User redirection via uncollapsed leading slashes
Published Aug 23, 2022 · Updated Dec 17, 2025
Open redirect in affected Python 3 http.server releases allows remote attackers to redirect users via paths beginning with multiple slashes. BaseHTTPRequestHandler.parse_request retains the leading slashes, allowing a directory redirect to emit a schemeless-host value in the 301 Location header. Exploitation requires an exposed http.server instance, a request resolving to an existing directory, and a user who follows the crafted URL; the redirect can send that user to an attacker-chosen host.
Summary
What happened
Open redirect in affected Python 3 http.server releases allows remote attackers to redirect users via paths beginning with multiple slashes. BaseHTTPRequestHandler.parse_request retains the leading slashes, allowing a directory redirect to emit a schemeless-host value in the 301 Location header. Exploitation requires an exposed http.server instance, a request resolving to an existing directory, and a user who follows the crafted URL; the redirect can send that user to an attacker-chosen host.
The record
- CVE
- CVE-2021-28861
- Published
- Aug 23, 2022
- Updated
- Dec 17, 2025
- Vendor
- Unknown vendor
- Product
- Unknown product
- Classifications
- CWE-601, T1204.001
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 23, 2022CVE publishedPublication date reported by the CVE source.
- Dec 17, 2025Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
What conditions does exploitation require?
What is affected?
Affected products and versions are unavailable in this record.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- http.server schemeless-host redirect regression testproof of concept · validated
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Technologies
Your stack
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Your stack
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