSL
 	backend at runtime.
 
+http.allowNTLMAuth::
+	Whether or not to allow NTLM authentication. While very convenient to set
+	up, and therefore still used in many on-prem scenarios, NTLM is a weak
+	authentication method and therefore deprecated. Defaults to "false".
+
 http.schannelCheckRevoke::
 	Used to enforce or disable certificate revocation checks in cURL
 	when http.sslBackend is set to "schannel" via "true" and "false",

credential.c

@@ -360,6 +360,9 @@ int credential_read(struct credential *c, FILE *fp,
 				credential_set_capability(&c->capa_authtype, op_type);
 			else if (!strcmp(value, "state"))
 				credential_set_capability(&c->capa_state, op_type);
+		} else if (!strcmp(key, "ntlm")) {
+			if (!strcmp(value, "allow"))
+				c->ntlm_allow = 1;
 		} else if (!strcmp(key, "continue")) {
 			c->multistage = !!git_config_bool("continue", value);
 		} else if (!strcmp(key, "password_expiry_utc")) {
@@ -420,6 +423,8 @@ void credential_write(const struct credential *c, FILE *fp,
 		if (c->ephemeral)
 			credential_write_item(c, fp, "ephemeral", "1", 0);
 	}
+	if (c->ntlm_suppressed)
+		credential_write_item(c, fp, "ntlm", "suppressed", 0);
 	credential_write_item(c, fp, "protocol", c->protocol, 1);
 	credential_write_item(c, fp, "host", c->host, 1);
 	credential_write_item(c, fp, "path", c->path, 0);

credential.h

@@ -175,6 +175,9 @@ struct credential {
 	struct credential_capability capa_authtype;
 	struct credential_capability capa_state;
 
+	unsigned ntlm_suppressed:1,
+		 ntlm_allow:1;
+
 	char *username;
 	char *password;
 	char *credential;

http.c

@@ -128,7 +128,8 @@ enum http_follow_config http_follow_config = HTTP_FOLLOW_INITIAL;
 
 static struct credential cert_auth = CREDENTIAL_INIT;
 static int ssl_cert_password_required;
-static unsigned long http_auth_methods = CURLAUTH_ANY;
+static unsigned long http_auth_any = CURLAUTH_ANY & ~CURLAUTH_NTLM;
+static unsigned long http_auth_methods;
 static int http_auth_methods_restricted;
 /* Modes for which empty_auth cannot actually help us. */
 static unsigned long empty_auth_useless =
@@ -429,6 +430,15 @@ static int http_options(const char *var, const char *value,
 		return 0;
 	}
 
+	if (!strcmp("http.allowntlmauth", var)) {
+		if (git_config_bool(var, value)) {
+			http_auth_any |= CURLAUTH_NTLM;
+		} else {
+			http_auth_any &= ~CURLAUTH_NTLM;
+		}
+		return 0;
+	}
+
 	if (!strcmp("http.schannelcheckrevoke", var)) {
 		if (value && !strcmp(value, "best-effort")) {
 			http_schannel_check_revoke_mode =
@@ -645,6 +655,11 @@ static void init_curl_http_auth(CURL *result)
 
 	credential_fill(&http_auth, 1);
 
+	if (http_auth.ntlm_allow && !(http_auth_methods & CURLAUTH_NTLM)) {
+		http_auth_methods |= CURLAUTH_NTLM;
+		curl_easy_setopt(result, CURLOPT_HTTPAUTH, http_auth_methods);
+	}
+
 	if (http_auth.password) {
 		if (always_auth_proactively()) {
 			/*
@@ -704,11 +719,11 @@ static void init_curl_proxy_auth(CURL *result)
 		if (i == ARRAY_SIZE(proxy_authmethods)) {
 			warning("unsupported proxy authentication method %s: using anyauth",
 					http_proxy_authmethod);
-			curl_easy_setopt(result, CURLOPT_PROXYAUTH, CURLAUTH_ANY);
+			curl_easy_setopt(result, CURLOPT_PROXYAUTH, http_auth_any);
 		}
 	}
 	else
-		curl_easy_setopt(result, CURLOPT_PROXYAUTH, CURLAUTH_ANY);
+		curl_easy_setopt(result, CURLOPT_PROXYAUTH, http_auth_any);
 }
 
