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Overview

sanitize-html is a library that allows you to clean up user-submitted HTML, preserving whitelisted elements and whitelisted attributes on a per-element basis

Affected versions of this package are vulnerable to Uncontrolled Recursion via its transitive dependency on deepmerge, which is affected by CVE-2026-93753. When sanitize-html merges configuration objects combining tag-specific and wildcard allowedClasses or allowedStyles entries, the deepmerge call could be exploited to cause a crash. The fix removes the deepmerge dependency entirely, replacing it with simple array concatenation.

Remediation

Upgrade sanitize-html to version 2.18.1 or higher.

References

• GitHub Advisory
• GitHub Commit
• GitHub Commit
• GitHub Release

Overview

Affected versions of this package are vulnerable to Prototype Pollution via mergeObject(), which fails to validate keys before writing them to target objects. An attacker can supply a malicious source object containing a __proto__ key in a merge operation, injecting attacker-controlled properties into the prototype of the returned object and causing applications to inherit unintended values when accessing properties without own-property checks.

Notes:

• The proposed fix in PR #274 denies proto by name in both merge loops and adds regression tests. The PR was closed without merging and the main branch still contains the vulnerable guard.
• The maintainer closed the issue as “completed” without a technical rebuttal. Per them, they will not review the fix
• Scope: According to the tests, a plain {} target is guarded. The global Object.prototype is not modified, and Impact is limited to the object returned by merge().

Details

Prototype Pollution is a vulnerability affecting JavaScript. Prototype Pollution refers to the ability to inject properties into existing JavaScript language construct prototypes, such as objects. JavaScript allows all Object attributes to be altered, including their magical attributes such as __proto__, constructor and prototype. An attacker manipulates these attributes to overwrite, or pollute, a JavaScript application object prototype of the base object by injecting other values. Properties on the Object.prototype are then inherited by all the JavaScript objects through the prototype chain. When that happens, this leads to either denial of service by triggering JavaScript exceptions, or it tampers with the application source code to force the code path that the attacker injects, thereby leading to remote code execution.

There are two main ways in which the pollution of prototypes occurs:

• Unsafe Object recursive merge

• Property definition by path

Unsafe Object recursive merge

The logic of a vulnerable recursive merge function follows the following high-level model:

merge (target, source)

  foreach property of source

    if property exists and is an object on both the target and the source

      merge(target[property], source[property])

    else

      target[property] = source[property]

When the source object contains a property named __proto__ defined with Object.defineProperty() , the condition that checks if the property exists and is an object on both the target and the source passes and the merge recurses with the target, being the prototype of Object and the source of Object as defined by the attacker. Properties are then copied on the Object prototype.

Clone operations are a special sub-class of unsafe recursive merges, which occur when a recursive merge is conducted on an empty object: merge({},source).

lodash and Hoek are examples of libraries susceptible to recursive merge attacks.

Property definition by path

There are a few JavaScript libraries that use an API to define property values on an object based on a given path. The function that is generally affected contains this signature: theFunction(object, path, value)

If the attacker can control the value of “path”, they can set this value to __proto__.myValue. myValue is then assigned to the prototype of the class of the object.

Types of attacks

There are a few methods by which Prototype Pollution can be manipulated:

Affected environments

The following environments are susceptible to a Prototype Pollution attack:

• Application server

• Web server

• Web browser

How to prevent

1. Freeze the prototype— use Object.freeze (Object.prototype).

2. Require schema validation of JSON input.

3. Avoid using unsafe recursive merge functions.

4. Consider using objects without prototypes (for example, Object.create(null)), breaking the prototype chain and preventing pollution.

5. As a best practice use Map instead of Object.

For more information on this vulnerability type:

Arteau, Olivier. “JavaScript prototype pollution attack in NodeJS application.” GitHub, 26 May 2018

Remediation

There is no fixed version for deepmerge.

References

• GitHub Issue
• Vulnerable Code

Overview

nodemailer is an Easy as cake e-mail sending from your Node.js applications

Affected versions of this package are vulnerable to Inefficient Algorithmic Complexity in the isPartialResponse function and the _onData method of SMTPConnection in src/smtp-connection/index.ts, where each continuation line is appended to the same response queue entry and the partial check re-runs str.slice(str.lastIndexOf('\n') + 1) over the whole accumulated string, forcing a full flatten of the V8 rope on every line. An attacker who controls or impersonates the SMTP peer can starve the event loop of the entire Node.js process, halting every other connection, request, and timer in it, and grow memory without bound, by streaming continuation lines such as 220-flood and never sending a completion line. This is reachable during the greeting and EHLO phases before the client authenticates or issues any command, the connection module applies no cap on accumulated reply size or line count, and the existing timers do not help, since greetingTimeout fires into an already starved loop and the inactivity socketTimeout resets on every inbound chunk.

