Symfony is a widely used PHP framework; this flaw enables small YAML inputs to expand into multi‑GB memory use causing service outages across many web apps — merits broad IT notification.
CVE-2026-45304
Symfony YAML parser recursive aliases allow memory‑exhaustion DoS.
Is CVE-2026-45304 being exploited?
Not confirmed. CVE-2026-45304 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.68% probability of exploitation in the next 30 days.
How severe is CVE-2026-45304?
CVE-2026-45304 is rated High with a CVSS score of 8.7. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-45304?
Yes. A fix has been recorded for CVE-2026-45304. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-45304 affect?
CVE-2026-45304 affects Symfony\Component\Yaml\Parser, Symfony versions < 5.4.52, Symfony versions < 6.4.40, Symfony versions < 7.4.12, and 1 further product or version. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-45304?
Upgrade to 5.4.52 / 6.4.40 / 7.4.12 / 8.0.12 and validate/limit YAML aliases
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Remote denial-of-service: crafted YAML with recursive aliases expands into multi‑GB structures, exhausting memory and crashing services.
Immediate action required
- affected< 5.4.52
- affected>= 6.0.0-BETA1 and < 6.4.40
- affected>= 7.0.0-BETA1 and < 7.4.12
- affected>= 8.0.0-BETA1 and < 8.0.12
- affected< 5.4.52
- affected>= 6.0.0-BETA1 and < 6.4.40
- affected>= 7.0.0-BETA1 and < 7.4.12
- affected>= 8.0.0-BETA1 and < 8.0.12
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.