PYSEC-2026-2067 
### Impact
During file downloads, yt-dlp or the external downloaders that yt-dlp employs may leak cookies on HTTP redirects to a different host, or leak them when the host for download fragments differs from their parent manifest's host.

This vulnerable behavior is present in all versions of [youtube-dl](https://github.com/ytdl-org/youtube-dl), [youtube-dlc](https://github.com/blackjack4494/yt-dlc) and [yt-dlp](https://github.com/yt-dlp/yt-dlp) released since 2015.01.25. All native and external downloaders are affected, except for `curl` and `httpie` (httpie version 3.1.0 or later).

At the file download stage, all cookies are passed by yt-dlp to the file downloader as a `Cookie` header, thereby losing their scope. This also occurs in yt-dlp's info JSON output, which may be used by external tools. As a result, the downloader or external tool may indiscriminately send cookies with requests to domains or paths for which the cookies are not scoped.

An example of a potential attack scenario exploiting this vulnerability:
1. an attacker has crafted a malicious website with an embedded URL designed to be detected by yt-dlp as a video download. This embedded URL has the domain of a trusted site that the user has loaded cookies for, and conducts an [unvalidated redirect](https://cheatsheetseries.owasp.org/cheatsheets/Unvalidated_Redirects_and_Forwards_Cheat_Sheet.html) to a target URL.
2. yt-dlp extracts this URL and calculates a `Cookie` header based on its domain for the file downloader to make its request(s) with.
3. the download URL redirects to a server controlled by the attacker, to which yt-dlp forwards the user's sensitive cookie information.

### Patches
yt-dlp version 2023.07.06 fixes this issue by doing the following:

- Remove the `Cookie` header upon HTTP redirects
- Have native downloaders calculate their own `Cookie` header from the cookiejar
- Utilize external downloaders' built-in support for cookies instead of passing them as header arguments
- If the external downloader does not have proper cookie support, then disable HTTP redirection (`axel` only)
- Process cookies passed as HTTP headers to limit their scope (`--add-header "Cookie:..."` is scoped to input URL domain only)
- Store cookies in a separate `cookies` field of the info dict instead of `http_headers` so as not to lose their scope

Patches for youtube-dl are expected and we will update this advisory when they are merged.

### Workarounds
It is recommended to upgrade yt-dlp to version 2023.07.06 as soon as possible.

For users who are not able to upgrade:

 - Avoid using cookies and user authentication methods (`--cookies`, `--cookies-from-browser`, `--username`, `--password`, `--netrc`). While extractors may set custom cookies, these usually do not contain sensitive information.
 - Avoid using `--load-info-json`

Or, if authentication is a must:

- Verify the integrity of download links from unknown sources in browser (including redirects) before passing them to yt-dlp
- Use `curl` as external downloader, since it is not impacted (`--downloader curl`)
- Avoid fragmented formats such as HLS/m3u8, DASH/mpd and ISM (use `-f "(bv*+ba/b)[protocol~='^https?$']"`)

### References
- https://github.com/yt-dlp/yt-dlp/security/advisories/GHSA-v8mc-9377-rwjj
- https://nvd.nist.gov/vuln/detail/CVE-2023-35934
- https://github.com/yt-dlp/yt-dlp/releases/tag/2023.07.06
- https://github.com/yt-dlp/yt-dlp-nightly-builds/releases/tag/2023.07.06.185519
- https://github.com/yt-dlp/yt-dlp/commit/1ceb657bdd254ad961489e5060f2ccc7d556b729
- https://github.com/yt-dlp/yt-dlp/commit/f8b4bcc0a791274223723488bfbfc23ea3276641
- https://github.com/yt-dlp/yt-dlp/commit/3121512228487c9c690d3d39bfd2579addf96e07
CVSS v3.1 score: 6.1