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Also guard against entity expansion attacks in attribute values (#149)
Looks like I missed this in the earlier implementation. This refactors the implementation a bit into a shape that also applies to any entities expanded as part of normalizing attribute values, both normal and default.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,55 @@ | ||
| import { EntityRefEvent } from './parserEvents'; | ||
| import { throwErrorWithContext } from './parsingAlgorithms'; | ||
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| /* | ||
| * Guard against entity expansion attacks by keeping track of the initial input | ||
| * length vs. the expanded input length. The latter includes the length of the | ||
| * replacement text for each processed entity reference. An attack is likely if | ||
| * the ratio between the two exceeds the maximum amplification factor AND the | ||
| * expanded input length exceeds a threshold. This approach and defaults are | ||
| * taken from libexpat's billion laughs attack protection. | ||
| */ | ||
| export default class EntityExpansionGuard { | ||
| private readonly _initialInputLength: number; | ||
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| private _expandedInputLength: number; | ||
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| private readonly _entityExpansionThreshold: number; | ||
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| private readonly _entityExpansionMaxAmplification: number; | ||
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| private _topLevelEntityRef: EntityRefEvent | null = null; | ||
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| private _depth = 0; | ||
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| public constructor( | ||
| initialInputLength: number, | ||
| entityExpansionThreshold: number, | ||
| entityExpansionMaxAmplification: number | ||
| ) { | ||
| this._initialInputLength = initialInputLength; | ||
| this._expandedInputLength = initialInputLength; | ||
| this._entityExpansionThreshold = entityExpansionThreshold; | ||
| this._entityExpansionMaxAmplification = entityExpansionMaxAmplification; | ||
| } | ||
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| public enter(event: EntityRefEvent, replacementTextLength: number): void { | ||
| const topLevelEntityRef = this._topLevelEntityRef ?? event; | ||
| this._expandedInputLength += replacementTextLength; | ||
| if (this._expandedInputLength > this._entityExpansionThreshold) { | ||
| const amplification = this._expandedInputLength / this._initialInputLength; | ||
| if (amplification > this._entityExpansionMaxAmplification) { | ||
| throwErrorWithContext('too much entity expansion', topLevelEntityRef); | ||
| } | ||
| } | ||
| this._topLevelEntityRef = topLevelEntityRef; | ||
| this._depth += 1; | ||
| } | ||
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| public exit(): void { | ||
| this._depth -= 1; | ||
| if (this._depth === 0) { | ||
| this._topLevelEntityRef = null; | ||
| } | ||
| } | ||
| } |
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