vulnerability / NVI-00007001 / Revision 4
Flarum: Path traversal in LESS parser via theme color settings (incomplete fix for CVE-2023-27577)
OSV carries this publisher advisory for flarum/core, flarum/core. This is an aggregated source copy; consult the original publisher and compare update dates.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
Published affected context helps an owner decide what evidence to gather. This report has not examined your application.
4.9 MEDIUM (3.1)
security-advisories@github.com severity; no combined Null risk score.
0.472%
FIRST EPSS model v2026.06.15; exploitation probability over 30 days, percentile 0.40789. Missing scores are unknown.
As of · FIRST EPSS
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
flarum/core
source listedECOSYSTEM: introduced 0; fixed 1.8.16
## Summary Flarum's patch for CVE-2023-27577 restricted the `@import` and `data-uri()` LESS features in the `custom_less` setting, but the same restriction was never applied to other settings registered as LESS config variables (for example `theme_primary_color` and `theme_secondary_color`, as well as any key registered via `Extend\Settings::registerLessConfigVar()`). Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary `@import` directive into the compiled `forum.css`. Because the underlying LESS parser honours `@import (inline) ''`, an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery). ## Impact An attacker who has compromised — or legitimately obtained — an administrator account can: - **Read arbitrary local files** reachable by the PHP process (e.g. `/etc/passwd`, `.env`, config files containing database credentials, OAuth secrets, API keys). - **Trigger outbound HTTP/HTT
flarum/core
source listedECOSYSTEM: introduced 2.0.0-beta.1; fixed 2.0.0-rc.1
## Summary Flarum's patch for CVE-2023-27577 restricted the `@import` and `data-uri()` LESS features in the `custom_less` setting, but the same restriction was never applied to other settings registered as LESS config variables (for example `theme_primary_color` and `theme_secondary_color`, as well as any key registered via `Extend\Settings::registerLessConfigVar()`). Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary `@import` directive into the compiled `forum.css`. Because the underlying LESS parser honours `@import (inline) ''`, an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery). ## Impact An attacker who has compromised — or legitimately obtained — an administrator account can: - **Read arbitrary local files** reachable by the PHP process (e.g. `/etc/passwd`, `.env`, config files containing database credentials, OAuth secrets, API keys). - **Trigger outbound HTTP/HTT
03 / Response
What to do next
- vendor patch
The original advisory lists fixed boundaries 1.8.16 for flarum/core. Confirm the publisher’s latest guidance.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. GitHub Advisory Database via OSV
- vendor patch
The original advisory lists fixed boundaries 2.0.0-rc.1 for flarum/core. Confirm the publisher’s latest guidance.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. GitHub Advisory Database via OSV
- investigate
Identify the exact component and deployed version, examine the linked publisher’s conditions, and retain evidence of any mitigation.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. FIRST EPSS
A fixed release is not deployment evidence.
Check current vendor support and compatibility. After making a change, verify the deployed build and relevant runtime conditions. Keep the evidence, owner and time. An updated lockfile alone does not prove what is running.
04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
- NVD lastModified does not establish freshness of every associated CPE record.
05 / Provenance
Sources you can inspect
Each assertion retains its source. Mirrored records may repeat one underlying publisher; additional links are not independent corroboration.
- GHSA-xjvc-pw2r-6878 ↗
GitHub Advisory Database via OSV · vendor advisory
Observed . GitHub-origin advisory, CC-BY-4.0; OSV is an aggregate copy, not independent corroboration.
- CVE-2026-41887 ↗
NIST National Vulnerability Database · official catalog
Observed . This product uses data from the NVD API but is not endorsed or certified by the NVD.
- CVE-2026-41887 ↗
FIRST EPSS · official catalog
Observed . FIRST EPSS; probability of exploitation in the next 30 days, not local compromise. Dataset date/model retained; FIRST services terms apply.
Public revision history
Only published revisions appear here. Routine fetches do not create a material-change event.
- Revision 4 ·
Dated EPSS estimate updated; this is not a new disclosure or proof of exploitation.
- Revision 3 ·
Source facts changed; compare the preserved prior revision.
- Revision 2 ·
Dated EPSS estimate updated; this is not a new disclosure or proof of exploitation.
- Revision 1 ·
First indexed by Null; source disclosure date is shown separately.