Suspicious input transformation (reflected)
Summary: PortSwigger describes suspicious input transformation as occurring when an application receives user input, transforms it - such as decoding UTF-8 or URL-encoding, or processing backslash escape sequences - and then performs further processing on the result. The transformation is not a vulnerability by itself, but can let an attacker bypass input filters by encoding payloads that are decoded after filtering, or consume the start of concatenated data by ending input with the start of a multi-character encoding or escape sequence.
Why it matters: The bypass hinges on ordering: if decoding happens after your input filters run, the payload your filter approved is not the payload the application processes.
Read the original at PortSwigger Research ↗
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