CWE-400
DiscouragedUncontrolled Resource Consumption
Abstraction: Class · Status: Draft
The product does not properly control the allocation and maintenance of a limited resource.
6465 vulnerabilities reference this CWE, most recent first.
GHSA-V6G3-9679-C8VH
Vulnerability from github – Published: 2022-05-24 17:19 – Updated: 2022-05-24 17:19An issue was discovered in Foxit PhantomPDF before 8.3.12. It allows stack consumption via nested function calls for XML parsing.
{
"affected": [],
"aliases": [
"CVE-2019-20815"
],
"database_specific": {
"cwe_ids": [
"CWE-400"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2020-06-04T16:15:00Z",
"severity": "MODERATE"
},
"details": "An issue was discovered in Foxit PhantomPDF before 8.3.12. It allows stack consumption via nested function calls for XML parsing.",
"id": "GHSA-v6g3-9679-c8vh",
"modified": "2022-05-24T17:19:13Z",
"published": "2022-05-24T17:19:13Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2019-20815"
},
{
"type": "WEB",
"url": "https://www.foxitsoftware.com/support/security-bulletins.php"
}
],
"schema_version": "1.4.0",
"severity": []
}
GHSA-V6G4-F4GG-HFQX
Vulnerability from github – Published: 2026-07-22 00:31 – Updated: 2026-07-22 00:31Vulnerability in the Oracle VM VirtualBox product of Oracle Virtualization (component: Core). The supported version that is affected is 7.2.12. Easily exploitable vulnerability allows low privileged attacker with logon to the infrastructure where Oracle VM VirtualBox executes to compromise Oracle VM VirtualBox. Successful attacks of this vulnerability can result in unauthorized ability to cause a hang or frequently repeatable crash (complete DOS) of Oracle VM VirtualBox. CVSS 3.1 Base Score 5.5 (Availability impacts). CVSS Vector: (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H).
{
"affected": [],
"aliases": [
"CVE-2026-47044"
],
"database_specific": {
"cwe_ids": [
"CWE-400"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-07-21T22:17:10Z",
"severity": "MODERATE"
},
"details": "Vulnerability in the Oracle VM VirtualBox product of Oracle Virtualization (component: Core). The supported version that is affected is 7.2.12. Easily exploitable vulnerability allows low privileged attacker with logon to the infrastructure where Oracle VM VirtualBox executes to compromise Oracle VM VirtualBox. Successful attacks of this vulnerability can result in unauthorized ability to cause a hang or frequently repeatable crash (complete DOS) of Oracle VM VirtualBox. CVSS 3.1 Base Score 5.5 (Availability impacts). CVSS Vector: (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H).",
"id": "GHSA-v6g4-f4gg-hfqx",
"modified": "2026-07-22T00:31:17Z",
"published": "2026-07-22T00:31:17Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-47044"
},
{
"type": "WEB",
"url": "https://www.oracle.com/security-alerts/cpujul2026.html"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
]
}
GHSA-V6H2-P8H4-QCJW
Vulnerability from github – Published: 2025-06-09 21:30 – Updated: 2025-06-11 20:54A vulnerability was found in juliangruber brace-expansion up to 1.1.11/2.0.1/3.0.0/4.0.0. It has been rated as problematic. Affected by this issue is the function expand of the file index.js. The manipulation leads to inefficient regular expression complexity. The attack may be launched remotely. The complexity of an attack is rather high. The exploitation is known to be difficult. The exploit has been disclosed to the public and may be used. Upgrading to version 1.1.12, 2.0.2, 3.0.1 and 4.0.1 is able to address this issue. The name of the patch is a5b98a4f30d7813266b221435e1eaaf25a1b0ac5. It is recommended to upgrade the affected component.
