NS_E_WMPCORE_CODEC_NOT_TRUSTED (0XC00D109A) Fix

What’s actually happening here

You’re trying to play a video or audio file in Windows Media Player (WMP) and instead of playing, you get:

The codec downloaded for this file does not appear to be properly signed, so it cannot be installed.
Error: 0XC00D109A

WMP 12 on Windows 7 through 10 has a codec signing check. When you play a file that needs a codec not already installed, WMP tries to download it from Microsoft’s servers. If that codec’s digital signature is missing, expired, or revoked, WMP kills the install and throws this error. You’ll see this most often with older media formats—like certain DivX or Xvid AVI files, or some MKV containers that use outdated codecs.

The fix is either getting a properly signed codec from a known source, or telling WMP to stop checking signatures. Both work. Pick whichever fits your tolerance for risk.

First fix: install a trusted codec pack

The real fix—the one that doesn’t weaken your system’s security—is to install a codec pack that Microsoft trusts. Windows 10 and 11 don’t ship with codecs for old formats like MPEG-2, DivX, or Xvid. WMP tries to pull them from the Microsoft Store or its own update service, but many third-party codecs never got signed by Microsoft’s catalog.

1. Download the K-Lite Codec Pack (Basic or Standard) from codecguide.com. I recommend the Standard pack—it includes Media Player Classic Home Cinema (MPC-HC), which sidesteps WMP entirely if you want. But if you’re sticking with WMP, the Basic pack works fine.
2. Run the installer. When asked about player preferences, choose Windows Media Player if you want to keep using it as default.
3. Restart WMP. That 0XC00D109A error should be gone for the files you tried before.

Why this works: K-Lite’s codecs are signed, but more importantly, they’re installed locally before WMP ever tries to auto-download anything. WMP sees the codec already present and skips its own download attempt entirely. No signing check needed. The error only triggers when WMP tries to fetch the codec itself.

Second fix: disable the codec signing check (registry)

If you can’t install a codec pack—maybe you’re on a locked-down corporate machine, or you’re certain the codec is safe—you can bypass the check. This lowers your security posture a bit, but for a single trusted file, it’s fine.

1. Press Win + R, type regedit, and hit Enter.
2. Navigate to this key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MediaPlayer\Player\Upgrade

3. If the Upgrade key doesn’t exist, right-click Player → New → Key, and name it Upgrade.
4. Inside Upgrade, right-click → New → DWORD (32-bit). Name it CodecDownloadBlocked.
5. Set its value to 0 (zero). Double-check it’s zero, not 1.
6. Close regedit and restart WMP.

What you’ve done: CodecDownloadBlocked set to 0 tells WMP not to block codec installation based on signature trust. WMP will still try to download the codec, but it won’t stop at the signing check. The reason step 5 sets it to 0 is historic—Microsoft’s internal logic treats 0 as “don’t block” and any non-zero as “block.” There’s no official documentation for this, but it’s been confirmed across many Windows 7–10 builds.

Caveat: This registry key doesn’t exist in Windows 11’s WMP replacement (the “Media Player” app from the Store). If you’re on Win11, you’re better off with the first fix or switching to a third-party player like MPC-HC or VLC.

Third fix: use a different media player

Not a fix for WMP itself, but the most reliable workaround: stop using WMP. It’s abandonware. Microsoft replaced it with the “Media Player” app (Groove Music rebranded) on Windows 11, and on Windows 10 it’s barely updated since 2012.

VLC Media Player plays almost anything out of the box. It bundles its own codecs, no system installs, no signing checks. You’ll never see 0XC00D109A because VLC doesn’t use WMP’s codec pipeline at all.

1. Download VLC from videolan.org.
2. Install, open the file. Done.

This is my recommended default for anyone not stuck on WMP for a specific reason (like corporate policy or a legacy app that integrates with WMP). VLC handles MKV, HEVC, VP9, and ancient formats equally well.

Quick-reference summary

Try the codec pack first. It’s the cleanest fix. Only touch the registry if you know what you’re doing and you trust the file. And if you’re on Windows 11, skip the registry—it won’t do anything.