Using Slashes in Alternate Encoding

What this technique does

Using Slashes in Alternate Encoding is a request-interpretation evasion technique. It hides slash characters in non-standard forms so filters, routing logic, or path validation may see a different string than the backend actually processes.

The core danger is not the encoding itself, but the mismatch it creates between validation and execution. If one layer inspects the raw request while another normalises it differently, an attacker can steer the request toward a different resource, path, or handler than the defender expected.

This pattern often appears alongside other input-interpretation flaws, especially where applications compare, decode, and route user input in multiple stages. Web Application Attacks Guide covers the broader family of attack patterns where inconsistent parsing enables bypasses and path manipulation.

Why alternate slash encodings matter

Slashes are structural characters in URL paths and file-system references, so changing how they are represented can alter routing, endpoint selection, or file access decisions. A security control that blocks the literal slash may still fail if the application later decodes an alternate representation into the same character.

That makes the technique useful for traversal-style abuse, hidden endpoint discovery, access control bypasses, and request smuggling between layers that do not agree on canonical form. CWE Top 25 Guide is a useful companion for understanding how input normalisation failures map to common weakness patterns such as improper neutralisation and alternate encoding handling.

Defenders should treat canonicalisation as part of the trust decision, not as a cosmetic transform. If decoding happens after allowlist checks, path restrictions, or security logging, the control boundary may be weaker than it appears.

How defenders should check exposure

Exposure usually exists wherever user-controlled paths, redirect targets, proxy rules, file retrieval endpoints, or framework routers accept encoded input. The practical question is whether every layer in the request path, including reverse proxies, application servers, and filesystem handlers, applies the same normalisation rules.

Security review should focus on inconsistent decode order, double decoding, and any component that accepts encoded delimiters differently from its neighbours. Reviewing vendor, framework, and gateway documentation is important because normalisation behaviour often differs by product and configuration.

Test cases should include both expected and rejected encodings, then confirm that the application validates after normalisation rather than before it. Where possible, compare the path seen by edge controls with the path interpreted by the application to identify trust-boundary gaps.

Prevention and response when the pattern appears

The safest design is to canonicalise once, validate once, and route once, using a single agreed interpretation of slash characters across the full stack. Ambiguous encodings should be rejected at trust boundaries instead of being passed downstream for later interpretation.

When a suspicious request is observed, defenders should review whether the application or gateway normalised the path before security decisions were made. If the issue is being exploited, look for unusual path shapes, bypassed denied routes, and requests that succeed only when encoded delimiters are used.

After hardening, reassess logging and detection rules so that the security team records the normalised form as well as the original request. That helps distinguish harmless malformed traffic from deliberate attempts to manipulate routing or file access.

Risk and Threat Considerations

Alternate slash encodings are dangerous because they exploit a gap between what security controls inspect and what the application ultimately executes. That gap can let attackers bypass path restrictions, reach hidden resources, or alter how downstream components resolve a request.

Failure mechanism: The attacker supplies an encoded slash that one component treats as inert input while another decodes it into a path separator, creating a control bypass through inconsistent normalisation.

Impact: The result can be unauthorised file access, endpoint discovery, route confusion, or traversal into resources that were meant to be blocked.

Practitioner Guidance

What to watch for: Treat any control that compares raw request text as suspect if later processing decodes, rewrites, or canonicalises the same input. The most reliable fix is to make validation consume the same canonical form that routing and authorisation will use.

Practitioner takeaway: If multiple layers disagree about path meaning, attackers will eventually find the version that benefits them.

Related resources from NHI Mgmt Group

• CVE-2026-20079 (Cisco Firewall Management Center Authentication Bypass Using an Alternate Path or Channel)
• CVE-2025-2746 (Kentico Xperience CMS Authentication Bypass Using an Alternate Path or Channel)
• CVE-2025-2747 (Kentico Xperience CMS Authentication Bypass Using an Alternate Path or Channel)
• CVE-2026-19490 (Citrix NetScaler Authentication Bypass Using an Alternate Path or Channel)