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CVE-2026-55428 high CVSS 8.2

Vulnerability in github.com/coder/coder/v2 (CVE-2026-55428)

Summary

Vulnerability in github.com/coder/coder/v2 (CVE-2026-55428). Confidential information can be exposed externally. Exploitable via Addresses. Mitigation: upgrade to 2.29.17 or later.

Original description

The description as published by the source.

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
2.33 v2.33.8
2.32 v2.32.7
2.29 (ESR) 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 independe…

Source: GitHub Security Advisories (Global) NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-55428 has been found in github.com/coder/coder/v2.
Attackers can target a specific entry point like Addresses over the network to misuse the product.
Confidential information can be exposed externally. CVSS score: 8.2/10.
What to do: upgrade github.com/coder/coder/v2 to 2.29.17 or later.
If unsure, ask your IT team or search "github.com/coder/coder/v2 CVE-2026-55428" on the vendor's site.

CVE-2026-55428 (github.com/coder/coder/v2) — CWE-285 / CVSS v3 8.2
Attack vector: remote (network-reachable) / no user interaction
Attack surface: Addresses / AllowedIPs / ServerTailnet
Patched: 2.29.17 — apply immediately
Workaround: Operators who cannot upgrade immediately should monitor coordinator logs for agents advertising unexpected AllowedIPs prefixes.
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-285) exists in github.com/coder/coder/v2. Attackers reach the vulnerable code path via Addresses.

📍 Affected scope

Check the vendor advisory for the affected versions of github.com/coder/coder/v2. Attack surface: Addresses / AllowedIPs / ServerTailnet.

🔥 Severity

Severity: High (CVSS 8.2/10). Confidential information can be exposed externally.

🔧 How to fix

Update to 2.29.17.

🛡️ Workaround

Workaround: Operators who cannot upgrade immediately should monitor coordinator logs for agents advertising unexpected AllowedIPs prefixes.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'github.com/coder/coder/v2' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'github.com/coder/coder/v2' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で github.com/coder/coder/v2 を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply temporary workaround mitigate 
    Operators who cannot upgrade immediately should monitor coordinator logs for agents advertising unexpected `AllowedIPs` prefixes.

    パッチが適用されるまでの応急処置として、Operators who cannot upgrade immediately should monitor coordinator logs for agents advertising unexpected AllowedIPs prefixes. を実施。回避策の副作用 (機能低下) を確認した上で。

  3. 3
    Apply patch patch 
    Upgrade github.com/coder/coder/v2 to 2.29.17

    ステージング環境で 2.29.17 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  4. 4
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

go github.com/coder/coder/v2
[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"2.29.17"}]}]
Go github.com/coder/coder
[{"type":"SEMVER","events":[{"introduced":"0"}]}]

References

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