V2: Coder: Route hijacking through lack of validation of agent-supplied AllowedIPs in tailnet coordinator (CVE-2026-55428)
### Summary 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. ### Impact 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. ### Patches The fix validates each `AllowedIPs` prefix against the authenticating agent's UUID just like `Addresses`. The fix was backported to all supported release lines: | Release line | Patched version | |---|---| | 2.34 | [v2.34.2](https://github.com/coder/coder/releases/tag/v2.34.2) | | 2.33 | [v2.33.8](https://github.com/coder/coder/releases/tag/v2.33.8) | | 2.32 | [v2.32.7](https://github.com/coder/coder/releases/tag/v2.32.7) | | 2.29 (ESR) | [v2.29.17](https://github.com/coder/coder/releases/tag/v2.29.17) | ### Workarounds Operators who cannot upgrade immediately should monitor coordinator logs for agents advertising unexpected `AllowedIPs` prefixes. ### Resources - Fix: #26144 ### Credits Coder would like to thank Anthropic's Security Team (ANT-2026-22451) for independently disclosing this issue!
AI Analysis
Technical Summary
The tailnet coordinator in Coder validates that an agent's Addresses correspond to its authenticated UUID but does not apply the same validation to AllowedIPs. As a result, a malicious workspace agent can advertise arbitrary AllowedIPs prefixes, including those belonging to other agents. Since routing is based on tailnet IPs, this enables the malicious agent to intercept and spoof traffic destined for other agents. Exploitation requires an authenticated user with a running workspace and a modified agent binary. The issue is fixed by validating AllowedIPs prefixes against the authenticating agent's UUID, with patches backported to all supported release lines.
Potential Impact
A malicious authenticated agent can hijack network routes by advertising IP prefixes assigned to other agents, allowing interception and spoofing of web terminal and workspace application traffic. This compromises confidentiality and integrity of communications within the tailnet. The vulnerability does not affect availability. Exploitation requires authentication and a modified agent binary, limiting the attack surface to insiders or compromised users.
Mitigation Recommendations
Official patches are available and have been backported to all supported release lines: 2.34.2, 2.33.8, 2.32.7, and 2.29.17. Operators should upgrade to these versions to remediate the vulnerability. For those unable to upgrade immediately, monitoring coordinator logs for agents advertising unexpected AllowedIPs prefixes is recommended as a temporary mitigation.
V2: Coder: Route hijacking through lack of validation of agent-supplied AllowedIPs in tailnet coordinator (CVE-2026-55428)
Description
### Summary 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. ### Impact 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. ### Patches The fix validates each `AllowedIPs` prefix against the authenticating agent's UUID just like `Addresses`. The fix was backported to all supported release lines: | Release line | Patched version | |---|---| | 2.34 | [v2.34.2](https://github.com/coder/coder/releases/tag/v2.34.2) | | 2.33 | [v2.33.8](https://github.com/coder/coder/releases/tag/v2.33.8) | | 2.32 | [v2.32.7](https://github.com/coder/coder/releases/tag/v2.32.7) | | 2.29 (ESR) | [v2.29.17](https://github.com/coder/coder/releases/tag/v2.29.17) | ### Workarounds Operators who cannot upgrade immediately should monitor coordinator logs for agents advertising unexpected `AllowedIPs` prefixes. ### Resources - Fix: #26144 ### Credits Coder would like to thank Anthropic's Security Team (ANT-2026-22451) for independently disclosing this issue!
CVSS v3.1
Score 8.2high
Affected software
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AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The tailnet coordinator in Coder validates that an agent's Addresses correspond to its authenticated UUID but does not apply the same validation to AllowedIPs. As a result, a malicious workspace agent can advertise arbitrary AllowedIPs prefixes, including those belonging to other agents. Since routing is based on tailnet IPs, this enables the malicious agent to intercept and spoof traffic destined for other agents. Exploitation requires an authenticated user with a running workspace and a modified agent binary. The issue is fixed by validating AllowedIPs prefixes against the authenticating agent's UUID, with patches backported to all supported release lines.
Potential Impact
A malicious authenticated agent can hijack network routes by advertising IP prefixes assigned to other agents, allowing interception and spoofing of web terminal and workspace application traffic. This compromises confidentiality and integrity of communications within the tailnet. The vulnerability does not affect availability. Exploitation requires authentication and a modified agent binary, limiting the attack surface to insiders or compromised users.
Mitigation Recommendations
Official patches are available and have been backported to all supported release lines: 2.34.2, 2.33.8, 2.32.7, and 2.29.17. Operators should upgrade to these versions to remediate the vulnerability. For those unable to upgrade immediately, monitoring coordinator logs for agents advertising unexpected AllowedIPs prefixes is recommended as a temporary mitigation.
Technical Details
- Gcve Source
- db.gcve.eu
- Osv Id
- GHSA-wrq8-fcv5-8hvp
- Osv Schema Version
- 1.4.0
- Aliases
- ["CVE-2026-55428"]
- Ecosystems
- ["Go"]
- Database Specific Severity
- HIGH
- Cvss Version
- 3.1
Threat ID: 6a4c340327e9c797195f5f45
Added to database: 07/06/2026, 23:02:27 UTC
Last enriched: 07/06/2026, 23:12:46 UTC
Last updated: 07/31/2026, 19:22:59 UTC
Views: 47
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