CVE-2026-58156: Apache Traffic Server Auth Bypass Flaw

CVE-2026-58156 Overview

CVE-2026-58156 affects Apache Traffic Server (ATS), an open-source, high-performance HTTP proxy and forward/reverse cache. The server mis-parses port numbers embedded in URLs and the userinfo component of URIs. This parsing flaw enables port-based access-control bypass, allowing clients to reach ports that operators intended to restrict. The issue impacts Apache Traffic Server 8.0.0 through 8.1.9, 9.0.0 through 9.2.14, and 10.0.0 through 10.1.3. Fixed versions are 9.2.15 and 10.1.4. The weakness is classified under [CWE-863] Incorrect Authorization.

Critical Impact
Attackers can bypass port-restriction access controls in Apache Traffic Server by crafting URLs with ambiguous port or userinfo components, exposing internal services intended to be blocked.

Affected Products

Apache Traffic Server 8.0.0 through 8.1.9
Apache Traffic Server 9.0.0 through 9.2.14
Apache Traffic Server 10.0.0 through 10.1.3

Discovery Timeline

2026-07-29 - CVE-2026-58156 published to the National Vulnerability Database
2026-07-30 - CVE-2026-58156 last updated in NVD

Technical Details for CVE-2026-58156

Vulnerability Analysis

Apache Traffic Server incorrectly parses the port component of URLs and the userinfo portion of URIs. When a client submits a request containing an ambiguous or malformed authority component, ATS extracts a different effective port than the one enforced by its access-control logic. The result is an authorization decision made against one parsed value while the request is routed based on another.

Operators commonly configure ATS with remap.config rules or ACLs that restrict which origin ports proxied traffic can reach. This weakness undermines that boundary. An unauthenticated remote attacker can reach otherwise restricted ports through the proxy without any user interaction.

Root Cause

The root cause is inconsistent URI parsing between the access-control layer and the request-forwarding layer. The userinfo field, which appears before the @ character in a URI authority, can contain characters that resemble a port separator. When ATS interprets these characters differently at different processing stages, the effective destination diverges from the value evaluated by policy. This is a classic [CWE-863] Incorrect Authorization pattern driven by parser differentials.

Attack Vector

Exploitation occurs over the network with low attack complexity and no privileges. An attacker crafts an HTTP request whose URL embeds a port and userinfo combination that ATS normalizes inconsistently. The proxy validates the request against one interpretation of the authority component and then forwards traffic based on a second interpretation. Because no user interaction or authentication is required, the flaw is reachable by any client able to send requests to the proxy. Full technical detail is available in the Apache Security Mailing List Thread.

Detection Methods for CVE-2026-58156

Indicators of Compromise

HTTP request lines containing userinfo components with embedded colon-port sequences, such as http://user:pass@host:port@target/
Proxy access logs showing successful connections to origin ports that are explicitly denied in remap.config or ACL configuration
Requests where the Host header and the parsed URI authority disagree on port value
Sudden traffic to internal management ports (for example, admin, database, or metrics endpoints) originating from the ATS forwarding path

Detection Strategies

Compare the port value in the request URI against the port value ATS actually connects to in upstream logs; discrepancies indicate parser differential exploitation
Alert on ATS proxy requests whose destination port is outside an explicit allowlist of business-approved origin ports
Inspect access logs for URIs containing multiple @ characters or colon-delimited userinfo, both of which are rare in legitimate traffic

Monitoring Recommendations

Ingest ATS access.log and error.log into a centralized analytics platform and retain for correlation
Monitor for outbound connections from ATS hosts to internal ports that should not be proxied
Track version strings of deployed Apache Traffic Server instances to identify unpatched nodes

How to Mitigate CVE-2026-58156

Immediate Actions Required

Upgrade Apache Traffic Server to version 9.2.15 or 10.1.4, which contain the parser fix
Inventory all ATS instances across environments and confirm the running version against the patched releases
Review remap.config and ACL definitions to ensure port-restriction rules match current policy
Restrict which clients can reach ATS listeners using network segmentation while patching is in progress

Patch Information

Apache Traffic Server 9.2.15 and 10.1.4 correct the URL and userinfo parsing behavior so that the port evaluated by access control matches the port used for upstream routing. Users on the 8.x branch should migrate to a supported fixed release, as 8.x does not appear in the fixed-version list. Details are published in the Apache Security Mailing List Thread.

Workarounds

Place a strict upstream firewall between ATS and internal networks that denies all origin ports except those explicitly required
Add explicit deny rules for sensitive internal ports at the network layer, independent of ATS ACLs
Reject requests at an upstream WAF or load balancer when the URI authority contains a userinfo component or more than one @ character

bash

# Example firewall workaround: allow ATS to reach only expected origin ports
# Replace 10.0.0.0/8 and ports with values that match your environment
iptables -A OUTPUT -o eth0 -p tcp -d 10.0.0.0/8 --dport 80 -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp -d 10.0.0.0/8 --dport 443 -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp -d 10.0.0.0/8 -j DROP