VulnerabilityPublished 16 Sep 2026

CVE-2026-58657: Stored CSS injection via Markdown image resize() bypasses prior media style sanitizers in Grav

Affects getgrav/gravSource GCVE Database

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Not known to be exploited · CVSS 4.8 Medium : upgrade to 2.0.0 when convenient.

Data as of 31 Oct 2026, 08:20 UTC

CVSS 3.1 base score

4.8Medium

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

EPSS: 0.2% chance of exploitation in 30 days – higher than 18% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.0.0

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: High
User Interaction: Required
Scope: Changed
Confidentiality: Low
Integrity: Low
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: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: High: The attacker is authorized with (i.e., requires) privileges that provide significant (e.g., administrative) control over the vulnerable component that could affect component-wide settings and files.
User Interaction: Required: Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited.
Scope: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
Confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on 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 16 Sep 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 30 Oct 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 getgrav/grav to 2.0.0.

Affected software

Product	Affected	Fixed in
Packagistgetgrav/grav	Affected >=2.0.0-rc.9 <2.0.0 =2.0.0-rc.9	Fixed in 2.0.0

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

A patch is available that properly sanitizes or type-normalizes all values before they are added to styleAttributes, including those passed through the resize() media action. Specifically, width and height values should be validated as numeric before appending 'px', or a shared CSS declaration builder should be used to reject unsafe characters such as semicolons and colons. Users should apply the official fix to affected Grav versions to remediate this vulnerability. Until patched, restrict Markdown editing privileges to trusted users to reduce risk.

Description

Grav versions 2.0.0-rc.9 and the current 2.0 branch contain a stored CSS injection vulnerability via the Markdown image resize() media action. This allows a lower-privileged content editor to inject arbitrary CSS declarations into the style attribute of rendered images, which are then executed in the context of higher-privileged users viewing the content. The vulnerability arises because the resize() method does not properly sanitize or validate input values before embedding them as CSS style attributes. This can lead to UI redress attacks such as overlays that manipulate the admin interface. A patch is available to properly sanitize these inputs.

Impact

A lower-privileged content editor can persistently inject CSS declarations that execute in the browser context of higher-privileged users (such as reviewers or admins) when they view the affected page or admin preview. This can lead to UI redress attacks, including overlays that obscure or manipulate the interface, potentially misleading or interfering with the admin's actions. The vulnerability does not require JavaScript execution but still poses a risk of content manipulation and user interface disruption in privileged sessions.

Summary

The vulnerability (CVE-2026-58657) in Grav 2.0.0-rc.9 and the current 2.0 branch allows stored CSS injection through the Markdown image resize() media action. While prior sanitizers block direct style= payloads and unsafe attribute() fallbacks, the resize() action improperly writes caller-controlled values directly into styleAttributes without validation. This enables a lower-privileged publisher who can edit page Markdown to craft an image URL with malicious CSS declarations injected into the final <img style=...> attribute. The injected CSS can create UI overlays or manipulate content when rendered by higher-privileged users. The root cause is that the resize() method appends 'px' to width and height values without verifying they are numeric and free of CSS delimiters, allowing injection of additional CSS declarations. The sanitizer for style() inputs is not applied to resize() values. The vulnerability has been demonstrated with a payload that injects position:fixed and other CSS properties to create a full-viewport overlay. A fix involves sanitizing or type-normalizing all values before adding them to styleAttributes, ensuring width and height are numeric before appending 'px'.

CWE-79Improper Neutralization of Input During Web Page Generation (Cross-site Scripting) (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #1OWASP Top 10 A05:2025: Injection (opens in a new tab)
The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.
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.
Implementation · Architecture and Design
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.
Architecture and Design · Implementation · Attack Surface Reduction
Understand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide…
OWASP Cheat Sheets
Cross Site Scripting Prevention (opens in a new tab)
DOM based XSS Prevention (opens in a new tab)
Injection Prevention (opens in a new tab)
Abuse Case (opens in a new tab)
Deserialization (opens in a new tab)
OWASP ASVS 4.0.3
V5.3.3 (opens in a new tab)
V1.5.1 (opens in a new tab)
Attack patterns
XSS Using MIME Type Mismatch (opens in a new tab)
DOM-Based XSS (opens in a new tab)
Reflected XSS (opens in a new tab)
Stored XSS (opens in a new tab)
Cross-Site Scripting (XSS) (opens in a new tab)
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 17 Sep 2026, 02:03 UTC

WeaknessesCWE-79

Technical details

References

Community reviews

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