CVE-2026-109449: Symfony: Unbounded Nesting in the Expression Parser Lets Untrusted Expressions Crash the Process
In Symfony Expression Language versions prior to 5.4.54, 6.4.48, 7.4.21, and 8.1.9, the parser of the ExpressionLanguage component built its node tree with no bound on its depth. An expression made of tens of thousands of nested unary operators, array literals, binary operators or property accesses crashed the PHP process, while the tree was being freed.
#[Assert\ExpressionSyntax] passes the validated value to ExpressionLanguage::lint(), so an application validating expressions written by its users could be crashed by a single request, and so could any code parsing untrusted expressions.
This issue affects symfony/expression-language through v5.4.53, v6.0.0-BETA1 through v6.4.47, v7.0.0-BETA1 through v7.4.20, and v8.0.0-BETA1 through v8.1.8.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
symfony/expression-languageto a version that resolves this vulnerability.Fixed in 5.4.54 - Upgrade
Upgrade
symfony/expression-languageto a version that resolves this vulnerability.Fixed in 6.4.48 - Upgrade
Upgrade
symfony/expression-languageto a version that resolves this vulnerability.Fixed in 7.4.21 - Upgrade
Upgrade
symfony/expression-languageto a version that resolves this vulnerability.Fixed in 8.1.9
Event History
Frequently Asked Questions
Which applications are realistically exposed to this issue?
Applications are exposed if they parse expressions supplied by untrusted users through the Symfony ExpressionLanguage component. This includes applications using Assert\ExpressionSyntax to validate user-written expressions, because it passes the value to ExpressionLanguage::lint().
What does an attacker need to do to trigger the crash?
An attacker needs to submit a crafted expression with tens of thousands of nested unary operators, array literals, binary operators, or property accesses. A single request can crash the PHP process when the parser's node tree is freed.
Which versions are affected and which versions contain the fix?
Affected versions are Expression Language through 5.4.53, 6.0.0-BETA1 through 6.4.47, 7.0.0-BETA1 through 7.4.20, and 8.0.0-BETA1 through 8.1.8. Fixed releases are 5.4.54, 6.4.48, 7.4.21, and 8.1.9.