VulnerabilityPublished 13 Nov 2026

CVE-2026-128121: sveltejs kit · Improper Encoding or Escaping of Output

Affects sveltejs kitSource CVE Database V5

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Not known to be exploited · CVSS 5.9 Medium : upgrade to 3.1.1 or 2.70.4 when convenient.

Data as of 13 Nov 2026, 20:04 UTC

CVSS 3.1 base score

5.9Medium

How severe the flaw is on the 0–10 CVSS scale.

Thin evidence

One scorer or none

CVSS scores by source
CNA (GitHub_M)	CVSS 3.1	5.9

Patch: Available – fixed in 3.1.1, 2.70.4, 7.0.2 +3

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

Attack Vector: Network
Attack Complexity: High
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
Integrity: None
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 13 Nov 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 13 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade kit to 3.1.1 or 2.70.4; adapter-vercel to 7.0.2 or 6.3.5; adapter-static to 4.0.1 or 3.0.11.

Affected software

Product	Affected	Fixed in
sveltejskit	Affected >=3.0.0-next.0 <3.1.1	Fixed in 3.1.1
sveltejskit	Affected <2.70.4	Fixed in 2.70.4
@sveltejsadapter-vercel	Affected >=7.0.0-next.0 <7.0.2	Fixed in 7.0.2
@sveltejsadapter-vercel	Affected <6.3.5	Fixed in 6.3.5
@sveltejsadapter-static	Affected >=4.0.0-next.0 <4.0.1	Fixed in 4.0.1
@sveltejsadapter-static	Affected <3.0.11	Fixed in 3.0.11

AI-suggested

Upgrade to @sveltejs/adapter-vercel versions 6.3.5 or later and 7.0.2 or later, and @sveltejs/adapter-static versions 3.0.11 or later and 4.0.1 or later. These versions include fixes that properly escape attacker-influenced links in the routing configuration, preventing unintended regex interpretation. No additional mitigation is required once these updates are applied.

Description

CVE-2026-128121 affects SvelteKit framework versions prior to certain fixed releases. The vulnerability involves improper escaping of attacker-influenced links during prerendering, which can cause unintended interpretation of routing paths as regular expressions. This may lead to exposure of content or behavior outside the intended application paths. The issue is resolved in specific patched versions of @sveltejs/adapter-vercel and @sveltejs/adapter-static.

Impact

An attacker can craft links that, when crawled during prerendering, cause the routing configuration to match unintended paths due to unescaped regex metacharacters. This can lead to exposure of application content or behavior that should be restricted, potentially leaking sensitive information. The CVSS score of 5.9 (medium severity) reflects a network attack vector with high complexity and no privileges required, impacting confidentiality but not integrity or availability.

Summary

SvelteKit versions before @sveltejs/adapter-vercel 6.3.5 and 7.0.2 and @sveltejs/adapter-static 3.0.11 and 4.0.1 improperly escape attacker-influenced links crawled during prerendering. These links are emitted into the generated Vercel routing configuration without escaping regular-expression metacharacters, causing the configuration to interpret paths as regex syntax. This can result in matching unintended application paths and exposing content or behavior beyond the intended scope. The vulnerability is fixed in the specified adapter versions.

CWE-116Improper Encoding or Escaping of Output (opens in a new tab)
Class weaknessLikelihood of exploit: HighOWASP Top 10 A05:2025: Injection (opens in a new tab)
The product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved.
Architecture and Design · Libraries or Frameworks
Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
Architecture and Design · Parameterization
If available, use structured mechanisms that automatically enforce the separation between data and code. These mechanisms may be able to provide the relevant quoting, encoding, and validation automatically, instead of relying on the developer to provide this capability at every point where output is generated.
Architecture and Design · Implementation
Understand the context in which your data will be used and the encoding that will be expected. This is especially important when transmitting data between different components, or when generating outputs that can contain multiple encodings at the same time, such as web pages or multi-part mail messages.
OWASP Cheat Sheets
Content Security Policy (opens in a new tab)
Injection Prevention (opens in a new tab)
SQL Injection Prevention (opens in a new tab)
Injection Prevention in Java (opens in a new tab)
Query Parameterization (opens in a new tab)
OWASP ASVS 4.0.3
V14.4.6 (opens in a new tab)
V14.4.4 (opens in a new tab)
V14.4.2 (opens in a new tab)
Attack patterns
Cross Zone Scripting (opens in a new tab)
User-Controlled Filename (opens in a new tab)
Web Server Logs Tampering (opens in a new tab)
AJAX Footprinting (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-625Permissive Regular Expression (opens in a new tab)
Base weakness
The product uses a regular expression that does not sufficiently restrict the set of allowed values.
Implementation
When applicable, ensure that the regular expression marks beginning and ending string patterns, such as "/^string$/" for Perl.
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 13 Nov 2026, 20:04 UTC

WeaknessesCWE-116, CWE-625

Technical details

References

Community reviews

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