Your AI Coding Assistant Can Be Hijacked Just By Asking It to Read a Webpage

by | Sep 9, 2026 | Articles | 0 comments

Your AI Coding Assistant Can Be Hijacked Just By Asking It to Read a Webpage

 

Why poor tech infrastructure can cost investors millions and how to spot it early 

If you or your dev use Claude Code, Anthropic’s AI coding agent, in its default “Auto Mode” on supported plans, a security researcher has shown it can be tricked into downloading and running attacker-controlled code up to 80% of the time.

The trigger is as mundane as asking it to summarise a website.

This isn’t a theoretical vulnerability. It’s a working exploit chain, published with a full technical write-up and video demo on 28 August 2026. Anthropic’s response, reported by The Register, was that the behaviour is “working as designed.”

What actually happened

Security researcher Johann Rehberger, known as “wunderwuzzi” and a repeat finder of prompt-injection flaws in AI coding tools, demonstrated that Claude Code running Opus 5 in Auto Mode can be manipulated through a chain of individually harmless-looking steps:

  1. You ask Claude Code to summarise a website that presents itself as an innocuous “archive of notebook records.”

  2. The site is built so that Claude’s normal web-fetch tool fails with a deliberate 415 error. That nudges the AI to fetch the page directly using a command-line tool (curl), without ever being told to do so.

  3. That request redirects to a ZIP file containing what looks like ordinary data plus a maliciously named Python file (struct.py). This exploits a technique called Python module shadowing, where a local file with the same name as a standard code library silently takes priority over the real one.

  4. Claude’s own safety guardrails correctly refuse to run the obviously suspicious file it was given. But in trying to be helpful, it decides to write its own replacement code to read the data. That self-written code is what triggers the malicious file instead.

  5. From there, the exploit can launch a hidden second AI agent with its own file and system access. In Rehberger’s tests, it could silently explore the machine, open applications and execute a real remote payload.

Across his tests, Rehberger reported success rates of 60–80%. Anthropic did not respond to The Register’s request for comment, but reportedly told Rehberger that Auto Mode’s built-in safety classifier is “a convenience feature … not a security guarantee,” and that the real defence has to be running these agents inside a sandboxed environment with restricted file and network access.

Why this matters if you’re not “technical”

This is exactly the kind of situation CYP’s clients can find themselves in. You’re not writing all the code yourself, an AI coding assistant is doing a lot of the heavy lifting, and you may have no realistic way to independently audit whether it’s configured safely.

“The AI wrote it” and “the AI secured it” are two different claims.

This incident shows that even a vendor’s own default settings can leave a real gap. It doesn’t require unusual behaviour from the user either. In this case, the trigger was simply asking the tool to look at a webpage, something founders and developers do regularly during research or competitor scanning.

The risk for an early-stage company is straightforward. An AI coding agent may have access to your codebase, your terminal and, in some cases, cloud credentials. A compromised agent session could exfiltrate API keys, customer data, or your source code itself.

What to actually do this week

You don’t need to stop using AI coding tools. You need to reduce what a compromised session can reach.

  1. Ask your dev, or check yourself, whether Claude Code or a similar agentic coding tool is running in an unrestricted “auto” mode that lets it execute shell commands and fetch arbitrary URLs without confirmation.

  2. Run agentic coding tools in a sandboxed or isolated environment where possible. That could be a disposable container or VM, rather than a machine with production credentials or customer database connection strings sitting in environment variables.

  3. Rotate any API keys or credentials that live in the same environment as an AI coding agent on a routine schedule, not just after an incident.

  4. Treat “summarise this website” requests to an AI coding agent as a genuine attack surface, not simply a harmless research shortcut. The trigger here required no unusual user behaviour at all.

Sources

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