# CVE-2026-121949: Symfony: HtmlSanitizer Does Not URL-Sanitize srcset and imagesrcset: Media Scheme and Host Allow-List Bypass
- **Severity:** Low 2.3 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 5 Nov 2026
- **Updated:** 5 Nov 2026
- **Also known as:** DEBIAN-CVE-2026-121949, GHSA-q8jq-xrf4-v5j8, UBUNTU-CVE-2026-121949
## What is CVE-2026-121949?
CVE-2026-121949 is a low-severity vulnerability in sensiolabs symfony, published on 5 Nov 2026.
### Official description
The Symfony HtmlSanitizer component does not apply the media URL policy to the srcset and imagesrcset attributes, allowing values that point to hosts or schemes not permitted by allowMediaSchemes() or allowMediaHosts() to bypass it, so that browsers load images from disallowed hosts in Symfony versions 6.1.x prior to 6.4.50, 7.x prior to 7.4.23, and 8.x prior to 8.1.11.The srcset attribute on img and source elements and the imagesrcset attribute on link elements are allowed by the sanitizer, but their URL candidates are never validated, so their values are kept verbatim while the same URL in src is dropped. The fixes for CVE-2026-45753 and CVE-2026-48761 extended the same attribute list but did not cover these two attributes. For example, with allowMediaSchemes(['https']) and allowMediaHosts(['trusted.example']), ![]()
survives sanitization, and the browser fetches the selected candidate as it would fetch src. Candidates with a scheme forbidden by the configuration, such as http, are kept as well.
Any application that sanitizes untrusted HTML and relies on the media schemes and hosts allowlists to restrict where images can be loaded from is affected. A single crafted value is enough to make the browser of a user viewing the sanitized content send a request to an attacker-controlled host, which can be used to track viewers or log their IP addresses. No authentication is needed if the application accepts HTML from unauthenticated users.
Weakness: CWE-184 (Incomplete List of Disallowed Inputs), CWE-1023 (Incomplete Comparison with Missing Factors)
## Which versions are affected by CVE-2026-121949?
## How do I fix CVE-2026-121949?
Upgrade to a fixed version.
## Is CVE-2026-121949 being exploited?
There is no known exploitation of CVE-2026-121949 in the wild and no public exploit code.
## Fix prompt for AI coding agents
```text
Check this codebase for CVE-2026-121949 and fix it if it is affected.
What it is: CVE-2026-121949 is a low-severity vulnerability in sensiolabs symfony, published on 5 Nov 2026.
Exploitation: There is no known exploitation of CVE-2026-121949 in the wild and no public exploit code.
Affected product: sensiolabs symfony. No package-level version data is available, so check the vendor advisory linked from the reference page.
Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.
Reference: https://insights.sbomify.com/CVE-2026-121949/ (Markdown: https://insights.sbomify.com/CVE-2026-121949/index.md)
```
## Sources
- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-121949
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-121949
- OSV: https://osv.dev/vulnerability/CVE-2026-121949
---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-121949/
Summaries are generated and may contain mistakes; the official description above is the record.
survives sanitization, and the browser fetches the selected candidate as it would fetch src. Candidates with a scheme forbidden by the configuration, such as http, are kept as well.
Any application that sanitizes untrusted HTML and relies on the media schemes and hosts allowlists to restrict where images can be loaded from is affected. A single crafted value is enough to make the browser of a user viewing the sanitized content send a request to an attacker-controlled host, which can be used to track viewers or log their IP addresses. No authentication is needed if the application accepts HTML from unauthenticated users.
Weakness: CWE-184 (Incomplete List of Disallowed Inputs), CWE-1023 (Incomplete Comparison with Missing Factors)
## Which versions are affected by CVE-2026-121949?
## How do I fix CVE-2026-121949?
Upgrade to a fixed version.
## Is CVE-2026-121949 being exploited?
There is no known exploitation of CVE-2026-121949 in the wild and no public exploit code.
## Fix prompt for AI coding agents
```text
Check this codebase for CVE-2026-121949 and fix it if it is affected.
What it is: CVE-2026-121949 is a low-severity vulnerability in sensiolabs symfony, published on 5 Nov 2026.
Exploitation: There is no known exploitation of CVE-2026-121949 in the wild and no public exploit code.
Affected product: sensiolabs symfony. No package-level version data is available, so check the vendor advisory linked from the reference page.
Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.
Reference: https://insights.sbomify.com/CVE-2026-121949/ (Markdown: https://insights.sbomify.com/CVE-2026-121949/index.md)
```
## Sources
- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-121949
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-121949
- OSV: https://osv.dev/vulnerability/CVE-2026-121949
---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-121949/
Summaries are generated and may contain mistakes; the official description above is the record.