这是到2024.4.1为止，我看过的liblzma后门相关的技术文章，做了部分摘录。

backdoor in upstream xz/liblzma leading to ssh server compromise - Andres Freund & Florian Weimer [2024-03-29]
https://www.openwall.com/lists/oss-security/2024/03/29/4
(第一个发现者，微软工程师)
(含有丰富的初步技术细节)

XZ Utils backdoor - Lasse Collin
https://tukaani.org/xz-backdoor/
(xz原作者Larhzu的一点说明)

Everything I Know About the XZ Backdoor - Evan Boehs [2024-03-29]
https://boehs.org/node/everything-i-know-about-the-xz-backdoor
(replaces safe_fprintf with an unsafe variant fprintf)
(梳理Jia Tan的来龙去脉)
(185.128.24.163 Singapore/Jia Cheong Tan)

FAQ on the xz-utils backdoor
https://gist.github.com/thesamesam/223949d5a074ebc3dce9ee78baad9e27
(IFUNC, a mechanism in glibc that allows for indirect function calls)
(diff build-to-host.m4)

xz/liblzma: Bash-stage Obfuscation Explained - Gynvael Coldwind [2024-03-30]
https://gynvael.coldwind.pl/?lang=en&id=782
(解释build-to-host.m4及bash脚本)
(用awk实现的RC4变体)
(获取liblzma_la-crc64-fast.o)

RC4 recognizer here - nugxperience
https://twitter.com/nugxperience/status/1773906926503591970

第二篇解释build-to-host.m4及bash脚本
https://pastebin.com/5gnnL2yT

第三篇解释build-to-host.m4及bash脚本 - Jonathan Schleifer [2024-03-30]
https://github.com/Midar/xz-backdoor-documentation/wiki

一张关于liblzma后门的总览图 - Thomas Roccia (@fr0gger_)
https://twitter.com/fr0gger_/status/1774342248437813525

XZ Backdoor: Times, damned times, and scams - Rhea Karty & Simon Henniger [2024-03-30]
https://rheaeve.substack.com/p/xz-backdoor-times-damned-times-and
(从时区角度看liblzma后门)
(作者认为Jia Tan试图让人判定他是中国人，但作者认为Jia更可能是在UTC+02/03工作)
(评论区有不同意见)

Backdoor in XZ Utils allows RCE: everything you need to know - Merav Bar, Amitai Cohen, Danielle Aminov [2024-03-30]
https://www.wiz.io/blog/cve-2024-3094-critical-rce-vulnerability-found-in-xz-utils
(混水摸鱼之作，标题党)

***

CVE-2024-3094 XZ Backdoor: All you need to know - Shachar Menashe, Jonathan Sar Shalom, Brian Moussalli [2024-03-31]
https://jfrog.com/blog/xz-backdoor-attack-cve-2024-3094-all-you-need-to-know/
(Timeline of the attack)

The payload hooks the RSA_public_decrypt function, a function originally used for validating RSA signatures. The malicious hook code examines the RSA public modulus ("N" value) passed inside the RSA struct (4th argument of RSA_public_decrypt). Note that this modulus is completely controlled by the connecting SSH client (in our case, the attackers). The malicious hook code decrypts the "N" value with a hardcoded decryption key (using the ChaCha20 symmetric stream cipher). The decrypted data is checked for validity by using the Ed448 elliptic curve signing algorithm. Note that since this is an asymmetric signing algorithm, the backdoor contains only the public (verification) key, ensuring that only the attackers can generate valid payloads for the backdoor. Furthermore, the signature is bound to the host's public key, meaning that a valid signature for one host cannot be reused on a different host. If the data is valid, the payload is executed as a shell command by passing it to system(). If the data is invalid in any way (malformed payload, invalid signature), the original implementation of RSA_public_decrypt is resumed in a transparent manner. This means the detection of vulnerable machines over the network may be impossible for anyone besides the attackers.

The sophisticated nature of this attack and the use of highly future proof crypto algorithms (Ed448 vs the more standard Ed25519) led many to believe that the attack may be a nation-state level cyberattack.

