Cisco IOS XR Software Image Verification Bypass Vulnerability

🚨SEVERITY: MEDIUM — CVSS 6.0Security Advisory

TL;DR 📌

  • A vulnerability in the installation process of Cisco IOS XR Software could allow an authenticated, local attacker to bypass Cisco IOS XR Software image signature verification and load unsigned software on an affected device. To exploit this vulnerability, the attacker must have root-system privileges on the affected device. This vulnerability is due to incomplete validation…
  • Highest CVSS: 6.0 (Medium).
  • Fix available — see the first fixed release below.
  • CVEs: CVE-2025-20248.

What it is

CVE-2025-20248 sits in the installation process for Cisco IOS XR Software, specifically in how the system validates the contents of an .iso image during install and activation. Validation of files inside the .iso is incomplete, which means a modified image can be installed and activated without every file in it being properly checked against Cisco’s signature.

Reaching this requires local access and root-system privileges on the device — this is not something exploitable over the network or by an unprivileged user. An attacker who already has root-system rights can modify the contents of an .iso file, then install and activate it, causing an unsigned file to be loaded as part of image activation. In effect, this bypasses Cisco’s image signature verification at the point of activation.

Cisco has raised the Security Impact Rating for this advisory from Medium to High, despite the CVSS base score of 6.0 (AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:N), because a successful bypass undermines the integrity guarantee that image signing is meant to provide.

The affected products are IOS XR platforms running vulnerable releases regardless of configuration: ASR 9000 Series (64-bit), IOS XR White box (IOSXRWBD), IOS XRv 9000, NCS 540 Series running an NCS 540-iosxr base image, NCS 560 Series, NCS 1000 Series (1001, 1002, 1004), NCS 5000 Series, NCS 5500 Series, NCS 5700 Series Line Cards and Routers running an NCS 5500 base image, and NCS 6000 Series. Cisco has confirmed IOS, IOS XE and NX-OS are not affected, nor is IOS XR running on 8000 Series Routers, NCS 540 Series on an NCS 540L-iosxr base image, NCS 1010, NCS 1014, or NCS 5700 Series on an NCS 5700 base image.

What to do

  • Patch to a fixed release where available: 24.2 trains should move to 24.2.21, and 24.4 trains to 24.4.2. For 7.10 and earlier, 7.11, and 24.3, the advisory states these should migrate to a fixed release — consult the advisory for the specific target version, as none is listed for these trains in the fixed releases table. 25.1 is not affected.
  • There are no workarounds; the only remediation path is upgrading.
  • No SMU has been produced for this vulnerability. Cisco notes that an attacker with the root-system privileges needed to exploit this could also remove an SMU before exploitation, so it was not considered a useful mitigation.
  • Given that exploitation requires root-system privileges, treat this primarily as a defence-in-depth issue tied to controlling who has that level of access, rather than a remote attack surface.
  • Cisco recommends manually verifying image integrity before installing any .iso: compare the MD5 or SHA512 checksum of the image on the device against the checksum published on Cisco.com (via run sha512sum or show md5 file on the device) before installing or activating it.
  • Monitor devices for unexpected downgrades from a fixed release, since the fix does not apply retroactively to older versions and a downgrade could reintroduce the exposure.

Fixed releases

Affected release First fixed release
24.2 24.2.21
24.4 24.4.2

For leadership 🧭

Executive summary. On core IOS XR routers such as ASR 9000, NCS 540/560/1000/5000/5500/6000 series, a privileged local user can install a tampered .iso and have it activate as if it were genuine signed Cisco software. There is no workaround, so patching to a fixed release is the only way to close it, and it should be scheduled into the next available maintenance window rather than left open indefinitely.

Why it matters:

  • Cisco has raised the Security Impact Rating from Medium to High because a successful bypass defeats the image signing guarantee that IOS XR relies on for trusted boot and upgrade integrity.
  • Affected platforms include widely deployed carrier and edge routers — ASR 9000, NCS 540 (iosxr base image), 560, 1000, 5000, 5500, 5700 (NCS 5500 base image) and 6000 series — regardless of how they are configured.
  • There are no workarounds and no SMU has been produced, because Cisco notes an attacker with root-system rights could simply remove an SMU before exploiting the flaw, leaving upgrade as the only fix.
  • The fix does not apply retroactively, so a device could be quietly downgraded from a patched release back into a vulnerable one without an obvious warning sign.

Now / Next / Later:

  • Now: Identify which IOS XR devices are running 7.10 and earlier, 7.11, 24.2 (pre-24.2.21), 24.3, or 24.4 (pre-24.4.2), and check who currently holds root-system privileges on each.
  • Next: Upgrade affected devices to 24.2.21, 24.4.2, or the appropriate fixed release for 7.10-and-earlier, 7.11, and 24.3 trains, verifying each .iso’s MD5/SHA512 checksum against Cisco.com before installing.
  • Later: Tighten and audit who is granted root-system access on IOS XR devices, and add monitoring for unexpected version downgrades from a fixed release since the patched behaviour does not carry backward.

Source