CVE-2026-53435 — Jenkins Deserialization → Arbitrary File Read (PoC)

First public proof-of-concept for CVE-2026-53435, built when only the advisory existed and no PoC was published anywhere. Reconstructed from a vague one-line advisory over ~8.5 hours on a Friday night — not by writing the exploit by hand, but by directing an AI through the dead ends, pivots, and verification until a working chain fell out.

The PoC reaching /etc/passwd on the controller — arbitrary file read confirmed.

Why this repo exists

This is an experiment in showing the process, not just the artifact.

For years, an offensive researcher's output was a single finished exploit — the 8 hours of wrong turns that produced it vanished. This repo keeps the wrong turns. The full, lightly-edited transcript of a human + AI working a fresh n-day from advisory to working PoC is included as an appendix.

The point it makes is deliberately not "the AI did it." The 8 hours are the evidence that it didn't. The AI never one-shot the chain. What carried the work was domain expertise — knowing the advisory was hiding a deserialization sink, knowing to look in Jenkins core types rather than plugins, knowing that a DescribableList pre-patch wouldn't enforce its element type, and knowing how to verify a claim instead of trusting it. The researcher's value didn't disappear; it moved from typing the exploit to steering, pruning, and validating.

That shift is the thing worth documenting.

The vulnerability (root cause)

Jenkins protects deserialization with a custom ClassFilter that only permits types defined in Jenkins core or plugins. CVE-2026-53435 is that this is not enough: an attacker who can POST a config.xml can get Jenkins to deserialize an arbitrary core/plugin type into a context that never expected it, and then reach that object over HTTP via Stapler routing.

This PoC plants a hudson.Plugin$DummyImpl (a Jenkins-core type, so it passes the location whitelist) into a ListView's <properties> list — a DescribableList<ViewProperty> that, pre-patch, does not enforce its element type. The planted object carries baseResourceURL=file:/. Routing an HTTP request to it then serves files straight off the controller's filesystem.

A generic ysoserial chain does not work here — the gadget must be a Jenkins/plugin-resident type to survive the filter. That constraint is the whole game.

Scope & responsible disclosure

• This PoC demonstrates the arbitrary file read impact only.
• The advisory also describes user impersonation and Script Console RCE via the same primitive. Those chains were reached during research but are intentionally withheld here: the CVE was patched only days before publication, and a turnkey RCE would mainly serve mass exploitation of unpatched instances. File read is sufficient to prove the impact.
• The vulnerability is already patched. All testing was done against a local, isolated lab (see below) on the author's own machine. No external or third-party systems were touched.

Use only against systems you own or are explicitly authorized to test.

Proof: it works on vulnerable, fails on patched

The exploit was run against two identical-config containers — vulnerable 2.555.2 and patched 2.555.3 — to confirm it exercises this CVE and not some unrelated file-read primitive:

The patched build accepts the request but the fix rejects the planted type at deserialization time, so the properties list comes back empty and there is nothing to route to. The failure lands exactly on the mechanism the advisory describes — which is what makes this a genuine PoC for CVE-2026-53435.

Usage

python3 exploit_cve_2026_53435_v2.py <base_url> <user> <pass> <remote_file> [view_name]

# example (against your own lab):
python3 exploit_cve_2026_53435_v2.py http://127.0.0.1:8080 <user> <pass> /etc/passwd

• base_url — http://host:port of the target (plain HTTP; do not prefix https unless the instance actually terminates TLS).
• requires an account with permission to POST a view config.xml (View/Configure), plus Overall/Read.

Reproduce the lab

docker compose -f lab/docker-compose.yml up -d
#   vulnerable:  http://127.0.0.1:8080   (Jenkins LTS 2.555.2)
#   patched:     http://127.0.0.1:8081   (Jenkins LTS 2.555.3, negative control)

The lab provisions a local-only user database via Groovy init so the authenticated-low-privilege path can be exercised without an external IdP.

Timeline — ~8.5 hours, one Friday night

2026-06-12 (Fri)  15:00 → 23:23   ≈ 8h 20m

  15:00–15:12   Lab built (docker: vuln 2.555.2 / patched 2.555.3) + first recon
  15:12–21:00   Deserialization sink & gadget analysis
                  (Commons-Collections bypass attempts failed → pivot
                   from properties to actions and back)
  21:00–22:30   ClassFilter analysis + gadget re-hunt, restricted to core types
  22:30–23:00   Working PoC (v1 → v2) + differential verification (vuln vs patched)

Most of those hours were analysis, not typing. The exploit itself came together in the last ~30 minutes — once the right sink and gadget were known.

How this was built

Built with Claude Code (Anthropic), driven interactively by the author:

• Main model: Claude Opus 4.8 (Claude Max)
• Portions: Claude Fable 5 (Mythos-class tier)

The complete, lightly-edited working transcript is in transcript/ — kept raw, including the dead ends. (Edited only to soften a few lines of tone and to mask local network details; nothing technical was removed.)

Disclaimer

For authorized security testing, research, and education only. The author and contributors accept no liability for misuse. The targeted vulnerability is patched — update to Jenkins LTS 2.555.3 / weekly 2.568 or later.