Remediation

Upgrade nodemailer to version 10.0.10 or higher.

References

• GitHub Advisory
• GitHub Commit

Overview

nodemailer is an Easy as cake e-mail sending from your Node.js applications

Affected versions of this package are vulnerable to Regular Expression Denial of Service (ReDoS) in the AUTH mechanism detection of SMTPConnection._actionEHLO in src/smtp-connection/index.ts, where all four patterns contain (?:(\s+|=)[^\n]*\s+|\s+|=) before the mechanism literal, so [^\n]* and the trailing \s+ compete for the same whitespace and the engine tries every partition of the run before failing. An attacker who controls or intercepts the SMTP peer can stall the whole Node.js process, suspending every other request, timer, and connection in it, by answering EHLO with a line such as 250-AUTH followed by a long whitespace run and a token that matches no mechanism. This is reached before authentication on the first EHLO against a server offering no STARTTLS, on the second after a STARTTLS upgrade, or on the first when ignoreTLS is set, and the synchronous stall cannot be interrupted by the socket timeout, with roughly 4,000 whitespace characters already costing tens of seconds per pattern.

Details

Denial of Service (DoS) describes a family of attacks, all aimed at making a system inaccessible to its original and legitimate users. There are many types of DoS attacks, ranging from trying to clog the network pipes to the system by generating a large volume of traffic from many machines (a Distributed Denial of Service - DDoS - attack) to sending crafted requests that cause a system to crash or take a disproportional amount of time to process.

The Regular expression Denial of Service (ReDoS) is a type of Denial of Service attack. Regular expressions are incredibly powerful, but they aren't very intuitive and can ultimately end up making it easy for attackers to take your site down.

Let’s take the following regular expression as an example:

regex = /A(B|C+)+D/

This regular expression accomplishes the following:

• A The string must start with the letter 'A'
• (B|C+)+ The string must then follow the letter A with either the letter 'B' or some number of occurrences of the letter 'C' (the + matches one or more times). The + at the end of this section states that we can look for one or more matches of this section.
• D Finally, we ensure this section of the string ends with a 'D'

The expression would match inputs such as ABBD, ABCCCCD, ABCBCCCD and ACCCCCD

It most cases, it doesn't take very long for a regex engine to find a match:

$ time node -e '/A(B|C+)+D/.test("ACCCCCCCCCCCCCCCCCCCCCCCCCCCCD")'
0.04s user 0.01s system 95% cpu 0.052 total

$ time node -e '/A(B|C+)+D/.test("ACCCCCCCCCCCCCCCCCCCCCCCCCCCCX")'
1.79s user 0.02s system 99% cpu 1.812 total

The entire process of testing it against a 30 characters long string takes around ~52ms. But when given an invalid string, it takes nearly two seconds to complete the test, over ten times as long as it took to test a valid string. The dramatic difference is due to the way regular expressions get evaluated.

Most Regex engines will work very similarly (with minor differences). The engine will match the first possible way to accept the current character and proceed to the next one. If it then fails to match the next one, it will backtrack and see if there was another way to digest the previous character. If it goes too far down the rabbit hole only to find out the string doesn’t match in the end, and if many characters have multiple valid regex paths, the number of backtracking steps can become very large, resulting in what is known as catastrophic backtracking.

Let's look at how our expression runs into this problem, using a shorter string: "ACCCX". While it seems fairly straightforward, there are still four different ways that the engine could match those three C's:

1. CCC
2. CC+C
3. C+CC
4. C+C+C.

The engine has to try each of those combinations to see if any of them potentially match against the expression. When you combine that with the other steps the engine must take, we can use RegEx 101 debugger to see the engine has to take a total of 38 steps before it can determine the string doesn't match.

From there, the number of steps the engine must use to validate a string just continues to grow.

By the time the string includes 14 C's, the engine has to take over 65,000 steps just to see if the string is valid. These extreme situations can cause them to work very slowly (exponentially related to input size, as shown above), allowing an attacker to exploit this and can cause the service to excessively consume CPU, resulting in a Denial of Service.