 static int has_cert_password(void)
@@ -1091,7 +1106,7 @@ static CURL *get_curl_handle(void)
 #endif
 
 	curl_easy_setopt(result, CURLOPT_NETRC, CURL_NETRC_OPTIONAL);
-	curl_easy_setopt(result, CURLOPT_HTTPAUTH, CURLAUTH_ANY);
+	curl_easy_setopt(result, CURLOPT_HTTPAUTH, http_auth_any);
 
 #ifdef CURLGSSAPI_DELEGATION_FLAG
 	if (curl_deleg) {
@@ -1461,6 +1476,8 @@ void http_init(struct remote *remote, const char *url, int proactive_auth)
 			ssl_cert_password_required = 1;
 	}
 
+	http_auth_methods = http_auth_any;
+
 	curl_default = get_curl_handle();
 }
 
@@ -1894,6 +1911,12 @@ static int handle_curl_result(struct slot_results *results)
 	} else if (missing_target(results))
 		return HTTP_MISSING_TARGET;
 	else if (results->http_code == 401) {
+		http_auth.ntlm_suppressed = (results->auth_avail & CURLAUTH_NTLM) &&
+					    !(http_auth_any & CURLAUTH_NTLM);
+		if (http_auth.ntlm_suppressed && http_auth.ntlm_allow) {
+			http_auth_methods |= CURLAUTH_NTLM;
+			return HTTP_REAUTH;
+		}
 		if ((http_auth.username && http_auth.password) ||\
 		    (http_auth.authtype && http_auth.credential)) {
 			if (http_auth.multistage) {
@@ -1903,6 +1926,16 @@ static int handle_curl_result(struct slot_results *results)
 			credential_reject(&http_auth);
 			if (always_auth_proactively())
 				http_proactive_auth = PROACTIVE_AUTH_NONE;
+			if (http_auth.ntlm_suppressed) {
+				warning(_("Due to its cryptographic weaknesses, "
+					  "NTLM authentication has been\n"
+					  "disabled in Git by default. You can "
+					  "re-enable it for trusted servers\n"
+					  "by running:\n\n"
+					  "git config set "
+					  "http.%s://%s.allowNTLMAuth true"),
+					http_auth.protocol, http_auth.host);
+			}
 			return HTTP_NOAUTH;
 		} else {
 			http_auth_methods &= ~CURLAUTH_GSSNEGOTIATE;

t/lib-httpd.sh

@@ -167,6 +167,7 @@ prepare_httpd() {
 	install_script error.sh
 	install_script apply-one-time-perl.sh
 	install_script nph-custom-auth.sh
+	install_script ntlm-handshake.sh
 
 	ln -s "$LIB_HTTPD_MODULE_PATH" "$HTTPD_ROOT_PATH/modules"
 

t/lib-httpd/apache.conf

@@ -151,6 +151,13 @@ SetEnv PERL_PATH ${PERL_PATH}
 	CGIPassAuth on
 	</IfDefine>
 </LocationMatch>
+<LocationMatch /ntlm_auth/>
+	SetEnv GIT_EXEC_PATH ${GIT_EXEC_PATH}
+	SetEnv GIT_HTTP_EXPORT_ALL
+	<IfDefine USE_CGIPASSAUTH>
+	CGIPassAuth on
+	</IfDefine>
+</LocationMatch>
 ScriptAlias /smart/incomplete_length/git-upload-pack incomplete-length-upload-pack-v2-http.sh/
 ScriptAlias /smart/incomplete_body/git-upload-pack incomplete-body-upload-pack-v2-http.sh/
 ScriptAlias /smart/no_report/git-receive-pack error-no-report.sh/
@@ -161,6 +168,7 @@ ScriptAlias /error_smart/ error-smart-http.sh/
 ScriptAlias /error/ error.sh/
 ScriptAliasMatch /one_time_perl/(.*) apply-one-time-perl.sh/$1
 ScriptAliasMatch /custom_auth/(.*) nph-custom-auth.sh/$1
+ScriptAliasMatch /ntlm_auth/(.*) ntlm-handshake.sh/$1
 <Directory ${GIT_EXEC_PATH}>
 	Options FollowSymlinks
 </Directory>

