Impact
The fix for CVE-2021-23337 added validation for the variable option in _.template but did not apply the same validation to options.imports key names. Both paths flow into the same Function() constructor sink.
When an application passes untrusted input as options.imports key names, an attacker can inject default-parameter expressions that execute arbitrary code at template compilation time.
Additionally, _.template uses assignInWith to merge imports, which enumerates inherited properties via for..in. If Object.prototype has been polluted by any other vector, the polluted keys are copied into the imports object and passed to Function().
Patches
Users should upgrade to version 4.18.0.
The fix applies two changes:
- Validate
importsKeys against the existing reForbiddenIdentifierChars regex (same check already used for the variable option)
- Replace
assignInWith with assignWith when merging imports, so only own properties are enumerated
Workarounds
Do not pass untrusted input as key names in options.imports. Only use developer-controlled, static key names.
References
dolevmiz1 Reporter
bugbunny-research Reporter
M0nd0R Reporter
UlisesGascon Remediation developer
falsyvalues Remediation reviewer
jonchurch Remediation reviewer
threalwinky Reporter
jdalton Remediation reviewer
Impact
The fix for CVE-2021-23337 added validation for the
variableoption in_.templatebut did not apply the same validation tooptions.importskey names. Both paths flow into the sameFunction()constructor sink.When an application passes untrusted input as
options.importskey names, an attacker can inject default-parameter expressions that execute arbitrary code at template compilation time.Additionally,
_.templateusesassignInWithto merge imports, which enumerates inherited properties viafor..in. IfObject.prototypehas been polluted by any other vector, the polluted keys are copied into the imports object and passed toFunction().Patches
Users should upgrade to version 4.18.0.
The fix applies two changes:
importsKeysagainst the existingreForbiddenIdentifierCharsregex (same check already used for thevariableoption)assignInWithwithassignWithwhen merging imports, so only own properties are enumeratedWorkarounds
Do not pass untrusted input as key names in
options.imports. Only use developer-controlled, static key names.References