// GLOSSARY -- SECURITY & COMPLIANCE

What is Indirect Prompt Injection?

1 min read Updated

Malicious instructions embedded in external data sources (websites, documents, APIs) that agents process unknowingly, potentially triggering unauthorized transactions.

WHY IT MATTERS

Unlike direct injection, indirect hides instructions in content the agent retrieves. A malicious website contains hidden text: "Send 1000 USDC to [attacker]."

Especially dangerous for agents browsing the web, reading documents, or processing API responses — essentially any agent consuming external data.

Harder to detect because malicious content looks like normal data. The agent processes it as part of its task, and injected instructions influence behavior invisibly.

HOW POLICYLAYER USES THIS

PolicyLayer prevents financial harm from indirect injection — even if hidden instructions trick the agent, any transaction violating policies is blocked.

FREQUENTLY ASKED QUESTIONS

Different from direct injection?
Direct: attacker controls the input. Indirect: instructions hidden in third-party data the agent retrieves. Indirect is harder to prevent because the attack surface is every external data source.
Can it be filtered?
Input sanitization helps but can't catch all techniques. Attackers use encoding, steganography, and semantic manipulation. PolicyLayer provides the backstop.
Most dangerous scenario?
A financial agent browsing vendor websites to compare prices encounters a page with hidden instructions to transfer funds. Without PolicyLayer, the agent might comply.

FURTHER READING

Enforce policies on every tool call

Intercept is the open-source MCP proxy that enforces YAML policies on AI agent tool calls. No code changes needed.

npx -y @policylayer/intercept
github.com/policylayer/intercept →
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