CVE-2025-20115: Cisco IOS XR BGP DoS Vulnerability

CVE-2025-20115 Overview

A vulnerability in the confederation implementation for the Border Gateway Protocol (BGP) in Cisco IOS XR Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition. This vulnerability is classified as a Buffer Overflow (CWE-120) and affects the processing of BGP update messages containing AS_CONFED_SEQUENCE attributes.

The vulnerability arises due to memory corruption that occurs when a BGP update is created with an AS_CONFED_SEQUENCE attribute containing 255 autonomous system numbers (AS numbers). An attacker could exploit this vulnerability by sending a crafted BGP update message, or the network could be designed in such a manner that the AS_CONFED_SEQUENCE attribute grows to 255 AS numbers or more.

Critical Impact

Successful exploitation causes memory corruption leading to BGP process restart, resulting in network routing disruption across affected Cisco IOS XR infrastructure.

Affected Products

• Cisco IOS XR versions 6.5.x through 6.9.x
• Cisco IOS XR versions 7.0.x through 7.11.x
• Cisco IOS XR versions 24.1.x through 24.2.x

Discovery Timeline

• March 12, 2025 - CVE-2025-20115 published to NVD
• August 1, 2025 - Last updated in NVD database

Technical Details for CVE-2025-20115

Vulnerability Analysis

This vulnerability exists in Cisco IOS XR's BGP confederation implementation, specifically in how the software processes the AS_CONFED_SEQUENCE path attribute within BGP UPDATE messages. BGP confederations are used to subdivide large autonomous systems into smaller sub-autonomous systems for improved scalability. The AS_CONFED_SEQUENCE attribute tracks the path through confederation members.

The memory corruption occurs when the AS_CONFED_SEQUENCE attribute reaches or exceeds 255 AS numbers. This boundary condition triggers improper memory handling within the BGP process, causing corruption that leads to process instability. The attack can be triggered either through maliciously crafted BGP updates or through legitimate but pathological network topologies.

To exploit this vulnerability, an attacker must control a BGP confederation speaker within the same autonomous system as the victim, or the network must be designed in such a manner that the AS_CONFED_SEQUENCE attribute grows to 255 AS numbers or more. This requirement limits the attack surface to entities with BGP peering relationships within the confederation.

Root Cause

The root cause is a classic buffer overflow condition (CWE-120) in the BGP process where the code does not properly validate or handle AS_CONFED_SEQUENCE attributes approaching the 255 AS number boundary. When the attribute reaches this length, memory operations exceed allocated buffer boundaries, resulting in corruption of adjacent memory regions.

The underlying issue appears to be insufficient bounds checking when processing or constructing BGP UPDATE messages with large confederation path segments. The 255 limit aligns with the single-byte length field commonly used in BGP path attribute encoding.

Attack Vector

The attack is network-accessible and can be executed by any entity that has established a BGP confederation peering session with the target router. The attacker crafts a BGP UPDATE message containing an AS_CONFED_SEQUENCE attribute with 255 or more AS numbers and transmits it to the vulnerable peer.

Upon receiving and processing the malformed update, the target router's BGP process experiences memory corruption. This corruption destabilizes the process, causing it to crash and restart. During the restart period, all BGP sessions on that router are disrupted, leading to route convergence issues and potential network outages.

The vulnerability can also be triggered unintentionally in networks with extremely deep confederation hierarchies where the AS path naturally accumulates to problematic lengths through normal route propagation.

Detection Methods for CVE-2025-20115

Indicators of Compromise

• Unexpected BGP process restarts on Cisco IOS XR routers configured for BGP confederation
• Core dump files indicating memory corruption in the BGP process
• BGP session flapping across confederation peers without apparent external cause
• System logs showing BGP daemon crashes with memory-related error messages

Detection Strategies

• Monitor syslog for BGP process crash events using patterns matching bgp and crash or restart
• Implement BGP session state monitoring to detect unexpected state transitions to IDLE
• Configure SNMP traps for BGP session state changes and process restart events
• Review BGP UPDATE messages at confederation boundaries for abnormally long AS_CONFED_SEQUENCE attributes

Monitoring Recommendations

• Enable detailed BGP logging on all confederation border routers
• Deploy network telemetry solutions to track BGP session stability metrics
• Configure alerts for BGP process memory utilization anomalies
• Implement route monitoring to detect sudden route withdrawals indicative of BGP process failures

How to Mitigate CVE-2025-20115

Immediate Actions Required

• Review the Cisco Security Advisory for fixed software versions applicable to your deployment
• Identify all Cisco IOS XR devices configured for BGP confederation participation
• Prioritize patching for devices at confederation boundaries with external peering relationships
• Consider implementing BGP route filtering to limit maximum AS path length where operationally feasible

Patch Information

Cisco has released software updates that address this vulnerability. Customers should consult the Cisco Security Advisory for IOS XR to determine the appropriate fixed software release for their specific IOS XR version. The advisory provides detailed fixed release information for all affected version trains from 6.5.x through 24.2.x.

Customers with active service contracts can obtain fixed software through their usual update channels. Those without contracts should contact the Cisco Technical Assistance Center for upgrade options.

Workarounds

• Evaluate network design to ensure AS_CONFED_SEQUENCE paths cannot naturally reach 255 AS numbers
• Implement strict BGP route policies limiting the maximum AS path length accepted from confederation peers
• Consider segmenting large confederations to reduce maximum path depths
• Monitor the APNIC Blog on BGP Paths for additional background on AS path manipulation techniques

# Example BGP route-policy to limit AS path length (adjust max-length as appropriate)
route-policy LIMIT_AS_PATH
  if as-path length ge 200 then
    drop
  endif
  pass
end-policy

# Apply to confederation peer sessions
router bgp [ASN]
 neighbor [PEER_IP]
  address-family ipv4 unicast
   route-policy LIMIT_AS_PATH in