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CVE-2026-55761 medium CVSS 5.9

Authentication Bypass in github.com/portainer/portainer (CVE-2026-55761)

Summary

Authentication bypass in github.com/portainer/portainer (CVE-2026-55761). Data can be tampered with by attackers. Exploitable via bouncer.PublicAccess. Mitigation: upgrade to 2.43.0 or later.

Original description

The description as published by the source.

Summary

Portainer supports restoring an instance from a backup archive via the /api/restore endpoint. This endpoint is intentionally unauthenticated to allow restoring before the first admin account is created, and remains accessible for the five-minute initialization window that opens each time Portainer starts. Any unauthenticated attacker with network access to a Portainer instance that has not yet been initialised can exploit this window to replace the Portainer database with a crafted archive containing attacker-controlled credentials and gain full administrative access. The same unauthenticated setup window also exposes the administrator-account-creation endpoint (/api/users/admin/init), which an attacker can call to create the first administrator directly; the fix gates both endpoints.

The attack requires the instance to be uninitialized, reachable by the attacker, and within the five-minute window. Once that window expires without initialization, Portainer locks its API and requires a restart to re-enable setup — each restart opens a fresh window. No credentials, session tokens, or local access are required.

Severity

High

The endpoint requires no authentication and no user interaction, but successful exploitation depends on three conditions holding simultaneously: the instance must be uninitialized, reachable from the attacker's network, and within the five-minute setup window that Portainer enforces before locking the instance pending a restart. Once t…

Source: NIST National Vulnerability Database 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-2026-55761 has been found in github.com/portainer/portainer.
Attackers can target a specific entry point like bouncer.PublicAccess over the network to misuse the product.
Data can be tampered with by attackers. CVSS score: 5.9/10.
What to do: upgrade github.com/portainer/portainer to 2.43.0 or later.
If unsure, ask your IT team or search "github.com/portainer/portainer CVE-2026-55761" on the vendor's site.

CVE-2026-55761 (github.com/portainer/portainer) — CWE-287 / CVSS v3 5.9
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: bouncer.PublicAccess / develop
Patched: 2.43.0 — apply immediately
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

Authentication bypass (CWE-287) exists in github.com/portainer/portainer. Attackers reach the vulnerable code path via bouncer.PublicAccess without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of github.com/portainer/portainer. Attack surface: bouncer.PublicAccess / develop.

🔥 Severity

Severity: Medium (CVSS 5.9/10). Data can be tampered with by attackers.

🔧 How to fix

Update to 2.43.0.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'github.com/portainer/portainer' . 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/portainer/portainer' . | 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/portainer/portainer 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 patch patch 
    Upgrade github.com/portainer/portainer to 2.43.0

    ステージング環境で 2.43.0 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。

  3. 3
    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/portainer/portainer
[{"type":"SEMVER","events":[{"introduced":"2.40.0"},{"fixed":"2.43.0"}]}]

References

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