What is CVE-2021-41270?

CVE-2021-41270 is a medium-severity security vulnerability in symfony/serializer (composer), affecting versions >= 5.0.0, < 5.3.12. It is fixed in 5.3.12, 4.4.35.

How severe is CVE-2021-41270?

CVE-2021-41270 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.

Which packages are affected by CVE-2021-41270?

symfony/serializer (composer) versions >= 5.0.0, = 4.1.0, < 4.4.35.

Is there a fix for CVE-2021-41270?

Yes. CVE-2021-41270 is fixed in 5.3.12, 4.4.35. Upgrade to this version or later.

Is CVE-2021-41270 exploitable, and should I be worried?

Whether CVE-2021-41270 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-2021-41270 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-2021-41270 in symfony/serializer

Q: How severe is CVE-2021-41270?

CVE-2021-41270 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.

Q: Which packages are affected by CVE-2021-41270?

symfony/serializer (composer) versions >= 5.0.0, = 4.1.0, < 4.4.35.

Q: Is there a fix for CVE-2021-41270?

Yes. CVE-2021-41270 is fixed in 5.3.12, 4.4.35. Upgrade to this version or later.

Q: Is CVE-2021-41270 exploitable, and should I be worried?

Whether CVE-2021-41270 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-2021-41270 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-2021-41270?

Upgrade: symfony/serializer to 5.3.12 or later, symfony/serializer to 4.4.35 or later, symfony/symfony to 4.4.35 or later, symfony/symfony to 5.3.12 or later.

Summary

CSV Injection in symfony/serializer

Description

CSV Injection, also known as Formula Injection, occurs when websites embed untrusted input inside CSV files. When a spreadsheet program opens a CSV, any cell starting with = is interpreted by the software as a formula and could be abused by an attacker.

In Symfony 4.1, we've added the opt-in csv_escape_formulas option in CsvEncoder, to prefix all cells starting by =, +, - or @ by a tab \t.

Since then, OWASP added 2 chars in that list:

Tab (0x09)
Carriage return (0x0D)

This makes our previous prefix char (Tab \t) part of the vulnerable characters, and OWASP suggests using the single quote ' for prefixing the value.

Resolution

Symfony now follows the OWASP recommendations and use the single quote ' to prefix formulas and adds the prefix to cells starting by \t, \r as well as =, +, - and @.

The patch for this issue is available here for branch 4.4.

Credits

We would like to thank Jake Barwell for reporting the issue and Jérémy Derussé for fixing the issue.

Impact

CVE-2021-41270 has a CVSS score of 6.5 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.3.12, 4.4.35); upgrading removes the vulnerable code path.

Affected versions

symfony/serializer (>= 5.0.0, < 5.3.12) symfony/serializer (>= 4.1.0, < 4.4.35) symfony/symfony (>= 4.1.0, < 4.4.35) symfony/symfony (>= 5.0.0, < 5.3.12)

Security releases

symfony/serializer → 5.3.12 (composer) symfony/serializer → 4.4.35 (composer) symfony/symfony → 4.4.35 (composer) symfony/symfony → 5.3.12 (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/serializer to 5.3.12 or later; symfony/serializer to 4.4.35 or later; symfony/symfony to 4.4.35 or later; symfony/symfony to 5.3.12 or later

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

Frequently Asked Questions

What is CVE-2021-41270? CVE-2021-41270 is a medium-severity security vulnerability in symfony/serializer (composer), affecting versions >= 5.0.0, < 5.3.12. It is fixed in 5.3.12, 4.4.35.
How severe is CVE-2021-41270? CVE-2021-41270 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
Which packages are affected by CVE-2021-41270?
- symfony/serializer (composer) (versions >= 5.0.0, < 5.3.12)
- symfony/symfony (composer) (versions >= 4.1.0, < 4.4.35)
Is there a fix for CVE-2021-41270? Yes. CVE-2021-41270 is fixed in 5.3.12, 4.4.35. Upgrade to this version or later.
Is CVE-2021-41270 exploitable, and should I be worried? Whether CVE-2021-41270 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-2021-41270 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-2021-41270?
- Upgrade symfony/serializer to 5.3.12 or later
- Upgrade symfony/serializer to 4.4.35 or later
- Upgrade symfony/symfony to 4.4.35 or later
- Upgrade symfony/symfony to 5.3.12 or later

Other vulnerabilities in symfony/serializer

CVE-IDSeverityPackage summary

GHSA-J68W-PG49-F6VXCriticalsymfony/serializer: Symfony XML decoding attack vector through external entities