{
"affected": [
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 2.0.1"
},
"package": {
"ecosystem": "npm",
"name": "brace-expansion"
},
"ranges": [
{
"events": [
{
"introduced": "2.0.0"
},
{
"fixed": "2.0.2"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 1.1.11"
},
"package": {
"ecosystem": "npm",
"name": "brace-expansion"
},
"ranges": [
{
"events": [
{
"introduced": "1.0.0"
},
{
"fixed": "1.1.12"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "npm",
"name": "brace-expansion"
},
"ranges": [
{
"events": [
{
"introduced": "3.0.0"
},
{
"fixed": "3.0.1"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"3.0.0"
]
},
{
"package": {
"ecosystem": "npm",
"name": "brace-expansion"
},
"ranges": [
{
"events": [
{
"introduced": "4.0.0"
},
{
"fixed": "4.0.1"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"4.0.0"
]
}
],
"aliases": [
"CVE-2025-5889"
],
"database_specific": {
"cwe_ids": [
"CWE-400"
],
"github_reviewed": true,
"github_reviewed_at": "2025-06-10T22:51:59Z",
"nvd_published_at": "2025-06-09T19:15:25Z",
"severity": "LOW"
},
"details": "A vulnerability was found in juliangruber brace-expansion up to 1.1.11/2.0.1/3.0.0/4.0.0. It has been rated as problematic. Affected by this issue is the function expand of the file index.js. The manipulation leads to inefficient regular expression complexity. The attack may be launched remotely. The complexity of an attack is rather high. The exploitation is known to be difficult. The exploit has been disclosed to the public and may be used. Upgrading to version 1.1.12, 2.0.2, 3.0.1 and 4.0.1 is able to address this issue. The name of the patch is `a5b98a4f30d7813266b221435e1eaaf25a1b0ac5`. It is recommended to upgrade the affected component.",
"id": "GHSA-v6h2-p8h4-qcjw",
"modified": "2025-06-11T20:54:10Z",
"published": "2025-06-09T21:30:51Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2025-5889"
},
{
"type": "WEB",
"url": "https://github.com/juliangruber/brace-expansion/pull/65/commits/a5b98a4f30d7813266b221435e1eaaf25a1b0ac5"
},
{
"type": "WEB",
"url": "https://github.com/juliangruber/brace-expansion/commit/0b6a9781e18e9d2769bb2931f4856d1360243ed2"
},
{
"type": "WEB",
"url": "https://github.com/juliangruber/brace-expansion/commit/15f9b3c75ebf5988198241fecaebdc45eff28a9f"
},
{
"type": "WEB",
"url": "https://github.com/juliangruber/brace-expansion/commit/36603d5f3599a37af9e85eda30acd7d28599c36e"
},
{
"type": "WEB",
"url": "https://github.com/juliangruber/brace-expansion/commit/c3c73c8b088defc70851843be88ccc3af08e7217"
},
{
"type": "WEB",
"url": "https://gist.github.com/mmmsssttt404/37a40ce7d6e5ca604858fe30814d9466"
},
{
"type": "PACKAGE",
"url": "https://github.com/juliangruber/brace-expansion"
},
{
"type": "WEB",
"url": "https://vuldb.com/?ctiid.311660"
},
{
"type": "WEB",
"url": "https://vuldb.com/?id.311660"
},
{
"type": "WEB",
"url": "https://vuldb.com/?submit.585717"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L",
"type": "CVSS_V3"
},
{
"score": "CVSS:4.0/AV:N/AC:H/AT:N/PR:L/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:P/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",
"type": "CVSS_V4"
}
],
"summary": "brace-expansion Regular Expression Denial of Service vulnerability"
}
GHSA-V6J3-7JRW-HQ2P
Vulnerability from github – Published: 2022-05-17 04:59 – Updated: 2023-08-25 23:09Rack before 1.1.3, 1.2.x before 1.2.5, and 1.3.x before 1.3.6 computes hash values for form parameters without restricting the ability to trigger hash collisions predictably, which allows remote attackers to cause a denial of service (CPU consumption) by sending many crafted parameters.
