CVE-2026-77103: Commvault Authentication Bypass Flaw

CVE-2026-77103 Overview

CVE-2026-77103 is an authentication bypass vulnerability in Commvault CommServe that affects access authorization and enables information disclosure. The flaw maps to [CWE-288] (Authentication Bypass Using an Alternate Path or Channel) and is exploitable over the network without user interaction or prior privileges. Commvault has published Security Advisory CV_2026_08_5 and directs customers to install the resolved maintenance release.

Critical Impact
Unauthenticated remote attackers can bypass authentication on CommServe and access restricted information managed by the backup infrastructure.

Affected Products

Commvault CommServe (see vendor advisory for fixed maintenance release)
Deployments on Microsoft Windows
Deployments on Linux

Discovery Timeline

2026-09-08 - CVE-2026-77103 published to the National Vulnerability Database
2026-09-09 - Last updated in the NVD database

Technical Details for CVE-2026-77103

Vulnerability Analysis

CommServe is the central management server in a Commvault deployment. It coordinates data protection jobs, stores policy metadata, and brokers authentication for administrative and agent-facing interfaces. An authentication bypass in this component undermines the trust boundary that protects the entire backup environment.

The issue permits an unauthenticated attacker with network access to CommServe to reach authorization-gated functionality. According to the vendor advisory, the impact is limited to confidentiality: an attacker can obtain information that should require valid credentials. Integrity and availability are not affected by this specific flaw.

Because CommServe manages backup catalogs, job history, and configuration for protected systems, disclosed data can include operational details useful for follow-on attacks against downstream backup targets.

Root Cause

The vulnerability is classified as [CWE-288], indicating that an alternate access path or channel bypasses the primary authentication check. The vendor advisory identifies the defect at a high level and does not publish deeper implementation details.

Attack Vector

Exploitation requires only network access to a vulnerable CommServe instance. No credentials, elevated privileges, or user interaction are required. Internet-exposed CommServe deployments carry the highest exposure, but internal attackers who reach the management network are equally positioned to exploit the flaw.

No public proof-of-concept, exploit code, or in-the-wild exploitation has been reported at the time of publication, and the CVE is not listed in the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2026-77103

Indicators of Compromise

Unexpected successful HTTP responses on CommServe administrative endpoints originating from clients that never completed an authentication exchange.
Access to authorization-gated CommServe resources without a corresponding valid session or token in application logs.
Anomalous outbound data volume from CommServe hosts to unfamiliar destinations following unauthenticated inbound requests.

Detection Strategies

Review CommServe application and web-tier logs for requests to protected paths that lack matching authentication events.
Correlate network flow telemetry with CommServe session logs to identify sessions that produced responses but never authenticated.
Alert on scanning patterns and enumeration behavior directed at CommServe management interfaces from non-administrative source ranges.

Monitoring Recommendations

Restrict and monitor inbound access to CommServe management interfaces at the network perimeter and internal firewalls.
Forward CommServe logs to a centralized analytics platform and baseline normal administrative access patterns.
Track version and patch level of every CommServe instance and alert when a host drifts off the fixed maintenance release.

How to Mitigate CVE-2026-77103

Immediate Actions Required

Upgrade CommServe to the resolved maintenance release identified in Commvault Security Advisory CV_2026_08_5.
Remove direct internet exposure of CommServe management interfaces and place them behind VPN or zero-trust access controls.
Rotate credentials, API keys, and service account secrets that may have been exposed through CommServe metadata.

Patch Information

Commvault has released a fixed maintenance version and documents the affected build ranges and remediation steps in the Commvault Security Advisory CV_2026_08_5. Software customers should upgrade CommServe to the resolved maintenance release listed in the advisory.

Workarounds

Restrict network access to CommServe to a small allow-list of administrative source addresses until the patch is applied.
Enable and enforce network segmentation between CommServe and untrusted network zones, including user VLANs and DMZs.
Increase log retention and monitoring on CommServe hosts during the remediation window to support post-incident review.

bash

# Configuration example
# Example host-based firewall rules restricting CommServe management access
# to a dedicated administrative subnet (adjust interface and CIDR as needed).

# Linux (iptables) - allow only 10.10.20.0/24 to reach CommServe TCP/8400-8403
iptables -A INPUT -p tcp -m multiport --dports 8400:8403 -s 10.10.20.0/24 -j ACCEPT
iptables -A INPUT -p tcp -m multiport --dports 8400:8403 -j DROP

# Windows (PowerShell) - restrict inbound to CommServe management ports
New-NetFirewallRule -DisplayName "CommServe-Admin-Allow" \
  -Direction Inbound -Protocol TCP -LocalPort 8400-8403 \
  -RemoteAddress 10.10.20.0/24 -Action Allow
New-NetFirewallRule -DisplayName "CommServe-Admin-Deny" \
  -Direction Inbound -Protocol TCP -LocalPort 8400-8403 -Action Block