Remediation

Upgrade nodemailer to version 10.0.13 or higher.

References

• GitHub Advisory
• GitHub Commit

Overview

nodemailer is an Easy as cake e-mail sending from your Node.js applications

Affected versions of this package are vulnerable to Inefficient Algorithmic Complexity in the parseHeaders method of MessageParser in src/dkim/message-parser.ts, whose unfold loop appends each continuation line onto the preceding one and then re-tests the anchored /^[ \t]/ against the growing merged line, forcing a full flatten of the accumulated rope on every iteration. A user who can influence the size of a header that folds into many continuation lines, most readily a large recipient list but equally a long Subject, References, or custom header, can starve the event loop of the single threaded sending process for seconds on every signed message, halting all other work in it. This requires DKIM signing to be enabled, and nothing bounds the input, since checkHeaders buffers the header block until the terminator and the signer's MAX_MESSAGE_SIZE and cacheTreshold apply to the body stream rather than the headers, while the MIME composer folds the value deterministically at column 76 so the line count scales with the supplied size.

Remediation

Upgrade nodemailer to version 10.0.10 or higher.

References

• GitHub Advisory
• GitHub Commit

Overview

handlebars is an extension to the Mustache templating language.

Affected versions of this package are vulnerable to Cross-site Scripting (XSS) in the quotedString() function in lib/handlebars/compiler/code-gen.js, which writes static template text into the JavaScript that Handlebars.precompile() generates without neutralizing a </script> sequence. An attacker can execute script in the browser under the embedding page's origin by placing </script> followed by markup in a template, which closes the script element early so the remainder is parsed as HTML. This requires the application to precompile templates the attacker controls and to embed the generated source directly in an inline <script> element, and the injected markup runs only when a user loads that page.

Workaround

This vulnerability can be avoided by serving precompiled templates as external JavaScript files rather than inlining the generated source in HTML.

Details

Cross-site scripting (or XSS) is a code vulnerability that occurs when an attacker “injects” a malicious script into an otherwise trusted website. The injected script gets downloaded and executed by the end user’s browser when the user interacts with the compromised website.

This is done by escaping the context of the web application; the web application then delivers that data to its users along with other trusted dynamic content, without validating it. The browser unknowingly executes malicious script on the client side (through client-side languages; usually JavaScript or HTML) in order to perform actions that are otherwise typically blocked by the browser’s Same Origin Policy.

Injecting malicious code is the most prevalent manner by which XSS is exploited; for this reason, escaping characters in order to prevent this manipulation is the top method for securing code against this vulnerability.

Escaping means that the application is coded to mark key characters, and particularly key characters included in user input, to prevent those characters from being interpreted in a dangerous context. For example, in HTML, < can be coded as &lt; and > can be coded as &gt; in order to be interpreted and displayed as themselves in text, while within the code itself, they are used for HTML tags. If malicious content is injected into an application that escapes special characters and that malicious content uses < and > as HTML tags, those characters are nonetheless not interpreted as HTML tags by the browser if they’ve been correctly escaped in the application code and in this way the attempted attack is diverted.

The most prominent use of XSS is to steal cookies (source: OWASP HttpOnly) and hijack user sessions, but XSS exploits have been used to expose sensitive information, enable access to privileged services and functionality and deliver malware.

Types of attacks

There are a few methods by which XSS can be manipulated:

Affected environments

The following environments are susceptible to an XSS attack:

• Web servers
• Application servers
• Web application environments

How to prevent

This section describes the top best practices designed to specifically protect your code:

• Sanitize data input in an HTTP request before reflecting it back, ensuring all data is validated, filtered or escaped before echoing anything back to the user, such as the values of query parameters during searches.
• Convert special characters such as ?, &, /, <, > and spaces to their respective HTML or URL encoded equivalents.
• Give users the option to disable client-side scripts.
• Redirect invalid requests.
• Detect simultaneous logins, including those from two separate IP addresses, and invalidate those sessions.
• Use and enforce a Content Security Policy (source: Wikipedia) to disable any features that might be manipulated for an XSS attack.
• Read the documentation for any of the libraries referenced in your code to understand which elements allow for embedded HTML.

Remediation

Upgrade handlebars to version 4.7.10 or higher.

References

• GitHub Advisory
• GitHub Commit
• GitHub PR
• GitHub Release
• Red Hat Bugzilla Bug

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