{
"affected": [
{
"package": {
"ecosystem": "RubyGems",
"name": "rack"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "1.1.3"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "RubyGems",
"name": "rack"
},
"ranges": [
{
"events": [
{
"introduced": "1.2.0"
},
{
"fixed": "1.2.5"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "RubyGems",
"name": "rack"
},
"ranges": [
{
"events": [
{
"introduced": "1.3.0"
},
{
"fixed": "1.3.6"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "Maven",
"name": "org.jruby:jruby-parent"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "1.6.5.1"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2011-5036"
],
"database_specific": {
"cwe_ids": [
"CWE-328",
"CWE-400"
],
"github_reviewed": true,
"github_reviewed_at": "2023-03-27T16:12:55Z",
"nvd_published_at": "2011-12-30T01:55:00Z",
"severity": "MODERATE"
},
"details": "Rack before 1.1.3, 1.2.x before 1.2.5, and 1.3.x before 1.3.6 computes hash values for form parameters without restricting the ability to trigger hash collisions predictably, which allows remote attackers to cause a denial of service (CPU consumption) by sending many crafted parameters.",
"id": "GHSA-v6j3-7jrw-hq2p",
"modified": "2023-08-25T23:09:49Z",
"published": "2022-05-17T04:59:13Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2011-5036"
},
{
"type": "WEB",
"url": "https://gist.github.com/52bbc6b9cc19ce330829"
},
{
"type": "WEB",
"url": "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/rack/CVE-2011-5036.yml"
},
{
"type": "WEB",
"url": "https://web.archive.org/web/20120201040317/http://jruby.org/2011/12/27/jruby-1-6-5-1"
},
{
"type": "WEB",
"url": "https://web.archive.org/web/20130213132312/http://archives.neohapsis.com/archives/bugtraq/2011-12/0181.html"
},
{
"type": "WEB",
"url": "http://www.debian.org/security/2013/dsa-2783"
},
{
"type": "WEB",
"url": "http://www.kb.cert.org/vuls/id/903934"
},
{
"type": "WEB",
"url": "http://www.nruns.com/_downloads/advisory28122011.pdf"
},
{
"type": "WEB",
"url": "http://www.ocert.org/advisories/ocert-2011-003.html"
}
],
"schema_version": "1.4.0",
"severity": [],
"summary": "Rack Gem Subject to Denial of Service via Hash Collisions"
}
GHSA-V6MH-52Q2-XJGF
Vulnerability from github – Published: 2022-05-12 00:02 – Updated: 2022-05-19 00:00Asus DSL-N14U-B1 1.1.2.3_805 allows remote attackers to cause a Denial of Service (DoS) via a TCP SYN scan using nmap.
{
"affected": [],
"aliases": [
"CVE-2021-3254"
],
"database_specific": {
"cwe_ids": [
"CWE-400"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2022-05-11T12:15:00Z",
"severity": "HIGH"
},
"details": "Asus DSL-N14U-B1 1.1.2.3_805 allows remote attackers to cause a Denial of Service (DoS) via a TCP SYN scan using nmap.",
"id": "GHSA-v6mh-52q2-xjgf",
"modified": "2022-05-19T00:00:17Z",
"published": "2022-05-12T00:02:04Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2021-3254"
},
{
"type": "WEB",
"url": "https://github.com/kaisersource/kaisersource.github.io/blob/main/_posts/2021-01-22-dsl-n14u.md"
},
{
"type": "WEB",
"url": "https://kaisersource.github.io/dsl-n14u-syn"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
]
}
GHSA-V6MW-4FWV-QVR5
Vulnerability from github – Published: 2025-07-04 18:30 – Updated: 2025-07-08 18:31Uncontrolled Resource Consumption vulnerability in Wikimedia Foundation Mediawiki - IPInfo Extension allows Excessive Allocation.This issue affects Mediawiki - IPInfo Extension: from 1.39.X before 1.39.13, from 1.42.X before 1.42.7, from 1.43.X before 1.43.2.
