# Server-side parameter pollution in a constructed request

An outer input injects or alters parameters in a server-created downstream request.

**Candidate CWE:** [CWE-235](https://cwe.mitre.org/data/definitions/235.html)

## Test hypothesis

Trace an observed service call to a local stub. Submit one outer value containing a controlled delimiter marker and compare the downstream request with normal and correctly encoded controls. Use a source-confirmed synthetic field to establish changed policy semantics.

## Evidence required

Outer request bytes, decoding and concatenation order, actual downstream parameter map, and a controlled action/data boundary crossed. Distinguish input delimiter injection from merely providing a documented extra top-level parameter.

## Rule out false positives

A marker remaining inside one correctly encoded value, an intentionally supported parameter, or changed output without an observed downstream request boundary is not enough.

## Playbook and command references

- [Request values: duplicate parameters, parser conflicts and downstream queries](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/modules/request-value-parsing.html) — Trace the value a security layer validates through to the value the application or downstream service actually consumes.
- [Trace input into queries, commands, and templates](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/modules/injection-tracing.html) — Prove interpretation at a specific sink using small paired experiments instead of payload spraying.

[Find the associated command snippets](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/commands.html?q=Request%20values%3A%20duplicate%20parameters%2C%20parser%20conflicts%20and%20downstream%20queries)

## Referenced by these target playbooks

[Web & API](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/web-api.html) · [OSS & self-hosted](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/oss.html) · [Cloud & containers](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/cloud.html) · [AI, agents & MCP](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/ai.html)

## Report checklist

- [ ] Capture the affected version, configuration, identities, and object ownership.
- [ ] Preserve a valid baseline and a negative control.
- [ ] Reproduce the forbidden data access, action, or state change on controlled fixtures.
- [ ] Explain the intended security rule and why the observed behavior violates it.
- [ ] State prerequisites, uncertainty, cleanup, and the smallest demonstrated impact.

## Conditional ATT&CK mapping

No direct ATT&CK mapping selected. Record the weakness and demonstrated impact without forcing an adversary-behavior classification. See the [mapping guide](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/attack.html).

## Primary references

- [PortSwigger server-side parameter pollution](https://portswigger.net/web-security/api-testing/server-side-parameter-pollution)
- [CWE-235 improper handling of extra parameters](https://cwe.mitre.org/data/definitions/235.html)

[Back to the complete vulnerability catalog](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/vulnerabilities.html)