***

It's RCE, not auth bypass, and gated/unreplayable - Filippo Valsorda [2024-03-31]
https://bsky.app/profile/filippo.abyssdomain.expert/post/3kowjkx2njy2b

The hooked RSA_public_decrypt verifies a signature on the server's host key by a fixed Ed448 key, and then passes a payload to system(). The payload is extracted from the N value (the public key) passed to RSA_public_decrypt, checked against a simple fingerprint, and decrypted with a fixed ChaCha20 key before the Ed448 signature verification. RSA_public_decrypt is a (weirdly named) signature verification function. Why "decrypt"? RSA sig verification is the same op of RSA encryption. The RSA_public_decrypt public key can be attacker-controlled pre-auth by using OpenSSH certificates. OpenSSH certs are weird in that they include the signer's public key. OpenSSH checks the signature on parsing. Here's a script by Keegan Ryan for sending a custom public key in a certificate, which on a backdoored system will reach the hooked function.

modify_ssh_rsa_pubkey.py
https://gist.github.com/keeganryan/a6c22e1045e67c17e88a606dfdf95ae4

Apparently the backdoor reverts back to regular operation if the payload is malformed or the signature from the attacker's key doesn't verify. Unfortunately, this means that unless a bug is found, we can't write a reliable/reusable over-the-network scanner. To clarify, by "gated" I mean it takes the attacker's private key to use the backdoor (it's NOBUS); by "unreplayable" I mean that even if we observe an attack against one host, we can't reuse it against another host (the attacker's signature is bound to the host public key, but not to the command).

***

Information about the liblzma (xz-utils) backdoor - karcherm [2024-03-31]
https://github.com/karcherm/xz-malware
(Stuff discovered while analyzing the malware hidden in xz-utils 5.6.0 and 5.6.1)
(从.o中还原了字符串)

I am a reverse engineer, and tried some static analysis on that code. One key feature is that the code does not contain any ASCII strings, neither in clear text nor in obfuscated form. Instead, it recognizes all relevant strings using one single deterministic finite automaton, a technique
commonly used to search for terms given by regular expressions.

I wrote a script that decodes the tables for the table-driven DFA and outputs the strings recognized by it accompanied with the "ID" assigned to the terminal accepting state that represents that string.

***

XZ Backdoor Analysis and symbol mapping - smx-smx
https://gist.github.com/smx-smx/a6112d54777845d389bd7126d6e9f504
(巨NB的逆向工程，解释5.6.0的.o中那些符号实际是啥意思)

看完所有liblzma后门的技术分析，有些感慨。Hook就位之后的逆向工程已经被一些天才的安全人员完成并公布，攻击方与分析方都巨NB。

作为围观群众，我在等待分析方展示一下，Hook是怎么安装上去的，就是说.so被加载后，怎么就call到那个.o里去了，我问的是第一次。换句话说，是不是与IFUNC相关，究竟怎么完成第一步Hook？

就目前展现出来的状态看，疑似国家级APT，不是一般的NB。但是，架不住全世界的顶级安全人员分析一个死样本啊。能分析清楚那个.o的，都是顶级水准，深表佩服。

必须感谢 Andres Freund & Florian Weimer，若非他们发现并披露了liblzma后门，灾难性的后果不敢想像。某种意义上，大伙儿这次运气不错，在真核弹级安全灾难出现前被摁住了。可，我们能总是这么运气不错吗？

从攻击方看，名字什么的隐隐指向东方，但个人不负责任地胡说两句，不像。咱就不说技术水平、细致程度这些方面，只说一个「隐忍」。这种「前人种树、后人乘凉、功成不必在我、风物长宜放眼量」的远见卓识及具体实施，放在1949年之前的心怀信仰的那个群体中，我信；放在当下东方体制内网络安全团队身上，说实话，我不大信。你要问我凭什么这么说，我还真不敢回答你。但我相信，会有不少我并不认识的同行认可我的观点。