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CVE-2026-48611 – Authentication Bypass – phpBB versions 3.3.0 through 3.3.16

Summary

CVE-2026-48611 is a critical authentication bypass vulnerability in phpBB, an open-source internet forum platform widely deployed across the internet, carrying a CVSS base score of 9.8. The flaw stems from improper authentication checks in phpBB’s OAuth implementation and can be exploited even when OAuth is not configured or enabled, meaning default installations are vulnerable out of the box. An unauthenticated attacker can send a single crafted HTTP request to obtain a valid session as any active user — including administrators — without knowing the target’s password and without requiring any user interaction.

Technical details

  • Root cause: Improper authentication checks in phpBB’s OAuth code path (CWE-287: Improper Authentication) that can be triggered regardless of whether OAuth is enabled or configured on the target instance.
  • Trigger conditions: No special server configuration is required; default database-authentication mode installations are fully exposed. The attack requires only a target’s username, which is publicly visible on forum member lists enabled by default.
  • Attack vector: A single unauthenticated HTTP request sent over the network (AV:N, AC:L, PR:N, UI:N). No prior credentials, no active victim session, and no user interaction are needed.
  • Impact: Full account hijacking — an attacker obtains a valid authenticated session as any chosen active user, including administrators, without supplying credentials. This results in complete compromise of confidentiality, integrity, and availability of the affected installation.

Affected software

  • phpBB versions 3.3.0 through 3.3.16

Severity

CVSS v3.0 Base Score: 9.8 (Critical)
Vector string: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate action: Upgrade to phpBB 3.3.17, released June 6, 2026. The phpBB development team has identified this as the only complete fix and has urged all administrators to upgrade immediately.
  • If immediate patching is not feasible: Restrict network-level access to phpBB installations (e.g., firewall rules or WAF policies) to limit exposure until the upgrade can be applied.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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How IONIX’s External Exposure Management Platform Detects and Validates
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1

Map your entire attack surface (continously)

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2

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3

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