What is CVE-2026-45070?

CVE-2026-45070 is a medium-severity security vulnerability in symfony/mime (composer), affecting versions < 5.4.52. It is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12.

Which packages are affected by CVE-2026-45070?

symfony/mime (composer) versions < 5.4.52; symfony/symfony (composer) versions < 5.4.52.

Is there a fix for CVE-2026-45070?

Yes. CVE-2026-45070 is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12. Upgrade to this version or later.

Is CVE-2026-45070 exploitable, and should I be worried?

Whether CVE-2026-45070 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

What actually determines whether CVE-2026-45070 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

CVE-2026-45070 in symfony/mime

Q: Which packages are affected by CVE-2026-45070?

symfony/mime (composer) versions < 5.4.52; symfony/symfony (composer) versions < 5.4.52.

Q: Is there a fix for CVE-2026-45070?

Yes. CVE-2026-45070 is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12. Upgrade to this version or later.

Q: Is CVE-2026-45070 exploitable, and should I be worried?

Whether CVE-2026-45070 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

Q: What actually determines whether CVE-2026-45070 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

Q: How do I fix CVE-2026-45070?

Upgrade: symfony/mime to 5.4.52 or later, symfony/symfony to 5.4.52 or later, symfony/mime to 6.4.40 or later, symfony/mime to 7.4.12 or later, symfony/mime to 8.0.12 or later, symfony/symfony to 6.4.40 or later, symfony/symfony to 7.4.12 or later, symfony/symfony to 8.0.12 or later.

Summary

Symfony has Email Header Injection via Non-Token Characters in Mime Parameter Names

Description

Symfony\Component\Mime\Header\ParameterizedHeader (and the related parameter handling reachable from Symfony\Component\Mime\Header\Headers) is responsible for serializing structured headers such as Content-Type and Content-Disposition, which carry key=value parameters (e.g. Content-Disposition: attachment; filename="x").

RFC 2045 / RFC 5322 require parameter names to be tokens: a restricted ASCII subset that excludes whitespace, CR/LF, and the tspecials set. Symfony's parameter handling validates and properly encodes parameter values, but does not validate parameter names: the supplied name is emitted verbatim into the serialized header.

A caller that derives a parameter name from untrusted input, e.g. an application that lets a user influence a Content-Disposition parameter name, can include \r\n or other non-token bytes inside the name, terminating the current header and injecting additional headers in the rendered message. This is the classic CRLF / header-injection primitive applied to the parameter-name slot.

Resolution

ParameterizedHeader now rejects parameter names that contain bytes outside the RFC token character class.

The patch for this issue is available here for branch 5.4.

Credits

Symfony would like to thank Fabian Fleischer for reporting the issue and Alexandre Daubois for fixing it.

Impact

Affected versions

symfony/mime (< 5.4.52) symfony/symfony (< 5.4.52) symfony/mime (>= 6.0.0, < 6.4.40) symfony/mime (>= 7.0.0, < 7.4.12) symfony/mime (>= 8.0.0, < 8.0.12) symfony/symfony (>= 6.0.0, < 6.4.40) symfony/symfony (>= 7.0.0, < 7.4.12) symfony/symfony (>= 8.0.0, < 8.0.12)

Security releases

symfony/mime → 5.4.52 (composer) symfony/symfony → 5.4.52 (composer) symfony/mime → 6.4.40 (composer) symfony/mime → 7.4.12 (composer) symfony/mime → 8.0.12 (composer) symfony/symfony → 6.4.40 (composer) symfony/symfony → 7.4.12 (composer) symfony/symfony → 8.0.12 (composer)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

Upgrade the following packages to resolve this vulnerability:

symfony/mime to 5.4.52 or later; symfony/symfony to 5.4.52 or later; symfony/mime to 6.4.40 or later; symfony/mime to 7.4.12 or later; symfony/mime to 8.0.12 or later; symfony/symfony to 6.4.40 or later; symfony/symfony to 7.4.12 or later; symfony/symfony to 8.0.12 or later

Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.

Frequently Asked Questions

What is CVE-2026-45070? CVE-2026-45070 is a medium-severity security vulnerability in symfony/mime (composer), affecting versions < 5.4.52. It is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12.
Which packages are affected by CVE-2026-45070?
- symfony/mime (composer) (versions < 5.4.52)
- symfony/symfony (composer) (versions < 5.4.52)
Is there a fix for CVE-2026-45070? Yes. CVE-2026-45070 is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12. Upgrade to this version or later.
Is CVE-2026-45070 exploitable, and should I be worried? Whether CVE-2026-45070 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
What actually determines whether CVE-2026-45070 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
How do I fix CVE-2026-45070?
- Upgrade symfony/mime to 5.4.52 or later
- Upgrade symfony/symfony to 5.4.52 or later
- Upgrade symfony/mime to 6.4.40 or later
- Upgrade symfony/mime to 7.4.12 or later
- Upgrade symfony/mime to 8.0.12 or later
- Upgrade symfony/symfony to 6.4.40 or later
- Upgrade symfony/symfony to 7.4.12 or later
- Upgrade symfony/symfony to 8.0.12 or later

Other vulnerabilities in symfony/mime

CVE-IDSeverityPackage summary

CVE-2026-45067Highsymfony/mime: Symfony has Email Header / SMTP Command Injection via CRLF in Symfony\Component\Mime\Address CVE-2019-18888Highsymfony/http-foundation: Argument injection in a MimeTypeGuesser in Symfony