[pp/exec] Restrict --exec template usage to safe conversions (#16883)

Authored by: bashonly

README.md

@@ -2332,8 +2332,8 @@ Some of yt-dlp's default options are different from that of youtube-dl and youtu
 * `certifi` will be used for SSL root certificates, if installed. If you want to use system certificates (e.g. self-signed), use `--compat-options no-certifi`
 * yt-dlp's sanitization of invalid characters in filenames is different/smarter than in youtube-dl. You can use `--compat-options filename-sanitization` to revert to youtube-dl's behavior
 * ~~yt-dlp tries to parse the external downloader outputs into the standard progress output if possible (Currently implemented: [aria2c](https://github.com/yt-dlp/yt-dlp/issues/5931)). You can use `--compat-options no-external-downloader-progress` to get the downloader output as-is~~
-* yt-dlp versions between 2021.09.01 and 2023.01.02 applies `--match-filters` to nested playlists. This was an unintentional side-effect of [8f18ac](https://github.com/yt-dlp/yt-dlp/commit/8f18aca8717bb0dd49054555af8d386e5eda3a88) and is fixed in [d7b460](https://github.com/yt-dlp/yt-dlp/commit/d7b460d0e5fc710950582baed2e3fc616ed98a80). Use `--compat-options playlist-match-filter` to revert this
-* yt-dlp versions between 2021.11.10 and 2023.06.21 estimated `filesize_approx` values for fragmented/manifest formats. This was added for convenience in [f2fe69](https://github.com/yt-dlp/yt-dlp/commit/f2fe69c7b0d208bdb1f6292b4ae92bc1e1a7444a), but was reverted in [0dff8e](https://github.com/yt-dlp/yt-dlp/commit/0dff8e4d1e6e9fb938f4256ea9af7d81f42fd54f) due to the potentially extreme inaccuracy of the estimated values. Use `--compat-options manifest-filesize-approx` to keep extracting the estimated values
+* yt-dlp versions from 2021.09.01 to 2022.11.11 (inclusive) applied `--match-filters` to nested playlists. This was an unintentional side-effect of [8f18ac](https://github.com/yt-dlp/yt-dlp/commit/8f18aca8717bb0dd49054555af8d386e5eda3a88) and is fixed in [d7b460](https://github.com/yt-dlp/yt-dlp/commit/d7b460d0e5fc710950582baed2e3fc616ed98a80). Use `--compat-options playlist-match-filter` to revert this
+* yt-dlp versions from 2021.11.10 to 2023.06.21 (inclusive) estimated `filesize_approx` values for fragmented/manifest formats. This was added for convenience in [f2fe69](https://github.com/yt-dlp/yt-dlp/commit/f2fe69c7b0d208bdb1f6292b4ae92bc1e1a7444a), but was reverted in [0dff8e](https://github.com/yt-dlp/yt-dlp/commit/0dff8e4d1e6e9fb938f4256ea9af7d81f42fd54f) due to the potentially extreme inaccuracy of the estimated values. Use `--compat-options manifest-filesize-approx` to keep extracting the estimated values
 * yt-dlp uses modern http client backends such as `requests`. Use `--compat-options prefer-legacy-http-handler` to prefer the legacy http handler (`urllib`) to be used for standard http requests.
 * The sub-modules `swfinterp`, `casefold` are removed.
 * Passing `--simulate` (or calling `extract_info` with `download=False`) no longer alters the default format selection. See [#9843](https://github.com/yt-dlp/yt-dlp/issues/9843) for details.
@@ -2342,8 +2342,8 @@ Some of yt-dlp's default options are different from that of youtube-dl and youtu
 For convenience, there are some compat option aliases available to use:
 
 * `--compat-options all`: Use all compat options (**Do NOT use this!**)
-* `--compat-options youtube-dl`: Same as `--compat-options all,-multistreams,-playlist-match-filter,-manifest-filesize-approx,-allow-unsafe-ext,-prefer-vp9-sort`
-* `--compat-options youtube-dlc`: Same as `--compat-options all,-no-live-chat,-no-youtube-channel-redirect,-playlist-match-filter,-manifest-filesize-approx,-allow-unsafe-ext,-prefer-vp9-sort`
+* `--compat-options youtube-dl`: Same as `--compat-options all,-multistreams,-playlist-match-filter,-manifest-filesize-approx,-allow-unsafe-ext,-prefer-vp9-sort,-allow-unsafe-exec-expansion`
+* `--compat-options youtube-dlc`: Same as `--compat-options all,-no-live-chat,-no-youtube-channel-redirect,-playlist-match-filter,-manifest-filesize-approx,-allow-unsafe-ext,-prefer-vp9-sort,-allow-unsafe-exec-expansion`
 * `--compat-options 2021`: Same as `--compat-options 2022,no-certifi,filename-sanitization`
 * `--compat-options 2022`: Same as `--compat-options 2023,playlist-match-filter,no-external-downloader-progress,prefer-legacy-http-handler,manifest-filesize-approx`
 * `--compat-options 2023`: Same as `--compat-options 2024,prefer-vp9-sort`
@@ -2358,7 +2358,12 @@ The following compat options restore vulnerable behavior from before security pa
 
     > :warning: Only use if a valid file download is rejected because its extension is detected as uncommon
     >
-    > **This option can enable remote code execution! Consider [opening an issue](<https://github.com/yt-dlp/yt-dlp/issues/new/choose>) instead!**
+    > **This option can enable remote code execution!** Consider [opening an issue](<https://github.com/yt-dlp/yt-dlp/issues/new/choose>) instead!
+
+* `--compat-options allow-unsafe-exec-expansion`: The `--exec` option allows output template syntax to be used in its commands; however, for security reasons the conversions that can be used are restricted to `i`/`d` (signed integer decimal), `f` (floating-point decimal) and `q` (shell-quoted). yt-dlp versions from 2021.04.11 to 2026.03.17 (inclusive) did not apply this restriction. This option reverts this restriction
+
+    > :warning: **This option can enable remote code execution!** Consider using `%()q` conversions in your exec command templates for any string values.
+
 
 ### Deprecated options