What CVEs are caused by CWE-177?

17 recorded CVEs are attributed to CWE-177, including CVE-2026-76504, CVE-2026-29045, CVE-2026-59083. 1 are listed in CISA's Known Exploited Vulnerabilities (KEV) catalog.

What are the consequences of CWE-177?

Exploiting CWE-177 can lead to: Unexpected State.

Is CWE-177 actively exploited?

Yes. 1 CWE-177 vulnerabilities are in CISA's KEV catalog of actively exploited flaws, out of 17 recorded CVEs.

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Variant weakness

Draft

CWE-177: Improper Handling of URL Encoding (Hex Encoding)

The product does not properly handle when all or part of an input has been URL encoded.

Last updated May 1, 2026

17

Recorded CVEs

With this primary weakness

1

KEVs

In the CISA KEV catalog

Not rated

Likelihood of exploit

Per MITRE

Variant

Abstraction

MITRE classification

Overview

CWE-177 (Improper Handling of URL Encoding (Hex Encoding)) is a variant-level software weakness catalogued by MITRE in the Common Weakness Enumeration (CWE). It describes a recurring type of mistake that can lead to exploitable security vulnerabilities.

Real-world CVEs

17 recorded CVEs are caused by CWE-177 (Improper Handling of URL Encoding (Hex Encoding)), including 1 in CISA's KEV (Known Exploited Vulnerabilities) catalog. KEVs are shown first. 12 new CWE-177 CVEs have been recorded so far in 2026 (1 in 2025).

CVE-2026-59083

Apache Tomcat: Incorrect URL decoding in RewriteValve may allow security control bypass

Critical · CVSS 9.1 · EPSS 29th

2026-07-14

CVE-2026-41041

Apache Gravitino: URL path injection via unencoded user-supplied identifiers in MCP REST client f-string URL construction, enabling path traversal to unintended API endpoints.

Critical · CVSS 9.1 · EPSS 47th

2026-07-13

CVE-2026-22031

Fastify Middie Middleware Path Bypass

High · CVSS 8.8 · EPSS 44th

2026-01-19

CVE-2026-22037

@fastify/express vulnerable to Improper Handling of URL Encoding (Hex Encoding)

High · CVSS 8.4 · EPSS 27th

2026-01-19

CVE-2026-96748

Connection redirection via percent-encoded delimiter injection in connection string hosts

High · CVSS 8.3 · EPSS 16th

2026-09-24

CVE-2026-15371

Velociraptor Stored XSS in URL column types

High · CVSS 8.1 · EPSS 29th

2026-08-18

CVE-2026-100409

OpenNext Cloudflare: Cache interception can bypass middleware protection for encoded paths

High · CVSS 7.5

2026-10-15

CVE-2026-76172

fast-uri vulnerable to host confusion via percent-encoded scheme normalization

High · CVSS 7.5 · EPSS 13th

2026-08-24

CVE-2022-27780

percent-encoded path separator in URL host

High · CVSS 7.5 · EPSS 85th

2022-06-01

CVE-2026-67448

Mailpit: WebSocket origin check bypass via percent-encoded path (regression of CVE-2026-22689)

Medium · CVSS 6.5 · EPSS 12th

2026-08-20

Showing 12 of 17 recorded CWE-177 CVEs. Get an alert for each new one, with AI-written analysis and fixes.

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Common consequences

What can happen when CWE-177 is exploited.

Unexpected State

Affects: Integrity

How it happens

When it is introduced

Typically introduced during these phases of the software lifecycle.

Implementation

How to prevent it

Practical mitigations for CWE-177, grouped by where in the lifecycle they apply.

Architecture and Design

Input Validation

Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.

Implementation

Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.

When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue."

Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

Implementation

Input Validation

Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.

Illustrative examples

Real CVEs that MITRE cites as examples of this weakness.

CVE-2000-0900 — Hex-encoded path traversal variants - "%2e%2e", "%2e%2e%2f", "%5c%2e%2e"
CVE-2005-2256 — Hex-encoded path traversal variants - "%2e%2e", "%2e%2e%2f", "%5c%2e%2e"
CVE-2004-2121 — Hex-encoded path traversal variants - "%2e%2e", "%2e%2e%2f", "%5c%2e%2e"
CVE-2004-0280 — "%20" (encoded space)
CVE-2003-0424 — "%20" (encoded space)
CVE-2001-0693 — "%20" (encoded space)
CVE-2001-0778 — "%20" (encoded space)
CVE-2002-1831 — Crash via hex-encoded space "%20".
CVE-2000-0671 — "%00" (encoded null)
CVE-2004-0189 — "%00" (encoded null)
CVE-2002-1291 — "%00" (encoded null)
CVE-2002-1031 — "%00" (encoded null)
CVE-2001-1140 — "%00" (encoded null)
CVE-2004-0760 — "%00" (encoded null)
CVE-2002-1025 — "%00" (encoded null)
CVE-2002-1213 — "%2f" (encoded slash)
CVE-2004-0072 — "%5c" (encoded backslash) and "%2e" (encoded dot) sequences
CVE-2004-0847 — web framework for .NET allows remote attackers to bypass authentication for .aspx files in restricted directories via a request containing a (1) "\" (backslash) or (2) "%5C" (encoded backslash)
CVE-2002-1575 — "%0a" (overlaps CRLF)

Terminology & mappings

Mapped taxonomies

PLOVER: URL Encoding (Hex Encoding)

Attack patterns

CAPEC attack patterns that exploit this weakness.

Frequently asked questions

Common questions about CWE-177.

What is CWE-177?: The product does not properly handle when all or part of an input has been URL encoded.
What CVEs are caused by CWE-177?: 17 recorded CVEs are attributed to CWE-177, including CVE-2026-76504, CVE-2026-29045, CVE-2026-59083. 1 are listed in CISA's Known Exploited Vulnerabilities (KEV) catalog.
How do you prevent CWE-177?: Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.
What are the consequences of CWE-177?: Exploiting CWE-177 can lead to: Unexpected State.
Is CWE-177 actively exploited?: Yes. 1 CWE-177 vulnerabilities are in CISA's KEV catalog of actively exploited flaws, out of 17 recorded CVEs.

References

Weakness data is sourced from the MITRE CWE catalog (v4.20). CVE associations are aggregated and kept current by RadicalNotion.AI.

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Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

How to prevent it

Illustrative examples

Related weaknesses

Parent

Terminology & mappings

Mapped taxonomies

Attack patterns

Frequently asked questions

References

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