CVE-2026-55428High· 8.2▾ TwilightCoder: Route hijacking through lack of validation of agent-supplied AllowedIPs in tailnet coordinator
▾ Twilight zone — High severity, or a signal on a lesser flaw
impact 45.1 · likelihood 0.1 · exploitation 0
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Exploit-prediction probability, daily snapshots since Jul 8.
Disclosure to exploitation, from the record and what we observed since indexing it.
Disclosed via GHSA
0.4%
The tailnet coordinator validates that an agent's Addresses derive from its authenticated UUID but applies no equivalent check to AllowedIPs. The coordinator forwards agent-supplied AllowedIPs verbatim to tunnel peers which install them into the WireGuard peer configuration.
A malicious workspace agent can advertise arbitrary AllowedIPs prefixes including another agent's tailnet address. Coder's ServerTailnet routes to agents by tailnet IP so an agent that claims a victim's prefix can intercept web terminal and workspace app traffic and serve spoofed content. Exploitation requires an authenticated user with a running workspace and a modified agent binary.
The fix validates each AllowedIPs prefix against the authenticating agent's UUID just like Addresses.
The fix was backported to all supported release lines:
Operators who cannot upgrade immediately should monitor coordinator logs for agents advertising unexpected AllowedIPs prefixes.
Coder would like to thank Anthropic's Security Team (ANT-2026-22451) for independently disclosing this issue!
github.com/coder/coder/v2 >= 2.34.0, < 2.34.2github.com/coder/coder/v2 >= 2.33.0, < 2.33.8github.com/coder/coder/v2 >= 2.30.0, < 2.32.7github.com/coder/coder/v2 < 2.29.17Upgrade to a patched release:
github.com/coder/coder/v2 2.34.2github.com/coder/coder/v2 2.33.8github.com/coder/coder/v2 2.32.7github.com/coder/coder/v2 2.29.17Connected by shared product, vendor, weakness, or advisory.
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