CVE-2026-19730: podman quadlet install --replace does not fully replace the old file

When running podman quadlet install --replace to replace a Quadlet file, if the original Quadlet is larger than the new Quadlet, the file would not be truncated and content from the original would be preserved. There is no risk of information leakage as the user already had access to the Quadlet in order to replace it, and in most cases, this would only lead to invalid Quadlet files (see https://github.com/podman-container-tools/podman/issues/29013). However, security-related options from the end of the old Quadlet could be included in the new Quadlet, and if the truncation resulted in a valid Quadlet file, this could result in undesirable behavior. For example, running podman quadlet install --replace to remove a single line from the end of a Quadlet - including security-sensitive content, like AddCapability - will fail, and the option will continue to be used. Further, with Volume Quadlets, this can include additional mounts which can cause content to be unintentionally exposed into containers. If, later, the image is updated then compromised content might be leaked to an attacker.

First introduced in commit 0ea4eaee70b7458bcc99d875529761faf5172651 with podman 5.7.0