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CVE Record

CVE-2021-23437: Regular Expression Denial of Service (ReDoS)

The package pillow 5.2.0 and before 8.3.2 are vulnerable to Regular Expression Denial of Service (ReDoS) via the getrgb function.

HighCVSS 7.5Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

CVE-2021-23437 is a denial-of-service issue in Pillow. A crafted color value handled by getrgb can cause excessive regular-expression processing, making an affected application spend CPU and potentially become unavailable. The business risk is service disruption, not data theft or system takeover based on the supplied sources.

Executive priority

Treat as high priority for internet-facing or customer-facing services using vulnerable Pillow color parsing. The expected impact is outage risk. Internal-only tools without untrusted color input can be scheduled after exposed systems, but should still be upgraded through normal dependency management.

Technical view

Pillow versions before 8.3.2 are reported vulnerable to ReDoS in the getrgb function. Exposure depends on whether an application passes attacker-controlled color strings into Pillow color parsing. CVSS 3.1 is 7.5 with network, low complexity, no privileges, no user interaction, and high availability impact.

Likely exposure

Python applications using Pillow before 8.3.2 are potentially exposed when untrusted input can reach getrgb. Common risk areas include image-processing services, upload workflows, rendering pipelines, and theming features that accept user-supplied color values.

Exploitation context

The source bundle does not show CISA KEV listing or cited evidence of active exploitation. The practical attack context is availability impact: an unauthenticated remote user may trigger expensive parsing if the application exposes vulnerable Pillow color parsing to network input.

Researcher notes

The evidence identifies the vulnerable function and fixed release but does not provide affected application patterns beyond getrgb. Validate reachability before assuming exploitability. Avoid reporting active exploitation unless supported by KEV or a cited advisory not present here.

Mitigation direction

  • Upgrade Pillow to 8.3.2 or a later fixed version.
  • Use fixed distribution packages from Fedora, Gentoo, Debian, or your vendor.
  • Check vendor advisories when pinned versions cannot be upgraded directly.
  • Restrict untrusted color input reaching Pillow until patched.
  • Prioritize externally reachable services that parse user-controlled color values.

Validation and detection

  • Inventory Pillow versions in application dependency manifests and runtime environments.
  • Confirm deployed packages are Pillow 8.3.2 or vendor-fixed builds.
  • Trace whether user-controlled input reaches getrgb or color parsing paths.
  • Review public endpoints that accept image, theme, or color parameters.
  • Monitor affected services for CPU spikes and availability degradation.
Prepared
Confidence
high
Sources
8

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

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CVE-2021-23437 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.5 (3.1)
Known Exploited
No
Published

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

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
0Timeline events
0ADP providers
8Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.5CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H3.93.6Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

7.5High
CVSS 3.1 vector shape for CVE-2021-23437Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

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

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
n/aPillow0, unspecifiedListed
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.