VulnerabilityPublished 14 Jul 2026

CVE-2026-49476: Soupsieve · Soup Sieve has Memory Exhaustion via Large Comma-Separated Selector Lists

Affects soupsieveSource GCVE Database

Mentioned vulnerabilities

CVE-2026-49477

Not known to be exploited · CVSS 7.5 High : upgrade to 2.8.4 soon.

Data as of 26 Oct 2026, 13:47 UTC

CVSS 3.1 base score

7.5High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.6% chance of exploitation in 30 days – higher than 50% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.8.4, 1.4.0_6, 0.52.1_2 +3

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 14 Jul 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade to a fixed release listed under Affected software.

Affected software

Product	Affected	Fixed in
PyPIsoupsieve	Affected <2.8.4	Fixed in 2.8.4
Homebrewadr-viewer`pkg:brew/adr-viewer`Other threats	Affected >=1.3.0 <1.4.0_6	Fixed in 1.4.0_6
Homebrewtoot`pkg:brew/toot`Other threats	Affected >=0.26.0 <0.52.1_2	Fixed in 0.52.1_2
Homebrewcheckov`pkg:brew/checkov`Other threats	Affected >=1.0.855 <3.2.530	Fixed in 3.2.530
Homebrewjupyterlab`pkg:brew/jupyterlab`Other threats	Affected >=3.3.2 <4.5.8	Fixed in 4.5.8
Homebrewpapis`pkg:brew/papis`Other threats	Affected >=0.14.1 <0.15.0_5	Fixed in 0.15.0_5

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

The Red Hat advisory references an update including the python-attrs RPMs (python3-attrs-26.1.0-3.hum1 noarch and python-attrs-26.1.0-3.hum1 src) but does not explicitly confirm that these updates fully remediate the vulnerabilities. No additional mitigation steps are provided by the vendor.

Description

### Summary The CSS selector parser in soupsieve (the CSS selector engine for Beautiful Soup 4) allocates unbounded memory when compiling large comma-separated selector lists. An attacker who can supply a crafted CSS selector string to `soupsieve.compile()` or Beautiful Soup's `.select()` / `.select_one()` can cause the application to allocate hundreds of megabytes of heap memory from a relatively small input, leading to memory exhaustion and denial of service. To be completely transparent, AI tools helped surface this issue. However, it was independently reproduced and carefully validated. Researchers follow responsible disclosure practices and originally shared this report privately. A **500 KB** selector string triggers allocation of approximately **244 MB** of heap memory - a 488x— amplification ratio**. ### Details **Affected code:** `soupsieve/css_parser.py`, lines ~204, 925, 1106 The soupsieve CSS parser splits comma-separated selector lists and creates one `CSSSelector` object per list item. Each `CSSSelector` object contains parsed selector data structures including `SelectorList`, `Selector`, and associated tag/attribute/pseudo-class metadata. When a selector string such as `a,a,a,...` (with 250,000 comma-separated items) is passed to `sv.compile()`, the parser: 1. Tokenises the entire string and identifies each comma-delimited segment (line ~1106) 2. Parses each segment into a full `Selector` object with all associated metadata (line ~925) 3. Stores all parsed selectors in a `SelectorList` (line ~204) **Root cause:** No limit is enforced on the number of selectors in a comma-separated list. The parser will attempt to parse and store an arbitrary number of selectors, with each selector object consuming approximately **976 bytes** of heap memory. The total allocation scales linearly with the number of list items, but the amplification ratio (output memory / input bytes) is extremely high because each single-character selector like `a` expands into a complex object graph. **Attack surface:** Any application that passes user-supplied CSS selectors to `soupsieve.compile()` or Beautiful Soup's `.select()` / `.select_one()`. ### Proof of Concept ```python import tracemalloc import soupsieve as sv tracemalloc.start() # Build a 500 KB selector string: "a,a,a,...,a" (250,000 items) count = 250_000 selector = ",".join("a" for _ in range(count)) print(f"Selector string size: {len(selector):,} bytes ({len(selector) / 1024:.0f} KB)") # Compile the selector â€” this allocates ~244 MB compiled = sv.compile(selector) current, peak = tracemalloc.get_traced_memory() tracemalloc.stop() print(f"Compiled selector count: {len(compiled.selectors):,}") print(f"Current memory: {current / 1024 / 1024:.1f} MB") print(f"Peak memory: {peak / 1024 / 1024:.1f} MB") print(f"Amplification ratio: {peak / len(selector):.0f}x") # Expected output: # Selector string size: 499,999 bytes (488 KB) # Compiled selector count: 250,000 # Current memory: ~244 MB # Peak memory: ~244 MB # Amplification ratio: ~488x ``` ### Impact **Severity: High** An attacker can exhaust available memory on any server-side Python application that compiles user-supplied CSS selectors via soupsieve. This can cause: - **OOM kills** in containerised deployments (Kubernetes pods, Docker containers) with memory limits - **Swap thrashing** on bare-metal servers, degrading performance for all co-located processes - **Process termination** via Python's `MemoryError` exception if the system runs out of addressable memory | Parameter | Value | |---|---| | Input size | ~500 KB selector string | | Memory allocated | ~244 MB | | Amplification ratio | ~488Ã— | | Per-object overhead | ~976 bytes per selector | | Authentication required | None | | User interaction required | None | **Scalability of attack:** The memory allocation scales linearly - doubling the selector count doubles memory usage. An attacker can tune the payload to exactly exhaust a target's memory limits. Multiple concurrent requests multiply the effect. **Downstream exposure:** soupsieve is an automatic dependency of `beautifulsoup4`, one of the most widely installed Python packages. Any web application accepting CSS selectors from users (e.g., web scraping APIs, content filtering tools, CMS preview features) is potentially affected. --- ### Credit Discovered by a security research team from the University of Sydney, focused on detecting open source software vulnerabilities. Liyi Zhou: https://lzhou1110.github.io/ Ziyue Wang: https://zyy0530.github.io/ Strick: https://str1ckl4nd.github.io/ Maurice: https://maurice.busystar.org/ Chenchen Yu: https://7thparkk.github.io/

