Symfony: HtmlSanitizer MetaRefreshAttributeSanitizer Matches Only a Narrow Subset of the Browser Refresh Grammar (Incomplete Fix for…
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 6.4.50, 7.4.23, 8.1.11
- Who is exposed
- Not reported
Do first: Upgrade to a fixed version: 6.4.50, 7.4.23, 8.1.11.
- Exploitation0/40No signal
- Exposure7/25Reachable over the network without a login · Data gap: No advisory context yet
- Reach10/20Affects Packagist packages
- Severity5.3/10CVSS 5.3
- No patch0/5No signal
What would change the score
- Would score about 77 if added to CISA KEV
- Would score about 72 if CISA's SSVC rates exploitation active
- Would score about 37 if advisories confirm default setups are exposed
CVE-2026-121373 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 5.3 (medium) and affects symfony; fixed in 6.4.50, 7.4.23 and 8.1.11.
- Last modified
- Where the record stands: published, rejected by its publisher, or superseded by another id.What do these mean? →
- Published
- Whether CISA lists it in its Known Exploited Vulnerabilities catalog: proof that attackers are using it.What do these mean? →
- Not listed in CISA KEV
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 22/100 · provisional
- Fix
- Fixed in 6.4.50, 7.4.23, 8.1.11
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
- Exploitation: none
- Automatable: no
- Technical impact: partial
Summary
The Symfony HtmlSanitizer component, used to clean HTML before it is displayed, can be bypassed through meta refresh tags so that a user is redirected to an untrusted site. An attacker can reach this remotely, without logging in, but the victim has to take an action, such as opening the affected page.
Read the original description
The Symfony HtmlSanitizer component MetaRefreshAttributeSanitizer, introduced to fix CVE-2026-48761, sanitizes the refresh URL of content attributes of meta tags using a regular expression that is too narrow, allowing sanitization to be bypassed, and resulting in potential URL redirection to untrusted sites 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 shared declarative refresh steps in the WHATWG HTML Standard require a broader approach: the separator after the time may be ASCII whitespace alone, with no ; or ,, and the url= keyword is optional. Directives the browser honours, but the regular expression misses are returned unchanged, so the attacker URL skips UrlSanitizer entirely and both allowLinkSchemes and allowLinkHosts are defeated for the refresh navigation. This is reachable under any configuration that allows meta with http-equiv and content, including the documented allowStaticElements() preset.
What to do
Upgrade to a fixed version: 6.4.50, 7.4.23, 8.1.11.
Timeline
- CVE published
- First seen by cve.now
- CISA SSVC assessment
Exploitation: none · Automatable: no · Technical impact: partial
- Record last updated
Affected versions
- symfony/html-sanitizerPackagist
From 6.1.0 · fixed in 6.4.50; From 7.0.0 · fixed in 7.4.23; From 8.0.0 · fixed in 8.1.11
Evidence 2
- Advisory
- Patch
Corrections
None so far.
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Questions about CVE-2026-121373
Is CVE-2026-121373 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
Which versions are affected by CVE-2026-121373?
symfony/html-sanitizer (Packagist): from 6.1.0 before 6.4.50; from 7.0.0 before 7.4.23; from 8.0.0 before 8.1.11.Source: cve.now feed data
How do I fix CVE-2026-121373?
Upgrade to a fixed version: 6.4.50, 7.4.23, 8.1.11.Source: cve.now feed data