Affected: Pylons webob

WebOb provides objects for HTTP requests and responses. Prior to 1.8.11, Response._make_location_absolute() in src/webob/response.py checks a Location value for a URI scheme or leading double slash before urllib.parse.urljoin() strips leading C0 control characters and spaces. An attacker-controlled value such as a space followed by a protocol-relative or absolute URL can therefore bypass SCHEME_RE and startswith("//") checks and be normalized to an off-host redirect. Request.relative_url() and webob.exc._HTTPMove subclasses, including HTTPFound, are also affected because they use the same unsafe URL joining behavior or bypass the earlier normalization path. An unauthenticated attacker who can influence an application's redirect target can send users to an attacker-controlled host for phishing or OAuth and SSO token theft, but exploitation requires the user to follow the redirect. This issue is fixed in version 1.8.11.

Affected: PyPI: webob

## Summary

This is a third follow-up to **CVE-2024-42353 / GHSA-mg3v-6m49-jhp3** and **CVE-2026-44889 / GHSA-fh3h-vg37-cc95**.

WebOb makes the `Location` header absolute when it serves a redirect. To stop a relative or protocol-relative target from redirecting users off-host, it checks the value for a URI scheme and for a leading `//`, then joins it against the request URI with `urllib.parse.urljoin()`. The previous fix additionally stripped ASCII tab/CR/LF from the value before those checks.

However, on Python 3.10+ `urllib.parse.urljoin()` (via `urlsplit()`) does more than remove tab/CR/LF: **it also strips leading and trailing C0 control characters (`U+0000`–`U+001F`) and spaces from the URL before parsing it.** Because WebOb's guard checks (`SCHEME_RE` and `startswith("//")`) run against the *un-stripped* value, a single leading space or control byte slips past them, and `urljoin()` then silently removes that byte and parses what remains as a protocol-relative — or even absolute — URL. The result is an open redirect to an attacker-controlled host.

## Details

`Response._make_location_absolute()` (in `src/webob/response.py`) performed, prior to the fix:

```python value = value.replace("\t", "").replace("\r", "").replace("\n", "")

if SCHEME_RE.search(value): # ^[a-z]+: -> already absolute, return as-is return value

if value.startswith("//"): # neutralize protocol-relative URLs value = f"/%2f{value[2:]}"

new_location = urlparse.urljoin(_…