{
"affected": [],
"aliases": [
"CVE-2025-53481"
],
"database_specific": {
"cwe_ids": [
"CWE-400"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2025-07-04T16:15:22Z",
"severity": "HIGH"
},
"details": "Uncontrolled Resource Consumption vulnerability in Wikimedia Foundation Mediawiki - IPInfo Extension allows Excessive Allocation.This issue affects Mediawiki - IPInfo Extension: from 1.39.X before 1.39.13, from 1.42.X before 1.42.7, from 1.43.X before 1.43.2.",
"id": "GHSA-v6mw-4fwv-qvr5",
"modified": "2025-07-08T18:31:06Z",
"published": "2025-07-04T18:30:56Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2025-53481"
},
{
"type": "WEB",
"url": "https://gerrit.wikimedia.org/r/q/I08a7154f8fa08bb6f0940e522075bdc2a3d4433f"
},
{
"type": "WEB",
"url": "https://gerrit.wikimedia.org/r/q/I474b7a1b3bc1e7597fee0826a18a0cf042359f0f"
},
{
"type": "WEB",
"url": "https://phabricator.wikimedia.org/T392976"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
]
}
GHSA-V6Q2-VRQ7-XPRQ
Vulnerability from github – Published: 2022-05-14 03:19 – Updated: 2022-05-14 03:19LiteCart before 2.1.2 allows remote attackers to cause a denial of service (memory consumption) via URIs that do not exist, because public_html/logs/not_found.log grows without bound, and is loaded into memory for each request.
{
"affected": [],
"aliases": [
"CVE-2018-10827"
],
"database_specific": {
"cwe_ids": [
"CWE-400"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2018-05-09T04:29:00Z",
"severity": "HIGH"
},
"details": "LiteCart before 2.1.2 allows remote attackers to cause a denial of service (memory consumption) via URIs that do not exist, because public_html/logs/not_found.log grows without bound, and is loaded into memory for each request.",
"id": "GHSA-v6q2-vrq7-xprq",
"modified": "2022-05-14T03:19:26Z",
"published": "2022-05-14T03:19:26Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2018-10827"
},
{
"type": "WEB",
"url": "https://github.com/litecart/litecart/issues/119"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
]
}
GHSA-V6RC-PPGP-5PMR
Vulnerability from github – Published: 2023-04-02 21:30 – Updated: 2023-04-07 21:30Uncontrolled resource consumption in the logging feature in Devolutions Gateway 2023.1.1 and earlier allows an attacker to cause a denial of service by filling up the disk and render the system unusable.
{
"affected": [],
"aliases": [
"CVE-2023-1580"
],
"database_specific": {
"cwe_ids": [
"CWE-400"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2023-04-02T21:15:00Z",
"severity": "HIGH"
},
"details": "Uncontrolled resource consumption in the logging feature in Devolutions Gateway 2023.1.1 and earlier allows an attacker to cause a denial of service by filling up the disk and render the system unusable.",
"id": "GHSA-v6rc-ppgp-5pmr",
"modified": "2023-04-07T21:30:15Z",
"published": "2023-04-02T21:30:17Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2023-1580"
},
{
"type": "WEB",
"url": "https://devolutions.net/security/advisories/DEVO-2023-0007"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
]
}
GHSA-V6RH-VJ3X-WP5C
Vulnerability from github – Published: 2023-08-14 18:32 – Updated: 2024-04-04 06:55IBM TXSeries for Multiplatforms 8.1, 8.2, and 9.1 is vulnerable to a denial of service, caused by improper enforcement of the timeout on individual read operations. By conducting a slowloris-type attacks, a remote attacker could exploit this vulnerability to cause a denial of service. IBM X-Force ID: 262905.
