CVE-2022-25881 ReDoS in http-cache-semantics: upgrade to v4.1.1

The Node.js package ecosystem picked up another ReDoS footnote in January 2023 when a Regular Expression Denial of Service affecting the widely used http-cache-semantics library was disclosed; the flaw, tracked as CVE-2022-25881, affects versions of http-cache-semantics prior to v4.1.1 and can be triggered by specially crafted HTTP request header values when a server uses the library to read cache policies.

Background / Overview​

The http-cache-semantics library implements logic to determine HTTP caching behavior from request and response headers. It's one of those small, focused dependencies that quietly sits under other higher-level modules (for example, HTTP clients and proxies) and makes decisions about whether a response is cacheable and for how long. Because it processes untrusted request header strings, a mistake in its parser or regular expressions can create an attack surface for ReDoS — a class of vulnerabilities where an attacker supplies input that causes a regular expression engine to consume excessive CPU cycles.
The official vulnerability descriptions agree on the essentials: versions older than 4.1.1 are vulnerable; the problem is a catastrophic regex/backtracking issue that can be exercised by manipulating header values; exploitation results in denial of service (excessive CPU and degraded availability). Trusted catalogues record the CVE and list the fix as upgrading to http-cache-semantics v4.1.1 or later.

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Why this matters: small library, big reach​

• Transitive exposure: Even if your application doesn't import http-cache-semantics directly, many HTTP-related packages do. That means a vulnerability can propagate transitively into a huge set of deployments.
• Network-exploitable: The attack vector is network-facing — an unauthenticated remote attacker can send malicious request headers. No credentials or prior access are required.
• Availability impact: While ReDoS does not grant data exfiltration or code execution, it can render services unresponsive by tying up CPU resources and event-loop capacity — especially dangerous for single-threaded Node.js processes. Multiple parallel requests amplify the effect and can cause sustained or persistent outages.

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Technical analysis: what fails and why​

The underlying flaw​

At its heart, CVE-2022-25881 is an instance of Inefficient Regular Expression Complexity (CWE-1333). The library uses a regular expression to parse certain header formats. Under benign inputs the regex behaves normally; under adversarially structured inputs the regex engine performs exponential backtracking and consumes large amounts of CPU—effectively a single-request denial of service. This is the classic ReDoS pattern.

Where the library is used​

http-cache-semantics is commonly used by caching layers and HTTP client libraries to compute cache policy decisions (for example, whether a response can be considered fresh, or whether a cached response satisfies a subsequent request). Any server that calls into the library with untrusted request headers to construct a cache policy is a candidate for exploitation. Several vendor advisories have noted the package appears as a dependency in larger products, which led to vendor-specific security bulletins.

Proof-of-concept and reproducibility​

Security researchers and Snyk published a straightforward PoC that demonstrates the timing spike caused by a crafted header value. The PoC constructs an increasingly long string designed to trigger catastrophic backtracking and measures the time taken by the library to parse it; runtimes grow rapidly with input length. The presence of a PoC confirms exploitability in controlled environments.

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Severity and scoring — why scores differ​

Different authorities assigned different severity metrics to this CVE:

• The NVD entry and multiple Linux distribution advisories associate a high impact profile (e.g., CVSS scores around 7.5) because NVD treats the availability impact as significant in network-exploitable components.
• Snyk reports a lower CVSS-like rating (Snyk's internal score ~5.3) and describes the attack as low complexity to carry out but limited to availability impact. Snyk also provides the PoC and exploitability context.

Why the difference? Scoring models and contextual assumptions vary: some systems emphasize potential service disruption for internet-facing middleware, while others focus on the exact scope of damage (confidentiality vs availability). Practitioners should treat the disagreement as a signal to prioritize remediation — the vulnerability is easy to weaponize against availability and affects internet-facing services.

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Real-world impact: what an attacker can do​

• Cause severe CPU spikes on Node.js processes that invoke the vulnerable code path, making the service slow or unresponsive.
• With a modest request volume, mount a sustained DoS (if the target has minimal horizontal scaling or is single-threaded).
• Force failovers, trigger autoscaling (increasing costs), or cause worker crashes if monitoring or watchdogs react poorly to high CPU.