Impact

The vulnerabilities can cause a denial of service condition affecting the availability of systems using the affected Red Hat Hardened Images RPMs, specifically the python-attrs package. There is no indication of confidentiality or integrity impact. No known exploits have been reported, suggesting limited or no active exploitation at this time.

Summary

The Red Hat security advisory RHSA-2026:23992 reports a high-severity vulnerability affecting Red Hat Hardened Images RPMs, particularly the python-attrs package versions python3-attrs-26.1.0-3.hum1 (noarch) and python-attrs-26.1.0-3.hum1.src (src). The vulnerabilities correspond to CVE-2026-49476 and CVE-2026-49477 and relate to resource management issues (CWE-770) and potentially improper handling of system resources (CWE-1333). The CVSS vector indicates network attack vector, low attack complexity, no privileges required, no user interaction, and impact limited to availability (denial of service). The advisory does not explicitly state that a patch is available or provide remediation instructions beyond referencing the update. No known exploits have been reported in the wild.

CWE-770Allocation of Resources Without Limits or Throttling (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #25
The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.
Requirements
Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.
Architecture and Design
Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.
Architecture and Design
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.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
OWASP ASVS 4.0.3
V8.1.4 (opens in a new tab)
V13.4.1 (opens in a new tab)
V11.1.4 (opens in a new tab)
Attack patterns
XML Ping of the Death (opens in a new tab)
Exponential Data Expansion (opens in a new tab)
Serialized Data Parameter Blowup (opens in a new tab)
Serialized Data with Nested Payloads (opens in a new tab)
Oversized Serialized Data Payloads (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-1333Inefficient Regular Expression Complexity (opens in a new tab)
Base weaknessLikelihood of exploit: High
The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.
Architecture and Design
Use regular expressions that do not support backtracking, e.g. by removing nested quantifiers.
System Configuration
Set backtracking limits in the configuration of the regular expression implementation, such as PHP's pcre.backtrack_limit. Also consider limits on execution time for the process.
Implementation
Do not use regular expressions with untrusted input. If regular expressions must be used, avoid using backtracking in the expression.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.1 (opens in a new tab)
Attack patterns
Regular Expression Exponential Blowup (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 30 Jul 2026, 23:07 UTC

WeaknessesCWE-770, CWE-1333

Technical details

References

Community reviews

Related threats