Anubis has a bot protection bypass when a sophisticated attacker asks to pass a challenge of difficulty 0

Vulnerability Description

Anubis is a tool that allows administrators to protect bots against AI scrapers through bot-checking heuristics and a proof-of-work challenge to discourage scraping from multiple IP addresses. Anubis allows attackers to bypass the bot protection by requesting a challenge, formulates any nonce (such as 42069), and then passes the challenge with difficulty zero. Commit e09d0226a628f04b1d80fd83bee777894a45cd02 fixes this behavior by not using a client-specified difficulty value.

References

• https://github.com/Xe/x/security/advisories/GHSA-56w8-8ppj-2p4f
• https://github.com/Xe/x/commit/7bd7b209f4f1b897de85ec8973458dc8be606a8b
• https://github.com/Xe/x/commit/e09d0226a628f04b1d80fd83bee777894a45cd02
• https://xeiaso.net/notes/2025/GHSA-56w8-8ppj-2p4f