Notably, there is no indication the flaw by itself enables data leakage, remote code execution, or privilege escalation. The primary consequence is availability degradation or denial. Several vendor advisories corroborate this availability-focused impact.

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Affected versions and dependencies​

• Affected: http-cache-semantics versions < 4.1.1.
• Fixed in: http-cache-semantics v4.1.1 (upgrading to this version or later mitigates the issue).
• Transitive reach: any package that resolves http-cache-semantics into its dependency tree (for example, some HTTP clients, caching proxies, or higher-level libraries) can be indirectly affected. Vendor advisories (IBM, SUSE, Oracle Linux, IBM Cloud Pak for Data, and others) list product-specific exposures where the vulnerable package is included transitively.

When auditing a codebase or container image, search package-lock.json, yarn.lock, or the build manifest for http-cache-semantics and check the resolved version. If you find versions earlier than 4.1.1, you need to plan remediation.

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Detection and hunting advice​

• Log scanning
• Inspect HTTP server logs for suspicious requests with unusually long or repetitive header values, especially in headers processed for caching (e.g., Cache-Control, Pragma, Vary, or custom headers used to build CachePolicy objects). Unusual whitespace patterns, long repeated characters, or header values that look like regex traps are strong indicators.
• Process metrics
• Correlate spikes in CPU utilization for Node.js worker processes with incoming request timestamps. If CPU rises sharply in conjunction with single requests, suspect a ReDoS-style input. Modern observability stacks (traces, APM) can tie a slow call to a specific request.
• IDS / WAF rules
• Add WAF/NGFW signatures to normalize or rate-limit suspicious header lengths and patterns before they reach application logic. If you maintain custom IDS rules, consider matching on unusually long Cache-Control header values or repeated whitespace sequences. Vendor advisories and PoCs provide examples of malicious inputs that can inform rule creation.
• Software composition analysis (SCA)
• Use SCA tools to identify transitive instances of vulnerable http-cache-semantics in build artifacts, containers, and package manifests. Prioritize internet-facing services and components that parse request headers.

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Mitigation and remediation steps​

The single most effective mitigation is to upgrade:

• Upgrade http-cache-semantics to v4.1.1 or later. This is the upstream fix and eliminates the vulnerable regex behavior. Ensure transitive dependencies are updated or the packages that depend on the library are upgraded to versions that incorporate the fixed version.

If you cannot upgrade immediately, consider layered mitigations:

• Input validation and sanitization:
• Enforce maximum header lengths for headers that get parsed as cache policies.
• Reject or trim headers with suspicious patterns (long runs of repeated characters or nested grouping tokens).
• Rate limiting:
• Apply rate limits per IP or per connection, so a single client cannot send many crafted requests in rapid succession.
• WAF / reverse proxy normalization:
• Put an upstream proxy or WAF that normalizes headers and enforces sensible limits on header length and character sets before requests reach the application layer.
• Isolate and scale:
• Run critical HTTP handling processes with isolation and quotas; run vulnerable code in separate worker pools that can be restarted automatically to limit blast radius.
• Monitoring and auto-heal:
• Set alerts for CPU spikes correlated with web requests and enable automated restart/rolling replacement for compromised workers.

Vendor advisories uniformly recommend updating; some vendors also released patches for their products that bundle the fixed dependency — consult product release notes if you consume vendor-supplied images or distributions.

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Practical remediation checklist (recommended order)​

• Inventory: locate all instances (direct or transitive) of http-cache-semantics in code, containers, and vendor packages.
• Prioritize: focus on internet-facing services and processes that construct cache policy objects from request headers.
• Upgrade: patch to http-cache-semantics v4.1.1+ or upgrade the consuming package to a release that depends on the fixed version.
• Compensate: deploy WAF/NGFW rules or header-length limits where upgrade is delayed.
• Test: run load tests with representative header inputs and measure CPU impact to ensure the fix prevents ReDoS behavior.
• Monitor: add observability checks for header anomalies and CPU spikes; maintain alerting on sustained high CPU correlated with request patterns.

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Code-level notes for developers​

• If your code constructs a CachePolicy from incoming request headers, treat those headers as untrusted input. Avoid passing raw, long header strings directly into third-party parsers without validation.
• Prefer defensive parsing: enforce an upper bound on header length and strip or reject excessive whitespace or suspicious sequences before passing to the cache-policy library.
• If you vendor or patch dependencies yourself, inspect the regex in question and validate the fix; the upstream commit that addressed the issue (and corresponding unit tests) is a primary artifact to review. Snyk and other vulnerability write-ups point to the specific lines and the commit that fixed the problematic regex.

