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CVE-2026-39923 – Authentication Bypass (Password Reset Token Expiry Bypass / Account Takeover) – Fla

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Summary

CVE-2026-39923 is a critical authentication bypass in the Flarum forum framework. The SavePasswordController::handle() method fails to validate the expiration of password reset tokens, allowing an unauthenticated attacker to reuse an expired reset token by submitting it directly to the reset processing endpoint and take over any user account. The flaw affects all versions of Flarum before 1.8.16 and is rated critical.

Technical details

  • Root cause: The password reset flow does not enforce token expiry. SavePasswordController::handle() accepts a reset token without checking whether it has passed its lifetime (CWE-324: Use of a Key Past its Expiration Date).
  • Trigger conditions: An attacker submits a previously issued but expired password reset token directly to the reset processing endpoint (POST /reset), bypassing the intended 24-hour token validity window.
  • Attack vector: Network, unauthenticated. No prior privileges or user interaction with the victim are required.
  • Impact: Reset of an arbitrary user’s password, leading to full account takeover, including administrator accounts, with corresponding loss of confidentiality and integrity.

Affected software

  • Flarum (flarum/framework) — all versions prior to 1.8.16.

Severity

  • CVSS v4.0: 9.2 (Critical) — CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
  • CVSS v3.1: 8.1 (High) — CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Upgrade to Flarum 1.8.16 or later, which enforces an expiry check on password reset token submission (PR #4545).
  • If no patch can be applied immediately: Restrict network access to the reset endpoint where feasible, monitor for repeated or anomalous requests to the password reset processing endpoint, and require affected users to request fresh reset tokens after upgrading.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • Raw response body: <div id="flarum-loading"
  • JavaScript objects present once the page runs: app.cache.discussionList, app.forum.freshness

References

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How IONIX’s External Exposure Management Platform Detects and Validates
Zero-Days to Shrink MTTR

1

Map your entire attack surface (continously)

IONIX uses multi-factor discovery methods, including DNS analysis, certificate mapping, metadata inspection, and more, to automatically map every internet-facing asset across your environment. This includes cloud instances, third-party platforms, shadow IT, and even forgotten infrastructure that traditional tools miss.

2

Monitor for new CVEs

Dozens of threat intel feeds using agentic technology are continuously analyzed to detect the appearance of proof-of-concept code, exploit kits, and indicators of active targeting. IONIX goes further by applying AI to proactively evaluate whether emerging vulnerabilities are likely to be exploited, even before PoCs go public.

3

Identify Potential External Exposures

Not all CVEs matter. IONIX filters vulnerabilities by asking attacker-centric questions: Can it be reached from the internet? Does it require authentication? Is it being exploited in the wild? This dramatically reduces noise and focuses teams on threats that can actually be weaponized.

4

Create Safe, Scalable Exploit Validations

IONIX transforms real-world PoCs into safe, non-intrusive test payloads that can be run in production environments without disruption. These simulations are precisely targeted to the systems that are vulnerable, ensuring rapid validation without unnecessary load.

5

Execute Exploit Validations

By combining context about software stack, versioning, exposure status, and reachability, IONIX ensures that only the right payloads are executed against the right assets, maximizing efficiency and minimizing risk.

6

Drive Fast and Actionable Remediation

Results are routed through integrations with ticketing, SOAR, and SIEM tools. Issues are written in plain language, bundled into remediation clusters, and prioritized based on asset criticality, exploitability, and blast radius. This shortens mean time to remediation (MTTR) and empowers teams to act with confidence.

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  • Mapping of all Assets with this Technology
  • Identification of potentially exposed assets to this CVE
  • Confirmation of verified exploitable assets
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