CVE-2020-5255 [Low] | Input Validation Bypass in Symfony

In Symfony before versions 4.4.7 and 5.0.7, when a `Response` does not contain a `Content-Type` header, affected versions of Symfony can fallback to the format defined in the `Accept` header of the request, leading to a possible mismatch between the response's content and `Content-Type` header. When the response is cached, this can prevent the use of the website by other users. This has been patched in versions 4.4.7 and 5.0.7.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2026-06-15	0.37%	1.30%	+0.92%
2	2025-03-30	0.57%	0.37%	-0.20%
3	2025-03-29	—	0.57%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2026-06-15

0.37%

1.30%

+0.92%

2025-03-30

0.57%

0.37%

-0.20%

2025-03-29

—

0.57%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
2.6	3.1	LOW	`CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:N/I:N/A:L` Click to expand Attack vector (AV:N) Could be attacked over the internet or any normal routed network—not just someone sitting at the machine. Attack complexity (AC:H) Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work. Privileges required (PR:L) A normal user session is enough; they don’t have to be admin. User interaction (UI:R) A real person has to do something—click, install, enable—otherwise it doesn’t land. Scope (S:U) Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems. Confidentiality (C:N) Doesn’t really leak secrets in a meaningful way. Integrity (I:N) Data isn’t meaningfully altered or forged. Availability (A:L) Might cause slowdowns, glitches, or partial disruption—not a full brick.	1.2	1.4	[email protected]
4.3	3.1	MEDIUM	`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L` Click to expand Attack vector (AV:N) Could be attacked over the internet or any normal routed network—not just someone sitting at the machine. Attack complexity (AC:L) Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup. Privileges required (PR:L) A normal user session is enough; they don’t have to be admin. User interaction (UI:N) Nobody has to click “OK” or open a trap file; it can work without a victim helping. Scope (S:U) Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems. Confidentiality (C:N) Doesn’t really leak secrets in a meaningful way. Integrity (I:N) Data isn’t meaningfully altered or forged. Availability (A:L) Might cause slowdowns, glitches, or partial disruption—not a full brick.	2.8	1.4	[email protected]
4.0	2.0	MEDIUM	`AV:N/AC:L/Au:S/C:N/I:N/A:P` Click to expand Access vector (AV:N) Can be exploited remotely over network reachability. Access complexity (AC:L) Exploitation conditions are straightforward and predictable. Authentication (AU:S) A single authentication is required. Confidentiality impact (C:N) No confidentiality impact. Integrity impact (I:N) No integrity impact. Availability impact (A:P) Partial availability impact.	8.0	2.9	[email protected]

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

2.6

3.1

LOW

CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:N/I:N/A:L Click to expand

Attack vector (AV:N): Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:H): Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
Privileges required (PR:L): A normal user session is enough; they don’t have to be admin.
User interaction (UI:R): A real person has to do something—click, install, enable—otherwise it doesn’t land.
Scope (S:U): Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N): Doesn’t really leak secrets in a meaningful way.
Integrity (I:N): Data isn’t meaningfully altered or forged.
Availability (A:L): Might cause slowdowns, glitches, or partial disruption—not a full brick.

1.2

1.4

[email protected]

4.3

3.1

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L Click to expand

Attack vector (AV:N): Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L): Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:L): A normal user session is enough; they don’t have to be admin.
User interaction (UI:N): Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U): Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N): Doesn’t really leak secrets in a meaningful way.
Integrity (I:N): Data isn’t meaningfully altered or forged.
Availability (A:L): Might cause slowdowns, glitches, or partial disruption—not a full brick.

2.8

1.4

[email protected]

4.0

2.0

MEDIUM

AV:N/AC:L/Au:S/C:N/I:N/A:P Click to expand

Access vector (AV:N): Can be exploited remotely over network reachability.
Access complexity (AC:L): Exploitation conditions are straightforward and predictable.
Authentication (AU:S): A single authentication is required.
Confidentiality impact (C:N): No confidentiality impact.
Integrity impact (I:N): No integrity impact.
Availability impact (A:P): Partial availability impact.

8.0

2.9

[email protected]

OS Trackers for CVE-2020-5255

vendor	priority	summary	link
`debian`	not yet assigned	CVE-2020-5255 not yet assigned priority: Debian including 1 source packages (symfony), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5.	https://security-tracker.debian.org/tracker/CVE-2020-5255
`ubuntu`	medium	CVE-2020-5255 medium priority: Ubuntu including 1 source packages (symfony), 10 status rows across 10 suites (bionic, eoan, focal, groovy, hirsute, impish, jammy, trusty, upstream, xenial): not-affected 7, DNE 1, ignored 1, released 1.	https://ubuntu.com/security/CVE-2020-5255

Affected software / configurations for CVE-2020-5255

Vendor	Product	Version	Raw CPE
sensiolabs	symfony	>= 4.4.0, < 4.4.7	cpe:2.3:a:sensiolabs:symfony::::::::
sensiolabs	symfony	>= 5.0.0, < 5.0.7	cpe:2.3:a:sensiolabs:symfony::::::::

References for CVE-2020-5255

URL	Tags
https://github.com/symfony/symfony/commit/dca343442e6a954f96a2609e7b4e9c21ed6d74e6	Patch Third Party Advisory
https://github.com/symfony/symfony/security/advisories/GHSA-mcx4-f5f5-4859	Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/C36JLPHUPKDFAX6D5WYFC4ALO2K7RDUQ/
https://symfony.com/blog/cve-2020-5255-prevent-cache-poisoning-via-a-response-content-type-header	Third Party Advisory

URL

CVE-2020-5255 | Prevent cache poisoning via a Response Content-Type header

Exploit prediction scoring system (EPSS) score for CVE-2020-5255

Common vulnerability scoring system (CVSS) metrics for CVE-2020-5255

Weakness enumeration for CVE-2020-5255

GitHub Security Advisory for CVE-2020-5255

OS Trackers for CVE-2020-5255

Affected software / configurations for CVE-2020-5255

References for CVE-2020-5255