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Detection signatures and examples (operational)​

• Example PoC behavior (conceptual): a Cache-Control header containing a large block of spaces or repeated characters followed by a mismatch character can force exponential regex backtracking. Snyk’s PoC shows a short test harness that times CachePolicy construction for increasing string sizes — a quick way to validate whether a build is vulnerable in your environment.
• IDS/WAF indicators to consider:
• Cache-Control headers with length > N (choose conservative default such as 1,024 bytes).
• Header values with repetitive patterns (e.g., >100 contiguous identical characters).
• High per-IP request rates that include long header values.

Be conservative when applying blocking rules in production — start by alerting rather than dropping to avoid false positives. Use a staged rollout: monitor, then block, then refine.

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Risk assessment — who should worry the most?​

High priority:

• Public-facing APIs and proxies that examine request headers to compute caching decisions.
• Server environments where Node.js processes run single-threaded worker models without horizontal redundancy.
• Vendor-provided appliances and containers that include older library versions and may not be updated frequently. Several vendor advisories (IBM, SUSE, Oracle Linux) list affected product lines.

Lower, but non-trivial priority:

• Internal services shielded behind robust perimeter controls and WAFs, but still worth inventorying and patching for defense-in-depth.

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Timeline and disclosure notes​

• Disclosure and advisories emerged in late January 2023; multiple security vendors and distribution maintainers published advisories and patches shortly after. The upstream author fixed the vulnerability in http-cache-semantics v4.1.1. NVD and Snyk track the CVE and provide PoC and remediation guidance.
• Some vendor bulletins that reference the package describe downstream impacts where the vulnerable dependency was bundled into larger products; these vendor updates are an important reminder that package fixes alone may not protect packaged product images until those images are rebuilt and redeployed.

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Strengths and limitations of the fix​

Strengths:

• The upstream remedy (v4.1.1) addresses the regex complexity at the source, which is the correct approach: fix the parser so crafted inputs cannot cause catastrophic backtracking. Upgrading removes the root cause.

Limitations and residual risks:

• Transitive dependencies and vendor bundles mean some installations will remain vulnerable until vendors rebuild images and administrators redeploy patched versions.
• WAF rules and header truncation may mitigate but are brittle and can create false positives if not tuned.
• Even after patching, obsolete runtime images (containers, VMs) may continue to run vulnerable code until redeployed — so inventory and lifecycle management are crucial.

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Final recommendations for WindowsForum readers (practical, prioritized)​

• Immediately inventory: find any direct or transitive uses of http-cache-semantics in your stacks (package-lock.json, yarn.lock, container manifests, vendor bundles).
• Upgrade first: move to http-cache-semantics v4.1.1+ wherever possible and rebuild images/services.
• If upgrade is delayed:
• Apply WAF/edge rate limits and header-length caps.
• Add observability hooks to detect CPU spikes tied to request timings.
• Coordinate with vendors: if you use vendor-supplied images, track vendor advisories and apply vendor patches when available — do not assume your application is safe until the vendor confirms the bundled dependency has been updated.
• Harden parsing code: treat all header values as untrusted input and apply defensive validation at the application boundary.
• Test post-fix: exercise fixed systems with both normal and intentionally malformed header values in a staging environment to ensure the fix behaves as expected. Use the Snyk PoC or a controlled equivalent for validation.

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Conclusion​

CVE-2022-25881 is a textbook ReDoS vulnerability: small root cause, outsized operational effect. The fix is simple in principle — upgrade to http-cache-semantics v4.1.1 — but the practical work is inventory, testing, and redeployment across all nodes where the library runs, direct or transitive. For organizations that depend on Node.js HTTP tooling, the incident is a reminder that even tiny packages deserve attention in your software composition and runtime monitoring programs. Prioritize patching internet-facing services, apply compensating controls where immediate upgrades aren't possible, and use this case as an operational test for your ability to detect and remediate availability-focused vulnerabilities.

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Source: MSRC Security Update Guide - Microsoft Security Response Center