t/lib-httpd/ntlm-handshake.sh

@@ -0,0 +1,38 @@
+#!/bin/sh
+
+case "$HTTP_AUTHORIZATION" in
+'')
+	# No Authorization header -> send NTLM challenge
+	echo "Status: 401 Unauthorized"
+	echo "WWW-Authenticate: NTLM"
+	echo
+	;;
+"NTLM TlRMTVNTUAAB"*)
+	# Type 1 -> respond with Type 2 challenge (hardcoded)
+	echo "Status: 401 Unauthorized"
+	# Base64-encoded version of the Type 2 challenge:
+	# signature: 'NTLMSSP\0'
+	# message_type: 2
+	# target_name: 'NTLM-GIT-SERVER'
+	# flags: 0xa2898205 =
+	#   NEGOTIATE_UNICODE, REQUEST_TARGET, NEGOTIATE_NT_ONLY,
+	#   TARGET_TYPE_SERVER, TARGET_TYPE_SHARE, REQUEST_NON_NT_SESSION_KEY,
+	#   NEGOTIATE_VERSION, NEGOTIATE_128, NEGOTIATE_56
+	# challenge: 0xfa3dec518896295b
+	# context: '0000000000000000'
+	# target_info_present: true
+	# target_info_len: 128
+	# version: '10.0 (build 19041)'
+	echo "WWW-Authenticate: NTLM TlRMTVNTUAACAAAAHgAeADgAAAAFgomi+j3sUYiWKVsAAAAAAAAAAIAAgABWAAAACgBhSgAAAA9OAFQATABNAC0ARwBJAFQALQBTAEUAUgBWAEUAUgACABIAVwBPAFIASwBHAFIATwBVAFAAAQAeAE4AVABMAE0ALQBHAEkAVAAtAFMARQBSAFYARQBSAAQAEgBXAE8AUgBLAEcAUgBPAFUAUAADAB4ATgBUAEwATQAtAEcASQBUAC0AUwBFAFIAVgBFAFIABwAIAACfOcZKYNwBAAAAAA=="
+	echo
+	;;
+"NTLM TlRMTVNTUAAD"*)
+	# Type 3 -> accept without validation
+	exec "$GIT_EXEC_PATH"/git-http-backend
+	;;
+*)
+	echo "Status: 500 Unrecognized"
+	echo
+	echo "Unhandled auth: '$HTTP_AUTHORIZATION'"
+	;;
+esac

t/t5563-simple-http-auth.sh

@@ -675,4 +675,33 @@ test_expect_success 'access using three-legged auth' '
 	EOF
 '
 
+test_lazy_prereq NTLM 'curl --version | grep -q NTLM'
+
+test_expect_success NTLM 'access using NTLM auth' '
+	test_when_finished "per_test_cleanup" &&
+
+	set_credential_reply get <<-EOF &&
+	username=user
+	password=pwd
+	EOF
+
+	test_config_global credential.helper test-helper &&
+	test_must_fail env GIT_TRACE_CURL=1 git \
+		ls-remote "$HTTPD_URL/ntlm_auth/repo.git" 2>err &&
+	test_grep "allowNTLMAuth" err &&
+
+	# Can be enabled via config
+	GIT_TRACE_CURL=1 git -c http.$HTTPD_URL.allowNTLMAuth=true \
+		ls-remote "$HTTPD_URL/ntlm_auth/repo.git" &&
+
+	# Or via credential helper responding with ntlm=allow
+	set_credential_reply get <<-EOF &&
+	username=user
+	password=pwd
+	ntlm=allow
+	EOF
+
+	git ls-remote "$HTTPD_URL/ntlm_auth/repo.git"
+'
+
 test_done