{
"affected": [],
"aliases": [
"CVE-2023-38741"
],
"database_specific": {
"cwe_ids": [
"CWE-400"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2023-08-14T18:15:11Z",
"severity": "HIGH"
},
"details": "\nIBM TXSeries for Multiplatforms 8.1, 8.2, and 9.1 is vulnerable to a denial of service, caused by improper enforcement of the timeout on individual read operations. By conducting a slowloris-type attacks, a remote attacker could exploit this vulnerability to cause a denial of service. IBM X-Force ID: 262905.\n\n",
"id": "GHSA-v6rh-vj3x-wp5c",
"modified": "2024-04-04T06:55:12Z",
"published": "2023-08-14T18:32:59Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2023-38741"
},
{
"type": "WEB",
"url": "https://exchange.xforce.ibmcloud.com/vulnerabilities/262905"
},
{
"type": "WEB",
"url": "https://www.ibm.com/support/pages/node/7025476"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
]
}
GHSA-V6WH-2WVH-C8X5
Vulnerability from github – Published: 2022-02-09 23:15 – Updated: 2023-09-11 23:10All versions of package djvalidator are vulnerable to Regular Expression Denial of Service (ReDoS) by sending crafted invalid emails - for example,
--@------------------------------------------------------------------------------------------------------------------------!.
{
"affected": [
{
"package": {
"ecosystem": "npm",
"name": "djvalidator"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"last_affected": "1.1.1"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2020-7779"
],
"database_specific": {
"cwe_ids": [
"CWE-400"
],
"github_reviewed": true,
"github_reviewed_at": "2021-04-13T18:54:11Z",
"nvd_published_at": "2020-11-26T11:15:00Z",
"severity": "HIGH"
},
"details": "All versions of package djvalidator are vulnerable to Regular Expression Denial of Service (ReDoS) by sending crafted invalid emails - for example, \n`--@------------------------------------------------------------------------------------------------------------------------!`.",
"id": "GHSA-v6wh-2wvh-c8x5",
"modified": "2023-09-11T23:10:32Z",
"published": "2022-02-09T23:15:46Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2020-7779"
},
{
"type": "WEB",
"url": "https://snyk.io/vuln/SNYK-JS-DJVALIDATOR-1018709"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
],
"summary": "Regular Expression Denial of Service in djvalidator"
}
Mitigation
Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended. A strong authentication and access control model will help prevent such attacks from occurring in the first place. The login application should be protected against DoS attacks as much as possible. Limiting the database access, perhaps by caching result sets, can help minimize the resources expended. To further limit the potential for a DoS attack, consider tracking the rate of requests received from users and blocking requests that exceed a defined rate threshold.
Mitigation
- Mitigation of resource exhaustion attacks requires that the target system either:
- The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question.
- The second solution is simply difficult to effectively institute -- and even when properly done, it does not provide a full solution. It simply makes the attack require more resources on the part of the attacker.
- recognizes the attack and denies that user further access for a given amount of time, or
- uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed.
Mitigation
Ensure that protocols have specific limits of scale placed on them.
Mitigation
Ensure that all failures in resource allocation place the system into a safe posture.
CAPEC-147: XML Ping of the Death
An attacker initiates a resource depletion attack where a large number of small XML messages are delivered at a sufficiently rapid rate to cause a denial of service or crash of the target. Transactions such as repetitive SOAP transactions can deplete resources faster than a simple flooding attack because of the additional resources used by the SOAP protocol and the resources necessary to process SOAP messages. The transactions used are immaterial as long as they cause resource utilization on the target. In other words, this is a normal flooding attack augmented by using messages that will require extra processing on the target.
CAPEC-227: Sustained Client Engagement
An adversary attempts to deny legitimate users access to a resource by continually engaging a specific resource in an attempt to keep the resource tied up as long as possible. The adversary's primary goal is not to crash or flood the target, which would alert defenders; rather it is to repeatedly perform actions or abuse algorithmic flaws such that a given resource is tied up and not available to a legitimate user. By carefully crafting a requests that keep the resource engaged through what is seemingly benign requests, legitimate users are limited or completely denied access to the resource.
CAPEC-492: Regular Expression Exponential Blowup
An adversary may execute an attack on a program that uses a poor Regular Expression(Regex) implementation by choosing input that results in an extreme situation for the Regex. A typical extreme situation operates at exponential time compared to the input size. This is due to most implementations using a Nondeterministic Finite Automaton(NFA) state machine to be built by the Regex algorithm since NFA allows backtracking and thus more complex regular expressions.