Using Escaped Slashes in Alternate Encoding

How escaped slashes in alternate encoding work

Escaping a slash in an alternate encoding, such as a percent-encoded or otherwise transformed form, lets an attacker make the input look harmless to one parser while another layer later interprets it as a real path separator. That mismatch is the core of the evasion.

This technique is especially effective where validation, routing, file access, and security checks do not all decode or normalise input in the same way. A request may appear to stay inside an allowed path when inspected early, then resolve to a different resource after decoding.

Why it bypasses path controls

The risk comes from parser confusion. One component may evaluate the raw string, another may decode it first, and a third may compare the resolved path against an allowlist or denylist. When those steps happen in the wrong order, the slash can reappear after validation and change the meaning of the path.

That is why path traversal, local file inclusion, and similar web input abuses often sit near encoding tricks in attack guidance. NHI Management Group’s Web Application Attacks Guide covers the broader attack families where alternate encodings are used to defeat input handling.

Defenders should treat encoding, decoding, and path canonicalisation as part of the same trust boundary, not separate concerns. If validation happens before normalisation, the application can approve one representation and execute another.

Where defenders should check exposure

Exposure usually exists wherever user-controlled paths are accepted, rewritten, proxied, or mapped to files, handlers, or backend resources. The relevant question is not only whether the application accepts slashes, but whether any intermediary, framework, or storage layer decodes them differently.

Testing should include multiple encoding forms, nested decoding cases, and comparisons between raw input and the final resolved path. NHI Management Group’s CWE Top 25 Guide is useful background for the underlying weakness patterns, including improper input validation, alternate encoding handling, and path equivalence failures.

In practice, the most important exposure checks are simple: confirm how the application canonicalises paths, verify what the reverse proxy or framework decodes, and compare the final resolved path rather than the user-supplied string.

Remediation and defensive testing

The safest design is to normalise input before validation, then validate the canonical form, and only then use it for routing or file access. Defences should also reject ambiguous encodings, not just the obvious raw slash character.

Security testing should explicitly include encoded delimiters, double-encoding attempts, and cases where one layer decodes more aggressively than another. This matters because a control that blocks the raw character can still fail if a later parser reconstructs it from encoded input.

For web-facing systems, this is part of a broader secure input-handling discipline, not a narrow edge case. The application should compare decoded paths, enforce allowlists on the final canonical resource, and avoid mixing raw and decoded values in different checks.

When the technique becomes a compromise path

Once alternate encoding defeats path checks, the attacker may reach files, directories, or handlers that were meant to be hidden or inaccessible. That can expose configuration files, source files, or internal application content, depending on how the target resolves the path.

Because the technique depends on ambiguity rather than brute force, it is often attractive in chained attacks where the initial payload is small but the downstream effect is significant. It is a parser-level abuse of trust, not just a formatting trick.

Path and encoding abuse also overlaps with other request-interpretation failures, including directory traversal and bypasses of route-based restrictions. The practical defence is consistent canonicalisation across the full request path, from ingress to backend resolution.

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)