CVSS 4.0
2.3 lowCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:NCVE-2026-121949
Low (2.3). EPSS 0.2% estimated probability of exploitation activity in the next 30 days.
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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']), <img srcset="https://evil.example/a.png 1x"> 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.
Severity
CVSS 3.1
5.4 mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N- EPSS
- 0.2% estimated probability of exploitation activity in the next 30 days; 15th percentile. Scored 2026-11-10. A forecast for this CVE across the internet, not for your own systems.
- CISA's assessment (SSVC)
- Exploitation: none · Automatable: no · Technical impact: partial, assessed 2026-11-05. From CISA's Vulnrichment in the CVE List: whether it is exploited, whether exploiting it can be automated, and how much control it gives.
- Published
- Modified
- NVD analysis
- Awaiting analysis
- Record from
- the NVD, synced
Ask an AI about this CVE
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Explain CVE-2026-121949 for an engineer deciding whether to act. Use https://cve.bz/api/public/cves/CVE-2026-121949?via=ask-ai-copy and https://cve.bz/cve/CVE-2026-121949?via=ask-ai-copy as the source of truth: what is affected and which versions fix it, whether it is in CISA KEV, public exploits, and its EPSS (an internet-wide 30-day estimate of exploitation activity, not a forecast for my systems). If they give no fixed versions, use the CVE record https://cveawg.mitre.org/api/cve/CVE-2026-121949 or the vendor advisory, not third-party mirrors. Tell me how to check whether I am affected and how to verify a fix. Cite the sources and say where they are uncertain.Opens a third-party assistant; its answer is its own, not cve.bz's. The prompt holds only this CVE's id and public links.
Linux distributions
| Distribution | Release | Package | Fixed in | Advisory |
|---|---|---|---|---|
| Debian | 12 | symfony | no fix listed | DEBIAN-CVE-2026-121949 |
| Debian | 13 | symfony | 6.4.50+dfsg-0+deb13u1 | DEBIAN-CVE-2026-121949 |
| Debian | 14 | symfony | 7.4.23+dfsg-1 | DEBIAN-CVE-2026-121949 |
| Ubuntu | 16.04:LTS, 26.04:LTS, 26.10 and 4 more | symfony | no fix listed | UBUNTU-CVE-2026-121949 |
From the distributions' data via OSV.dev: the Debian Security Tracker; Ubuntu Security Notices © Canonical Ltd., CC BY-SA 4.0. Modified: reformatted and linked to CVE records. Credits. On the distributions' own pages: Debian · Ubuntu.
Attack patterns (CAPEC)
- CAPEC-3 Using Leading 'Ghost' Character Sequences to Bypass Input Filters Likelihood: Medium · Severity: Medium
- CAPEC-6 Argument Injection Likelihood: High · Severity: High
- CAPEC-15 Command Delimiters Likelihood: High · Severity: High
- CAPEC-43 Exploiting Multiple Input Interpretation Layers Likelihood: Medium · Severity: High
- CAPEC-71 Using Unicode Encoding to Bypass Validation Logic Likelihood: Medium · Severity: High
- CAPEC-73 User-Controlled Filename Likelihood: High · Severity: High
- CAPEC-85 AJAX Footprinting Likelihood: High · Severity: Low
- CAPEC-120 Double Encoding Likelihood: Low · Severity: Medium
- CAPEC-182 Flash Injection Likelihood: High · Severity: Medium
The attack patterns MITRE's CAPEC relates to this record's weaknesses, as MITRE names them. Credits.