Vulnerability report

Active exploitation observed Confirmed confidence Not in CISA KEV

CVE-2026-28496

FOSSBilling Information Disclosure

FOSSBilling · affected before 0.8.0

Watch CVE-2026-28496 Browse KEV feed
Severity
CVSS 9.4 · Critical
Confidence
Confirmed
Exploit status
Observed in sensors
EPSS
1.9%
First observed
19 Jul 2026
Last observed
11 Oct 2026

Decision summary

What security teams need to know first

Direct answers before the deeper technical record.

What it is

CVE-2026-28496 is a vulnerability affecting FOSSBilling. FOSSBilling is a free, open-source billing and client management system. Versions prior to 0.8.0 have a Server-Side Template Injection (SSTI) vulnerability in...

Is it exploited?

Yes. Previdian sensors observed exploitation attempts with confirmed confidence. Also confirmed by third-party sources.

Who is affected?

FOSSBilling affected before 0.8.0.

What should we do?

Patch immediately, validate internet-facing exposure, and monitor for matching requests.

Overview

FOSSBilling is a free, open-source billing and client management system

FOSSBilling is a free, open-source billing and client management system. Versions prior to 0.8.0 have a Server-Side Template Injection (SSTI) vulnerability in the template rendering system. Administrators with access to features that render Twig templates (email templates, mass mail campaigns, custom payment adapters, and the `string_render` API endpoint) can inject arbitrary Twig expressions, leading to information disclosure and remote code execution. The vulnerability exists because Twig templates are rendered without a sandbox, allowing access to the full Twig environment, API context, and the application's dependency injection container.

Version 0.8.0 patches the issue. Some workarounds are available. Audit existing email templates for suspicious Twig expressions, rotate all admin and client API tokens, and/or block external access to /api/system/* at reverse proxy/WAF to mitigate chaining with GHSA-78x5-c8gw-8279.

FOSSBilling Affected
FOSSBilling
Before 0.8.0
Published
23 Jun 2026
Exploited Since
19 Jul 2026
Attack vector
Remote
Complexity
Low
Privileges
High
User interaction
None

Tags

nuclei_scanner

CVE References

Exploitation evidence

Why Previdian marks this CVE as exploited

Third-party attestation and first-party sensor observation are shown separately so teams can judge the evidence chain.

Exploited in the wild

The Shadowserver

Recorded 03 Aug 2026

Previdian independently recorded this as exploited after first seeing it in honeypot sensors.

Active exploitation observed

Previdian sensor

First observed 19 Jul 2026

Previdian first observed exploitation attempts targeting this vulnerability in our honeypot sensors.

Proof of concept available

Nuclei Templates

Recorded 24 Jun 2026

Public scanner or PoC coverage increases practical exploitability.

Known exploited vulnerability sources

Per-source evidence links for KEV attestations are available through the Previdian Pro API.

Learn about Pro API access
Source Added
The Shadowserver First 2026-08-03 00:00 UTC
Previdian Sensors 2026-08-04 13:25 UTC

Operational indicators for this CVE are listed under Detection.

Sensor telemetry

First-party evidence of exploitation activity

Previdian sensors recorded exploitation attempts targeting this vulnerability. The cards and chart show volume, unique attackers, and daily activity.

28

Attempts observed

17

Unique attacker IPs

11

Attacker countries

AF · BG · CH · ER · FR · ID · JP · NL · SC · US · YE

17

Sensors observed

CVE-2026-28496 exploitation attempts over the last 91 days

Daily events observed by Previdian sensors

Updated 17 Oct 2026

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19 Jul 7 Aug 26 Aug 14 Sep 17 Oct

First observed 19 Jul 2026 · Last observed 11 Oct 2026

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Detection

Operational artifacts and observed signals

Make the evidence actionable in scanner, SOC, and edge-control workflows.

Observed signals

Request targets
User-Agents
Callback hosts
1
7
0

Request targets and User-Agents available in Pro. Callback host details available in Enterprise.

Virtual patch status

No Previdian virtual patch is currently available. Future rules ship for ModSecurity, Cloudflare, and AWS WAF.

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Attacker IP Indicators

Attacker IP indicators observed · available in Pro and Enterprise.

Sensor-derived attacker IP indicators are available to Pro and Enterprise accounts under Detection and through the Pro API.

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Scanner Artifacts

Scanner and exploit-framework references linked to this CVE.

Scanner Reference Detected
Nuclei https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2026/CVE-2026-28496.yaml 24 Jun 2026

Risk and context

Severity, weaknesses, and research context

CVSS v4.0

9.4 Critical

Potential damage if exploited. Separate from whether attackers are using it.

CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H

EPSS

1.9%

Estimated chance of exploitation in the next 30 days. Previdian's warning comes from evidence, not this score.

Weaknesses

Improper Neutralization of Special Elements Used in a Template Engine CWE-1336

Potential Proof of Concepts

These PoCs are unverified and could contain malware. Use at your own risk.

FOSSBilling - Server-Side Template Injection

nuclei · Created Unknown

Timeline

From disclosure to observed exploitation

  1. 00:00 UTC

    Added to Previdian KEV Feed

    High-confidence, third-party attested exploitation

  2. 02:12 UTC

    Indicators of compromise added (17)

    Indicators of compromise recorded

  3. 02:12 UTC

    Observed by Previdian sensors

    Evidence-backed exploitation signal

  4. 04:30 UTC

    Public PoC available

    Public proof-of-concept code published

  5. 04:30 UTC

    Nuclei template available

    Scanner coverage available

  6. 14:20 UTC

    CVE published

    Vulnerability disclosed publicly

  7. 20:57 UTC

    CVE ID reserved

    Identifier reserved by the CNA

Pro API

Automate this intelligence

Confidence, exploit status, sensor telemetry, PoCs, scanner integrations, mentions, and tags are available programmatically for VM, SOC, and CTI workflows.

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  • Evidence confidence and provenance
  • First-party sensor telemetry
  • PoC and scanner references
  • Affected versions and enrichment
  • Automation-ready JSON delivery

GET /api/v2/pro/kevs/CVE-2026-28496

Sample API response 
{
  "cve_id": "CVE-2026-28496",
  "confidence": "Confirmed",
  "cvss_score": 9.4,
  "cvss_estimated": false,
  "epss_score": 0.01905,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": true
  },
  "sensor_telemetry": { "attempts": 28, "sensors": 17 }
}